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Introduction and General Information

The persistence and spread of raccoon rabies in eastern North America indicates that rabies virus is transmitted effectively by raccoons. 
  • Transmission of raccoon rabies presumably usually occurs by bite, although it is possible that other routes of transmission occur also. Both normal and rabies-related abnormal behaviours of raccoons may be involved in transmission of raccoon rabies among raccoons and from raccoons to other species. Additionally, the behaviour of other species may affect the likelihood of their being infected by a rabid raccoon. It is important to consider the transmission of raccoon rabies in relation to normal behaviours and raccoon ecology, as well as the behavioural effects of rabies in the host in increasing likelihood of transmission. This page is designed to consider all these factors and their interrelationships.

It should be noted that in designing a model for raccoon rabies transmission, the transmission rate had to be estimated indirectly (by inference from estimates of "the threshold density of raccoons below which rabies is unable to persist in the population") (J13.50.w1), as there were no studies quantifying intraspecific or interspecific transmission rates in raccoons. (J1.38.w1)

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Routes of Transmission of Rabies in Raccoons

It is probable that the main route of transmission for raccoon rabies is biting, inoculating virus from the rabid host into the susceptible animal.

  • Raccoons naturally infected with raccoon rabies in Florida (Long Boat Key) were noted to uniformly have rabies virus in their saliva, at high titres, indicating "a highly infective transmission of rabies by their bite". (J1.6.w3)
  • The likelihood of transmission of rabies during contact may be affected by the amount of virus in saliva, the frequency of shedding in saliva and the clinical manifestation of the disease. (J308.5.w1)
  • Note: raccoons may have rabies virus in their saliva before the onset of clinical signs. (B358.4.w4)

Possible alternative routes of transmission through ingestion, mutual grooming (ingestion of saliva), contact with urine of infected raccoons, transplacental transmission and transmission via the milk from mother to cubs should also be considered:

  • There is no evidence for or against natural oral or other mucosal transmission of rabies in raccoons. However:
    • Rabies can be transmitted by the oral route (see Rabies - Life Cycle and Transmission (Viral Reports));
    • Oral absorption of rabies virus has been demonstrated in raccoons during vaccine trials with attenuated or inactivated rabies strains. (J1.25.w4, J1.28.w12,  J1.38.w2), and absorption from the intestines has been demonstrated for inactivated rabies virus. (J1.28.w12)
    • Transmission of rabies can occur by ingestion of infected material (B352.5.w5, J35.135.w1) and raccoons may sometimes eat other raccoons, at least as carrion. See: Common Raccoon Procyon lotor - Natural Diet (Literature Reports)
    • Virus has been isolated from the bladder of a raccoon; this indicates the possibility of transmission of rabies via mucosal contamination by urine, with a susceptible raccoon contacting the urine with its nose or tongue. (B358.4.w4)
  • There is no evidence for transmission of rabies virus via milk in raccoons. However, there is some evidence of transmission from nursing dams to offspring in other species (J35.135.w1).
  • There is no evidence for transplacental transmission of rabies virus in raccoons, but there is some evidence for this occurring, at least occasionally, in other species. (J13.42.w1, J35.135.w1)

However, it is probable that non-bite routes of transmission, if they occur, as in humans are not epidemiologically important. (B360.29.w29)

For further information on rabies transmission see: Rabies - Life Cycle and Transmission (Viral Reports)

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Behaviours which may affect Intraspecific Transmission

"The frequency of raccoon-to-raccoon contact is related to daily and seasonal movements of individual animals, their overall population abundance and distribution, and their behavior." (J308.5.w1)
  • Spatial and temporal patterns of food supply, food type and habitat characteristics may affect raccoon intraspecific contact rates. (P103.1983.w1)

Assuming that the main route of transfer of rabies from an infected to a susceptible raccoon is by inoculation of saliva into a bite wound, it is necessary for the infected raccoon to bite the susceptible raccoon, and to be shedding rabies virus in its saliva at this time. Biting could occur related to normal raccoon behaviour, or related to abnormal raccoon behaviour. 

  • Transmission may occur related to normal behaviour, since raccoons have been confirmed laboratory positive for rabies without showing any behavioural signs of rabies (J1.6.w3) and rabies virus can be present in the saliva before the onset of clinical signs in the infected raccoon. (B358.4.w4)
  • Note: there is only limited direct information about normal raccoon intraspecific interactions in their natural habitats, due to the difficulties involved in watching these nocturnal animals. (J1.38.w1, J30.56.w3, J332.37.w1, J364.4.w1)
Normal raccoon behaviour
  • In general, raccoons appear to avoid one another during their normal nightly activities. However, raccoons do meet and agonistic behaviour, including actual fighting, may then occur (e.g. at aggregated feeding resources - see below).
  • Female raccoons and their offspring remain together for several months - usually until at least fall, often coming together again for winter to den before dispersing in spring. The prolonged time for which this social unit stays together means that there is a high potential for transmission of rabies between adult females and their young. (J1.42.w3) For more information on parental behaviour see: Common Raccoon Procyon lotor - Parental Behaviour (Literature Reports)
    • In Ontario, 1999-2003, there were increased reports of rabid juvenile raccoons in fall (autumn) compared to other times of year; it was suggested this was most likely to be due to transmission from adult females. However, adult males have been seen in close proximity to adult females and their offspring at this time of year, so transmission from adult males could occur also. (J1.42.w3)
Effects of linear habitats and runways
Behaviour at aggregated feed sources

At aggregated feeding resources, several raccoons may be present at the food resource at one time. Interactions between raccoons at such sites may be peaceful, or only threats may be employed in agonistic interactions. However, at least in some circumstances, fighting, with biting, occurs at such sites.

  • Early outbreaks in Florida were noted to occur in large, dense raccoon populations, in close association with humans, making use of human food sources such as garbage cans, dumps and landfills. (B358.4.w4, J1.6.w3)
    • On Long Boat Key, large artificial feeding stations were available (food provided by a local restaurant and by residents), providing an "overcrowded" population with the potential for increased competition and contact. (B358.4.w4)
  • A study at a winter feeding site in Nebraska, USA, where corn was provided for grouse and pheasants, found that from November to January, in apparently competitive encounters between groups of adult raccoons, an arriving group of raccoons made threats and was then accepted to feed, with the population feeding peacefully. However, in February, as the breeding season started, fighting would occur, with characteristic growling and squealing audible. (J332.37.w1)
  • A study at a rural garbage dump in Ontario, Canada, (mid June to early September) found that biting of raccoons by raccoons was relatively common. It was considered possible that at similar feeding sites in urban habitats, where raccoon concentrations tend to be higher, even higher rates of intraspecific contact might occur. (J1.38.w1)
  • For further information on use of and interactions at aggregated food resources see: Common Raccoon Procyon lotor - Social Behaviour - Territoriality - Predation - Learning (Literature Reports) - Interactions with the same Species
Effects of population density

Higher population densities enhance the transmission of rabies (and other infectious diseases). High population density may increase opportunities for contacts between individuals and may also increase stress and lower resistance to disease. (J1.21.w6, J40.67.w2, J308.5.w1) Population density is thought to play a role in raccoon rabies epidemiology.

  • In Florida, a study of raccoon rabies between 1953 and 1972 showed that outbreaks in raccoon populations occurred in artificially concentrated by habitat loss and food availability around human habitation; the fastest spread of rabies occurred in the densest population. (J101.98.w1)
    • Reported epizootics of rabies in raccoons in the southeastern USA appear to have occurred "where the raccoon population has become very dense because of overreproduction and/or reduction of available habitat." (J100.125.w1)
    • On Long Boat Key (where an outbreak of raccoon rabies occurred) , natural habitats were rapidly destroyed by construction projects and large artificial feeding stations were available, providing an "overcrowded" population with the potential for increased competition and contact, which may have made the raccoons more susceptible to pathogens and increased transmission potential. (B358.4.w4)
  • Raccoon population densities are higher in urban areas than in rural ones due to higher survival rates and greater site fidelity, but high density populations are more susceptible to disease outbreaks, which reduce survivorship. (J40.67.w2)
    • Aggregate distribution leads to more intra-species contact, increasing disease transmission. (J40.67.w2)
  • Population concentrations may be very high in suburban and urban areas close to parkland, which is likely to facilitate rabies transmission in such areas. (J308.5.w1)
  • A study of rabies in Loudoun county, Virginia, 1981 to 1982, noted that the outbreak may have been linked to environmental and geographical factors: rainfall had been low for September 1980 to July 1981, this would have decreased favourable habitat and led to an influx of raccoons into the Goose Creek watershed area, which may have led to physiological stress - increased intra-specific competition and hormonal and neurological changes - which may lower resistance to introduced disease. (J1.21.w6)
  • Further information on raccoon population density is provided in: Common Raccoon Procyon lotor - Social Behaviour - Territoriality - Predation - Learning (Literature Reports)
Seasonal activity and breeding behaviour
  • "The breeding cycle (mating, pregnancy and parturition) appears to be a major factor in the transmission of the virus." (J1. 21.w6)
It is possible that in raccoons, as in foxes, variation in contact rates due to breeding behaviour may result in a seasonal pattern to rabies transmission. (J1.28.w10) 
  • Some, but not all, analyses of data on the occurrence of raccoon rabies, epizootic or enzootic, indicate a peak of incidence of rabies in the breeding season in spring. (B358.4.w4, J1.26.w10, J1.28.w10, J4.197.w1, J4.200.w3, J4.201.w2, J4.203.w2, J4.205.w2, J4.207.w1, J4.209.w1, J4.211.w2, J4.213.w3, J4.215.w1, J4.217.w1, J4.219.w5, J4.221.w10, J4.223.w1, J4.225.w1, J4.227.w1J101.98.w1, J101.126.w1, N7.37.w1, N7.37.w1, N7.38.w1, N7.41.w1, N7.45.w1)
  • Factors which might increase rabies transmission during the mating season include:
    • Larger home ranges and potentially higher movement rates for males during the mating season may lead to increased social interactions and greater chance of contracting disease. (J40.63.w2, J101.98.w1)
    • Increases in contact rates and aggression among raccoons. (J1.28.w10, J1.42.w3)
    • An increase in fighting between males during the mating season would increase the chance for rabies virus transmission. (J101.98.w1)
    • The increased interactions between males and females could increase the chance of contracting epizootic disease. (J40.63.w2)
    • A sexually mature male may mate with more than one female and therefore may transmit the virus to more than one female if infected. (J1.21.w6)
      • However, prevalence in juvenile females was 72% (during a study in Loudoun County, Virginia, 1981-1982), indicating breeding not being a major factor for the high prevalence rate in females in this outbreak. (J1.21.w6)
    • Increased virus circulation due to increased contact and rate of interaction between raccoons of both sexes. (J101.98.w1)
    • Hormonal changes of females during oestrus or parturition. (J101.98.w1)
    • Stress of pregnancy may make females more susceptible to infection. (J1.21.w6)

Note: increased incidence of reported cases of raccoon rabies could also occur during the mating season because increased movement and activity patterns of raccoons during the breeding season making raccoons more obvious and likely to be observed by humans. (J101.98.w1)

Linking incidence of raccoon rabies with the breeding season may be complicated by:

  • Variation in the length of incubation of rabies in raccoons: the incubation period for raccoon rabies may vary from 10 days to more than three months. (See: Rabies - Incubation Period (Disease Reports)) Therefore, infection acquired during mating may result in clinical disease at parturition, or during the fall, when juveniles are dispersing. (J308.5.w1)
  • Geographic variations in the timing of the breeding season. (B358.4.w4)
  • Other factors affecting timing of raccoon rabies, such as what time of year an epizootic arrives in a given area. (J270.10S4.w1)
Other factors which may affect transmission
  • Dispersal:
    • It is possible that in raccoons, as in foxes, variation in contact rates due to juvenile dispersion may result in a seasonal pattern to rabies transmission. (J1.28.w10)
    • It was considered possible that peaks of major reporting of rabies in raccoons in fall 1993 might possibly be related to fall dispersal of juveniles. This was noted to be in agreement with independent assessments of raccoon activity based on trapping and road-kills. (J4.205.w2)
  • Mutual grooming:
    • Limited mutual grooming of raccoons does occur, including facial rubbing. (B402.4.w3) This raises the possibility of transmission of saliva without biting.
  • Denning and den sharing: 
    • Activities in the pre-denning and denning period may increase transmission. (J1.42.w3)
    • In northern latitudes, raccoons compete for denning sites during the fall. (J1.42.w3)
    • Transmission could occur during denning (J1.42.w3). Raccoons are known to share dens. Generally this involves a pair of raccoons in the mating season, mother and offspring, or siblings. However, unrelated raccoons which are not paired do share dens sometimes. Den sharing is particularly common in winter. See: Common Raccoon Procyon lotor - Social Behaviour - Territoriality - Predation - Learning (Literature Reports) - Interactions with the same species
  • Stress:
    • Undetermined stress factors may increase susceptibility to disease (J1.6.w2) and this may allow the occurrence of localised epizootics of raccoon rabies in an enzootic area. (D222.3.w3)
      • A study of the rabies epidemic in Loudoun county, Virginia, 1981-1982, noted that the outbreak may have been linked to environmental and geographical factors: rainfall had been low for September 1980 to July 1981, this would have decreased favourable habitat and led to an influx of raccoons into the Goose Creek watershed area, which may have led to physiological stress - increased intra-specific competition and hormonal and neurological changes - which may lower resistance to introduced disease. (J1.21.w6)
Effects of rabies on behaviour
Rabies commonly affects the behaviour of the rabid host in a manner likely to increase transmission of the rabies virus, particularly by increased aggression in the furious form of rabies. (J214.187.w1, J214.187.w2)
  • Abnormal behaviour is commonly noted in rabid raccoons, including aggressive behaviour, (P66.1.w1, J1.21.w6) although not all confirmed rabies-positive raccoons show behavioural signs of rabies. (J1.6.w3)
    • Rabid raccoons have been reported attacking other raccoons. (B358.4.w4) This would increase the likelihood of transmission of rabies.
    • The potential transmission of rabies from a given individual raccoon may be affected by active, aggressive versus passive, paralytic rabies. (J308.5.w1)
    • Animals may be more likely to attack conspecifics which are behaving abnormally (V.w5, V.w6). Abnormal behaviour of rabies-infected raccoons could therefore lead to fighting with other raccoons even if the rabies-infected raccoon was not specifically aggressive. 
  • It is known that some rabid animals wander long distances beyond their usual home range. (J15.23.w4) It is not known how much rabies affects raccoon movement. However:
    • During a study in new Jersey, two rabid raccoons had dispersed seven to eight kilometres over a period of four to 10 months. In general, raccoons in the study moved a mean of 1.2 km, with a maximum distance moved of 11.4 km. (P102.4.w2) Since agonistic behaviour is more likely to lead to fighting between raccoons which do not know each other than between neighbours (J22.185.w1), increased wandering could increase the likelihood of the rabid raccoon fighting and transmitting rabies.
    • In eastern Ontario, an adult female raccoon captured, ear-tagged, vaccinated and released had moved 1.7 km when recaptured one day later. One the second occasion it was euthanased due to definitely abnormal behaviour - agitation, high-pitched whining and repeatedly turning upside down - and it was found to be rabid; it was noted that average raccoon density in this area was 10 raccoons per kmē, and that therefore there was a high potential for the rabid raccoon to have transmitted rabies to other raccoons. A few kilometres away, a juvenile male had moved 800 m between capture and being found dying of rabies 17 days later. (J435.119.w2)
    • For five ear-tagged raccoons, distances moved by the time the raccoons were reported rabid were 1.564 - 4.143 km, over periods of 14 to 1,092 days). It was noted that these were mid to low range of maximum distances reported to be moved by normal (non-rabid) raccoons, but higher than the median distances moved by non-rabid raccoons. (J1.42.w3)
  • Aimless wandering noted in some raccoons with rabies (J1.37.w2) could also result in their moving into new areas and being more likely to become involved in conflict with other raccoons.
  • For information on the effects of rabies on raccoon behaviour see: 
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Behaviours which may Affect Interspecific Transmission

Both the behaviour of raccoons (normal or rabid) towards other species and the behaviour of other species towards normal or rabid raccoons must be considered in interspecies transmission of raccoon rabies. (J330.75.w1)
  • Additionally, it should be remembered that, while there is no evidence for raccoon rabies variant circulating independently in any other species such as skunks, (J84.9.w24) individual cases in other species such as rodents or domestic animals may be transmitted via a skunk (or other animal) with raccoon rabies. (J101.126.w2)
  • Note: there is only limited direct information about normal raccoon interactions with other species in their natural habitats, due to the difficulties involved in watching these nocturnal animals.
Normal behaviour of raccoons and other animals
Species vary in their tendency to be infected with rabies virus: inquisitive species are more likely to be bitten by a rabid animal than are timid species, and very small species are unlikely to survive a bite from a rabid animal, therefore are unlikely to develop rabies. (J15.23.w3)
Interactions with skunks (particularly Mephitis mephitis - Striped skunk)
Raccoon rabies is common in skunks in areas where this rabies variant occurs; in some states with raccoon rabies, there are more reported cases of rabies in skunks than in raccoons. This indicates effective transmission of this rabies variant to skunks. (J4.197.w1, J4.215.w1, J84.9.w24, J91.57.w1)
Interactions with Lynx rufus - Bobcats
Lynx rufus - Bobcats are (after the known carnivore reservoirs have been discounted) one of the carnivore species most commonly diagnosed with rabies. In the 1990s, most cases of rabies in bobcats occurred in the southeastern US, as spillover from raccoon rabies. (J1.39.w3) Further information on the occurrence of raccoon variant rabies in this species is provided in Rabies virus - Definitive Mammal Host Species (Viral Reports)
  • Bobcats are known to be one of the major predators of raccoons. (B180.w1, B402.7.w6)
Interactions with Rodents and Lagomorphs
The majority of cases of rabies reported in rodents (Rodentia - Rodents (Order)) and lagomorphs (Lagomorpha - Lagomorphs (Order)) in the USA from the 1980s onwards have occurred in states with raccoon rabies, have been temporally and spatially linked with reports of raccoon rabies, and are considered to be caused mainly by spillover of raccoon rabies. (J1.22.w6, J1.33.w11) Further information on the occurrence of raccoon variant rabies in these species is provided in Rabies virus - Definitive Mammal Host Species (Viral Reports)
Interactions with domestic animals

There have been considerable increases in rabies in domestic animals, such as cats, associated with the expanding raccoon rabies epizootic in the USA, and a clear geographical concordance between areas with raccoon rabies and areas with rabies in domestic animals. Typing of rabies virus isolates from domestic animals has confirmed raccoon rabies virus variant in a number of cases. (B360.3.w3 ,J4.202.w2, J4.203.w2, J4.205.w1, J4.207.w1, J4.209.w1, J4.211.w2, J4.213.w3, J4.215.w1, J4.217.w1, J4.218.w4, J4.219.w5, J4.221.w10, J4.223.w1, J4.225.w1, J4.227.w1, J19.132.w1, J101.126.w1, P66.1.w1) Further information on the occurrence of raccoon variant rabies in these species is provided in Rabies virus - Definitive Mammal Host Species (Viral Reports)

  • High population densities of raccoons in urban parks and suburban areas increase the probability of encounters between domestic animals and rabid raccoons. (J1.39.w4)
  • A study of raccoon rabies in cats in Maryland noted that "cats are at high risk of exposure to rabid raccoons because of their small size and free-roaming, predatory nature." It was also noted that all the infected cats were free-roaming, and 84% spent more than half of their time outdoors. (J4.202.w2)
  • Cattle may be diagnosed with rabies more commonly than other livestock due to an increased risk of exposure to wild animals while foraging on pasture. (D239). It has been suggested that most cases of rabies in cattle in the USA, from raccoons or skunks, occur when the bovine, being by nature curious, investigates a rabid animal which is behaving strangely, and gets bitten on the muzzle. (B353.w6)
  • Note: non-rabid as well as rabid raccoons may have contact with domestic animals, particularly with dogs. A study in Virginia, Maryland and Pennsylvania 1982 - 1983 found that 54 submitted rabies negative raccoons had had contact with dogs and four had had contact with cats. (J101.126.w1)
Interactions of raccoons with humans

While there has been only one case of raccoon rabies virus variant recorded in a human, thousands of humans have been given PEP after direct or indirect contact with rabid or possibly rabid raccoons, or indirect exposure via a domestic animal. (J4.202.w2, J4.225.w1, J84.5.w4, J84.8.w16, J84.8.w16, N7.52.w7) Further information is provided in Rhabdoviridae- Rabies virus (with special reference to raccoon rabies variant) (Viral Species) - Degree of Hazard (Risk to Humans / other Species)

  • Humans may be more likely to be exposed to rabies from raccoons than from skunks or foxes, not due so much to different behaviour of the rabid animal, but to closer associations between humans and raccoons, their being more likely to be kept as pets, and a greater likelihood that a human will approach a sick raccoon than a skunk or fox. (B358.4.w4, J100.123.w1)
  • It has been noted that the majority of the exposures of humans to rabid or potentially rabid raccoons are due to people failing to use their common sense in interacting with wild animals. (J101.126.w1)
  • Orphaned raccoons may be handled by large numbers of people, particularly children if taken to a school. One hand-reared kitten which died and was found to be rabies-positive had been responsible for 250 potential exposures, including the members of the family which were rearing it who, when it became ill and reluctant to suck, had been placing their fingers, dipped in milk, in the raccoon's mouth. (J4.163.w4)
  • Raccoons are cute and playful, particularly as cubs, and therefore may frequently be adopted as pets after being found in the wild. There are several known incidences in which such pet raccoons, after shorter or longer periods, became ill and/or aggressive and were found to be rabid (in one case, a raccoon found as a kit, but which did not develop clinical rabies until "adult size"). Between one and more than 100 individuals have been determined to be potentially exposed and required post-exposure prophylaxis in such incidents. (D222.4.w4)
  • In general, humans who have been bitten by a wild carnivore such as a raccoon in the USA are likely to seek medical attention and receive post-exposure prophylaxis for rabies. For this reason, deaths due to rabies transmitted directly from wild carnivores [including raccoons] in the USA are rare. (J308.6.w1)
  • In areas with raccoon rabies, both epizootic and enzootic, cases of rabies in other species are increased as well as cases in raccoons, increasing the concern for rabies following bites of humans. (J4.217.w1, J4.217.w2)
  • Both rabid and non-rabid raccoons may bite or scratch humans. (J101.98.w1)
  • Exposure to raccoon rabies may also occur by indirect contact, via pets. (J91.57.w1)
  • High proportions of raccoons submitted due to being found close to human dwellings during daylight were rabid, in Loudoun County, Virginia, 1981-1982. (J1.21.w6)
  • In the early years (1977 - 1983) of the mid-Atlantic rabies outbreak, a study noted that potential exposures of humans to raccoon rabies resulted from single (four people, 6%) or multiple (one person) bites, saliva on an open wound (two people), as well as 13 people 19% who had skinned or dressed a rabid animal. A further 31 people (42) were given PEP having only touched a rabid animal and 22 people (30%) had indirect contact - e.g. handing a dog or touching an object which a rabid raccoon had touched. Just five people (9%) of those given PEP reported having been attacked by a rabid raccoon, while seven (12%) had petted, 16% (28%) had fed, seven (12%) had lifted, eight (14%) had played with a raccoon and 15 (26%) had skinned a raccoon. (P66.1.w1)
  • Data for 1,610 raccoons submitted for testing between 1st January 1982 and 20th September 1983, from the mid-Atlantic raccoon rabies focus states of Virginia, West Virginia and Maryland, found that for people potentially exposed to rabies through a raccoon, fox or skunk, 25% had been exposed through bites, scratches or saliva in open wounds, 25% while skinning or dressing a carcass and nearly 50% of those receiving treatment had "questionable" exposure: touching an animal, or indirect exposure via a dog or inanimate object (in 17% of exposures, the potentially rabid animal had attacked the person). (J101.126.w1)
  • Note: Raccoons have successfully adapted to suburban and urban environments, where they may be found at high densities, increasing the risk of human exposure. (J91.57.w1) They may be found in places such as yards of houses. This increases the likelihood of interaction with humans. Data from about 700 raccoons from six counties and cities in four states affected early in the mid-Atlantic raccoon rabies outbreak found that: 
    • Submitted raccoons were collected most commonly from yards (61%) and 88.3% of these were rabid; a further 16% were collected from buildings (78.5% rabid), 17% from cultivated areas (82.3% rabid), only 2% from uncultivated areas (100% rabid) and 16% for "other" locations (78.9% rabid). (P66.1.w1)
    • Residential areas were the origin of 68% of submitted raccoons (62.6% rabid), 13% were found in recreational areas (63.6% rabid), 9% in agricultural (96.6% rabid), 5% in commercial (16.7% rabid), 1% in industrial (75.0% rabid) and 11% in "other" areas (9.1% rabid). (P66.1.w1)
    • Further information on types of habitats used by raccoons is provided in: Common Raccoon Procyon lotor - General Habitat Type (Literature Reports)
  • Many raccoon populations are semidomesticated, living in close proximity with humans. (B358.4.w4, J100.123.w1, J101.98.w1)
    • "The danger of the rabies enzootic in raccoons is enhanced by the number of rabid raccoons found in residential areas and, specifically, in private yards." Because raccoons will search for food in such places, garbage control is important to keep a residence or area free of raccoons. (J4.201.w2)
    • Common features of epizootics of raccoon rabies in Florida in the late 1960s/early 1970s included close associations with humans, the raccoons using garbage cans as a feeding resource, and concentrations around garbage dumps and landfills. (B358.4.w4)
    • The close association of semi-domesticated raccoons and humans in such places results in large numbers of exposures of humans to rabid raccoons. (B358.4.w4, J101.98.w1)
Effects of rabies on interspecific behaviour
  • "Since rabies affects the central nervous system, rabid animals may exhibit aberrant behaviours, leading to increased contact between the species and cross-species transmission of the virus." (J84.9.w24)

Rabid raccoons often show altered behaviour. Behaviours seen in rabid raccoons than might increase transmission of rabies to other species include:

  • Actively aggressive behaviour
    • Actively aggressive behaviour including unprovoked or provoked attacks on humans or other animals, has been noted in many, although by no means all, raccoons with rabies. (B358.4.w4, B360.16.w16, J1.6.w3, J1.38.w1, J1.21.w6, J4.157.w2, J101.126.w1, J196.58.w1, P66.1.w1)
  • Failure to avoid other species 
    • It is possible that rabid raccoons lose their fear of dogs, leading to more encounters between rabid raccoons and dogs than between normal, healthy raccoons and dogs. Dogs may be particularly likely to be bitten by rabid raccoons when they harass sick raccoons which have wandered into towns or other communities. (J1.21.w6, J101.126.w1, J330.75.w1)
    • There are few reports of raccoon rabies in porcupines (Erethizon dorsatum - Erethizontidae - New-world porcupines). However, interactions between raccoons and porcupines might increase when raccoons have clinical signs of rabies. In one study in Ontario, Canada, four of seven raccoons with embedded porcupine quills were rabies-positive. (J1.42.w3)
      • In eastern Ontario, an adult female raccoon captured, ear-tagged, vaccinated and released had porcupine quills in her nose when recaptured one day later. One the second occasion she was euthanased due to definitely abnormal behaviour, and was found to be rabid. (J435.119.w2)
  • Interaction with humans
    • Behavioural changes induced by rabies infection mean that rabid raccoons are more likely to come into contact with humans than are healthy raccoons. (J100.125.w1)
      • Some raccoons are actively aggressive and may attack humans directly. (B358.4.w4, J101.126.w1)
      • Rabid raccoons may attack domestic animals such as dogs (B358.4.w4) or cats [assumed from the cases of raccoon rabies in domestic cats], and humans may then become exposed directly (trying to intervene) or indirectly (due to fresh saliva from the rabid animal on the pet's fur getting into an open wound on a person) or from a bite of the infected pet. (J84.5.w4, J84.8.w16)
      • Humans may approach raccoons which appear to be either unusually tame, or sick, and may then be bitten or scratched. (J1.6.w3, J100.123.w1, J330.75.w1)
  • For further information on behavioural signs of raccoons with rabies see: 
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How Raccoon Rabies Spreads in Raccoon Populations in their Habitats

Rabies virus is transmitted directly from animal to animal. Geographical spread of raccoon rabies must therefore involve movement of infected raccoons, either as normal behaviour, or after the onset of clinical signs, or by human action. The geographical spread of raccoon rabies within raccoon populations is affected by a number of factors including raccoon ecology and behaviour, geographical features and human interventions, whether deliberate or accidental. This section explores these factors, and their interactions.

  • Spread of rabies in a wildlife population, from an initial focus, may be basically radial. However, physiogeographic features may affect the rate of spread of rabies, so that, for example, rapid linear spread may be noted along valleys where the habitat is suitable and the density of the reservoir/vector species is high, while other features, such as ridges or expansive mature forests may, by physically disrupting suitable habitat and sustaining only lower density populations, act as barriers to movement of reservoir/vector species, slowing transmission. (J91.27.w1)
  • While some of the factors enhancing or restricting the spread of raccoon rabies are easily identified, other factors are more obscure. It is not known what barriers, physical or biological, held back the spread of raccoon rabies into the Florida panhandle until 1970, nor what restricted movement of the disease into Alabama while it was spreading northward through Georgia to South Carolina in the 1970s. (B358.4.w4, V.w42)
    • Part of the delay in recorded spread of raccoon rabies into South Carolina may be due to limitations of the passive surveillance system. (CDC)
  • Long-distance translocation events (LDTs), if they result in rabies becoming established at a new location, may allow rabies to cross barriers, whether natural (e.g. rivers) or artificial (vaccine belts). (J4.215.w2, J67.71.w1, J249.111.w2)
    • Long-distance translocation of raccoons for hunting is considered to be the method by which raccoon rabies reached the mid-Atlantic states. 
    • Long distance translocation events do not always result in successful development of a new disease focus: data from Connecticut indicated that some apparent LDTs were either false or failed, in that an initial single reported rabid raccoon (or occasionally, two reports) at a location in advance of the main raccoon rabies front was then not followed by further reported cases for some time, or possibly were successful but further cases were not detected by surveillance. (J67.71.w1)
Observed speed of spread of raccoon rabies
Speed of spread of rabies has been noted to vary in association with known movement rates of the main hosts. (J14.15.w2) Calculations using mean, minimum and maximum rabies incubation period and mean, minimum and maximum raccoon movements provide a suggestion that raccoon rabies could expand by as little as 3 km per year up to more than 400 km per year, with a mean of 30-50 km per year. This fits with the observed spread, which has however been restricted by various barriers. (J308.5.w1)
  • In the southeastern states in the 1970s, raccoon rabies spread at about 25 miles per year (40 km/yr). (J308.5.w1, P66.1.w1) In the mid-Atlantic and north-eastern states, spread has been observed varying from about 25 to 50 km/yr. (J1.34.w8, J4.200.w3, J4.201.w2, J19.132.w1, J91.57.w1, N7.32.w1)
Dispersal and long distance movements of raccoons
Information from radio-tracking studies suggests relatively small home range sizes and short average dispersal movements in raccoons. However, information from ear-tagging studies involving large numbers of raccoons in Ontario, Canada, suggest greater mobility. (P102.8.w1) Long distance movements of raccoons are not common, but do occur; movements of about 265 km (165 miles) by young male raccoons have been recorded and one study in Canada found that as many as 56% moved more than 10 km and 35% more than 20 km. (J187.64.w1, J332.42.w1, J332.48.w1)
  • An analysis of detected cases of raccoon rabies in Ontario, Canada, indicated that the incubation period may be prolonged (e.g. 19 weeks) in some cases and it was noted that long incubation periods increased the risk of an infected individual moving a long distance before transmitting the disease. (J67.56.w1)
  • It is known that some rabid animals wander long distances beyond their usual home range. (J15.23.w4) Increased movement may increase the likelihood of a rabid raccoon encountering naive hosts (J1.34.w8) and could increase the rate of movement of a raccoon rabies epidemic along the wave front.
    • In New Jersey during one study, two rabid raccoons had dispersed seven to eight kilometers over an observation period of four to ten months. (J308.5.w1)
    • In New Jersey during a vaccination study, while 19 radio-collared raccoons moved a mean straight line distance of 1.5 +/- 0.5 km (range 0.8 - 2.58 km), five raccoons (including one radiocollared) trapped and ear tagged while clinically normal, and later diagnosed as rabid, had moved a mean of 8.4 +/- 4.5 km (range 1.9 - 12.9 km), and the maximum linear movement of the rabid raccoons was significantly higher (P = 0.001) than that of non-rabid collared raccoons in this area. The two longest distance movements, 11.6 and 12.9 km, were recorded in raccoons recovered three and two months after original trapping respectively. (J1.34.w8)
  • Projections predicting spatial dynamics for raccoon rabies through Ohio, based on a stochastic model with parameters calculated using data from Connecticut and New York State, noted that long-distance transmission (LDT) events were likely to assist rapid spread of raccoon rabies across central Ohio. (J363.3.w1) [Such events could arise through natural or human-assisted (see below) movements of raccoons]
  • Further information on raccoon movements is provided in Common Raccoon Procyon lotor - Social Behaviour - Territoriality - Predation - Learning (Literature Reports) - Home Ranges and Distances Travelled
Geographic features enhancing spread
  • Rivers encountered parallel to the direction of spread can increase rate of spread, along corridors of prime raccoon habitat. (J197.271.w1)
    • Early studies of raccoon rabies in Florida found a "time and space relationship" between reports of rabies along river systems/waterways; data indicated that in the seven years following the first reported case in 1947, rabies had spread through raccoon populations north and south along major waterways. (J101.98.w1, J329.75.w1)
    • Increased rate of spread appears to have occurred in New York State along the Hudson River. (J197.271.w1)
    • In Maryland from 1981-1987, it was noted that the disease had spread through Maryland raccoons at about 15 to 25 miles per year in an east-northeasterly direction, with spread facilitated by waterways. (J4.201.w2)
    • In New Jersey, it was noted that the raccoon rabies epidemic moved from north-west New Jersey to the south of New Jersey along watersheds. North to south river drainages were thought to be specifically the route by which the vaccination area in southern New Jersey was breeched: 13 of the 16 rabid raccoons found in the vaccination area were detected along this route, as were eight other rabid animals. (J1.34.w8)
    • In Vermont, (first case in northwest Vermont in February 1992, found in southwestern Vermont in June 1994), it was noted that the epidemic had been spreading northwards up the Champlain Basin and the Connecticut River valley. [1997](N7.45.w1)
    • Note: Raccoons feed along water courses and may need to pass one another along the shore or bank. Agonistic encounters in such situations probably do not normally involve actual fighting (J332.28.w1) but increased aggression in a raccoon with rabies could result in biting. 
Geographic features acting as barriers
While it has been noted that raccoon rabies has spread at about 30-50 km per year since it began to expand from central Florida, this spread has not been unrestricted. Both from the southeast focus, starting in Florida, and from the mid-Atlantic focus, starting on the Virginia/West Virginia border, spread of raccoon rabies has been affected by barriers, some of which appear to be absolute, others which have been proved only temporary. 
  • Geographical features such as rivers and mountain ranges have acted as barriers to the spread of raccoon rabies. (J4.201.w2, J67.71.w1, J197.271.w1, J308.5.w1)
  • Absolute barriers to date have included large bodies of water such as the Great Lakes and the Chesapeake Bay, and the Appalachian Mountains. (J308.5.w1)
  • Relative barriers have included major rivers e.g. the Susquehanna, Potomac, Delaware and Hudson. (J197.271.w1, J308.5.w1)
Barriers may (a) prevent access of infected raccoons to susceptible animals; (b) affect population density (J308.5.w1): mountainous upland areas support a lower, and less productive, population of raccoons than do bottomlands by permanent water sources (13 to 23 times lower, in one study in western Tennessee). (J308.5.w1, P101.42.w1) For further information on the population effects, see below.
  • The observed spread of raccoon rabies has been restricted by various barriers: large bodies of water such as the Great Lakes provide a barrier, major rivers and highways, particularly when found together (multi-lane highways paralleling river banks) appear to act as a barrier which is not insurmountable but may be effective for some years. Mountains also act as a barrier, not just in physical terms but because raccoon population densities are reduced in upland, mountainous areas. (J308.5.w1)
  • In Connecticut, the Connecticut River, perpendicular to the direction of spread of raccoon rabies, appears to have not only slowed spread of rabies across the state but also to have deflected the movement to a more northerly, rather than easterly, direction. (J91.57.w1, J179.271.w1) Cases east of the river were initially limited to a small area (whereas cases west of the river were occurring along a longer front). (J91.57.w1)
  • In Maryland from 1981-1987, it was noted that spread of raccoon rabies was slowed by mountains. Spread to the Eastern Shore was delayed for several years by the presence of two major roads within two miles of one another, plus the 100-yard-wide Chesapeake-Delaware Canal. However, once past these barriers in 1989, raccoon rabies spread once more, reaching halfway down the Eastern Shore peninsula by 1992.  (J4.201.w2)
  • Projections predicting spatial dynamics for raccoon rabies through Ohio, based on a stochastic model with parameters calculated using data from Connecticut and New York State, considered it reasonable to assume that the Ohio River would serve as a barrier to movement of raccoons (and thus of raccoon rabies). (J363.3.w1)

Movements across natural barriers:

  • Some natural barriers, such as rivers and large roads, are partial rather than absolute barriers.
    • Four raccoons ear-tagged in Ontario, Canada, were recaptured in New York State, USA, while a further 12 individuals ear-tagged on the Ontario mainland were recaptured on islands within the St Lawrence River. (J187.64.w1)
    • In Maryland, while spread to the Eastern Shore was delayed for several years by the presence of two major roads within two miles of one another, plus the 100-yard-wide Chesapeake-Delaware Canal, rabies passed these barriers in 1989. (J4.201.w2)
    • Even large rivers only partially restrict spread of raccoon rabies: infection reached Ontario, Canada, in 1999, despite separation from northern New York State by the St Lawrence River, which is about 1 km wide at this point; ear-tagged raccoons have been known to cross the river in this area. (J279.2.w3, J1.37.w2)
    • Major rivers including the Delaware, Hudson and Connecticut rivers were all crossed by raccoon rabies. (J4.213.w4)
    • Projections predicting spatial dynamics for raccoon rabies through Ohio, based on a stochastic model with parameters calculated using data from Connecticut and New York State, recognised that rabid raccoons could potentially cross on bridges over the Ohio River. (J363.3.w1)
Effects of population density
Higher population densities may facilitate transmission of raccoon rabies (see above: Behaviours which may affect Intraspecific Transmission)
  • High population density was noted as one of the common features in outbreaks of raccoon rabies in Florida (Long Boat Key, Marco Island and Tyndall Air Force Base) in the late 1960s/early 1970s. (B358.4.w4)
  • Rapid spread of raccoon rabies in the mid-Atlantic region may be related to relatively dense raccoon populations found in urban and suburban areas where there is an abundance of food combined with shelter in adjacent parklands. (J308.5.w1)
  • A raccoon rabies front may not be propagated effectively through an area with a low raccoon population density. (J67.71.w1)
  • One way in which mountains act as a barrier to the spread of raccoon rabies is because raccoon population densities are reduced in upland, mountainous areas. (J308.5.w1)
    • In New York state, spread of raccoon rabies slowed on reaching the Adirondack Mountains area. This area represented a change to a less-preferred habitat for raccoons, coniferous forest (rather than deciduous forest habitat which the disease had previously been spreading through) and was likely to have affected raccoon densities and movements. However, there was also a vaccination campaign in the area at the same time and it was not possible to distinguish the relative effects of the habitat change and the vaccination programme in reducing the rate of spread. (J179.271.w1)
  • Other unfavourable habitats may also result in reduced population densities.
    • In Loudoun county, Virginia in 1981 to 1982,the low number of raccoons submitted from the southeastern section of the county was considered likely to be related to the lowland flat, less favourable habitat with few streams, compared to the western and northern areas, with rolling to steep topography with many streams as well as cultivated land (corn), pastures and hardwood areas. (J1.21.w6)
  • It is probable that population dynamics and changes in the number of susceptible individuals in the population influence temporal trends in wildlife rabies. (J1.28.w10)
  • Note: the presence of raccoons may not be the only requirement for propagation of raccoon rabies. It was noted that, while raccoon populations were "abundant" along the coast of the Florida panhandle, and raccoon rabies was spreading north in Florida, with 48% of cases reported in the northern part of Florida in 1960-1965, the disease did not spread westwards into the Florida panhandle until November 1970. (B358.4.w4, J100.123.w1) Similarly, while raccoon rabies spread through Georgia starting in the early 1960s, (B358.4.w4, J1.6.w3, J100.123.w1) it did start expanding though Alabama until the late 1970s (D235, D236), and although it reached the Georgia-South Carolina border in 1971/1972 (B358.4.w4), expansion through South Carolina was recorded only starting in 1977. (D235, D236)
Human mediated translocation: deliberate and accidental

Raccoons can travel very long distances with the deliberate or accidental assistance of humans. This may allow transport of diseases, including rabies, over long distances. (J4.215.w2, J249.111.w2)

  • There is considerable potential for accidental translocations of rabid raccoons to breach natural barriers such as major rivers. (J279.2.w3)
  • Even local translocation may allow rabies to cross geographic barriers or immunological barriers created by oral vaccination, or simply to spread more rapidly into contiguous populations. (J4.215.w2, J67.71.w1, J249.111.w2)
Deliberate
  • Deliberate translocation of raccoons from the south-eastern USA is considered to be the most likely source of mid-Atlantic/north-eastern USA rabies epizootic in raccoons. (J4.215.w2, J100.149.w1, J237.36.w1)
    • Direct spread from the southeastern states to the mid-Atlantic focus is unlikely: no cases of raccoon rabies were reported from the 300 miles between these points. (J100.149.w1)
    • Thousands of raccoons have been imported into the mid-Atlantic area for hunting purposes yearly. (J100.149.w1, J237.36.w1) More than 3,500 raccoons were moved from Florida to Virginia with legal permits in 1977 to 1981. (J101.126.w1)
    • Rabid raccoons were detected in raccoon shipments from Florida to the mid-Atlantic area. (J100.149.w1) Rabies was detected in a raccoon in a shipment from Florida to Virginia in 1981; there are also reports of rabid raccoons in shipments to North Carolina. (J101.126.w1)
      • Rabies was detected in two raccoons from Florida, found dead on arrival in North Carolina in the late 1970s; other raccoons from the same shipments (of 137 and 86 raccoons), which could have been infected, were released before the rabies was diagnosed in the dead individuals. It was noted that raccoons had been held together in a large pen pre-shipment, and shipped in cages each containing eight raccoons, therefore an unknown number of the released raccoons could have been incubating rabies. (J323.69.w1)
      • More than 3,500 raccoons were translocated to southwest Virginia from Florida and it has been confirmed that there were rabid raccoons in such shipments (despite legal permits and health certificates for the raccoons). (J270.10S4.w1)
    • The rabies isolates from the mid-Atlantic focus raccoons have been found to be identical to isolates from raccoons from the southeastern raccoon rabies endemic area, but different from those from bats, or from terrestrial vertebrate hosts from other locations. This data supports the theory that the mid-Atlantic outbreak originated from infected animals transported from the south-eastern United States. (J100.149.w1, D239) - See: Surveillance for Rabies in Raccoons
  • An unknown but large number of raccoons caught as "nuisance" animals are translocated varying distances both by individual members of the public and by animal control agencies.
    • Relocation is considered a practical method of dealing with live-trapped nuisance animals. (P70.5.w1)
    • Nearly 6,000 "nuisance" raccoons were officially translocated to woodlots or forest preserves in rural areas in Illinois in 1994. Studies indicate that such raccoons generally leave the release site and disperse into the surrounding area. (J40.63.w1)
    • In New Jersey, while raccoons in the northern Cape May Peninsula were being vaccinated to control the spread of raccoon rabies southwards into the peninsula, two homeowners were noted to be illegally relocating raccoons - one moving them 53 km from north of the vaccination area to well south of the area (releasing them at the tip of the Cape May Peninsula), and the second 18 km to a wildlife management area on the southern border of the vaccination area. (J1.34.w8)
    • Despite provincial legislation under the Fish and Wildlife Conservation Act making it illegal to relocate raccoons in Ontario, Canada, they are still relocate by members of the public and by many animal control agencies. (J1.37.w2)
    • It was noted [1999] that raccoons were being translocated long distances within Ontario by animal control agencies and rehabilitators. (J59.25.w1)
Accidental
  • Long-distance transportation of raccoons is known to occur within vehicles transporting household refuse: raccoons have been observed to spill out of trucks when refuse is dumped. (J91.57.w1, J179.271.w1)
    • It appears that raccoon rabies "jumped" 100 km from Connecticut northeastwards to northcentral Massachusetts via raccoons transported inadvertently by refuse trucks: cases apparently occurred first at a landfill site serving towns in western Connecticut where rabies was being reported in raccoons, and drivers of garbage trucks reported that they regularly found live raccoons in the refuse being dumped at the landfill site. (J1.37.w2, J91.57.w1)
  • It was hypothesised that a rabid raccoon found on Long Island, New York (where rabies had been absent for decades), may have been transported inadvertently in a vehicle. (W27.12Aug04.R1, W27.14Aug04.R1)
  • During the period 1996 to 1998, 15 incidents were reported in which raccoons were transported from the raccoon rabies enzootic area of the USA to the Greater Toronto area of Ontario, Canada, via tractor trailers. (J1.37.w2)
  • By 1999, at least six raccoons were known to have reached Toronto in transport trucks from locations within the raccoon rabies areas of the USA. (J59.25.w1)
  • In Ontario, 14 different raccoons were noted moving an average of 479 km from the US into Ontario, Canada, in the back of transport trailers. (J435.X.w1)
  • An example of the possible distances which may be travelled by raccoons with inadvertent human assistance is the individual which exited from a container in Le Havre harbour, Normandy, France, having survived since the container was sealed five weeks earlier in Houston, Texas, USA. (W27.18Aug04.R1)
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Maintenance of Rabies in the Raccoon Population

Once rabies has been established within a species in a particular geographical area, transmission within that species can persist at enzootic levels for decades and perhaps for centuries. (J4.213.w3)

In small areas, raccoon rabies does not necessarily persist in a population. (J364.4.w1) However, in general, raccoon rabies has been found to persist in a given state or larger region, either as an enzootic disease (e.g. in Florida) or as repeated epizootics (e.g. in mid-Atlantic/northeastern states). (B360.16.w16, J1.6.w3, J1.26.w6, J4.202.w2, J4.227.w1, P10.19.w1) Further information on occurrence of rabies in different areas is provided in Rabies Virus - Distribution and Geographical Occurrence (Viral Reports)

Possible mechanisms for development of epizootics of rabies in raccoons include:

  • Introduction of the virus into a naive population. (J1.6.w2)
  • Increased contact between raccoons due to high population density. (J1.6.w2)
    • When the raccoon rabies epidemic developed in the Mid-Atlantic states, it was noted that the urban areas contained large, dense and susceptible raccoon populations, giving ideal conditions for rabies transmission. (P66.1.w1)
    • "Large increases in numbers of reported cases of rabies in states where the disease is enzootic among raccoons result from cyclic or periodic reemergence when populations of raccoons, decimated by a previous epizootic, again reach densities sufficient to support epizootic transmission of rabies." (J4.213.w3)
  • Stress associated. (B358.4.w4)
    • "Reactivation of chronic latent infections after a prolonged period appears to be associated with physiologic changes during breeding activities and stress." (B358.4.w4)
  • "All the conditions of the natural host population." (J1.6.w2)
  • Temporary or artificial increases in population density due to human-related factors such as destruction of habitat, or to environmental factors reducing useable habitat, may be involved in epizootics:
    • Outbreaks on Long Boat Key, Marco Island and Tyndall Air Force Base, all in the coastal habitat of the Gulf Coast of Florida, were associated with disturbance and habitat destruction, close association with humans, feeding from garbage cans and concentrations around dumps and landfills. (B358.4.w4)
    • In Loudoun County, Virginia, in 1981-82, it was noted that the outbreak may have been linked to environmental and geographical factors: rainfall had been low for September 1980 to July 1981, this would have decreased favourable habitat and led to an influx of raccoons into the Goose Creek watershed area, which may have led to physiological stress - increased intra-specific competition and hormonal and neurological changes - which may lower resistance to introduced disease. (J1.21.w6)
    • In Posey Hollow, at the National Zoological Park's Conservation and Research Center, Virginia, where a raccoon rabies epidemic occurred in 1980-1981, a severe drought in summer 1980 led to disappearance of most running water and extreme clumping of water-related food resources for the raccoons. It was noted that such conditions could lead to prolonged co-occurrence of raccoons at food sites and intensification of social interactions. (J363.4.w1, P103.1983.w1) See above: Behaviours which may affect Intraspecific Transmission

The main factor responsible for the decline in raccoon rabies following an epizootic peak is thought to be a reduction in the number of susceptible individuals remaining: deaths of raccoons due to rabies would probably reduce the abundance of raccoons, thus there would be fewer infectious contacts and fewer new cases. This would explain why, following local epizootic rabies activity, cases then rapidly decreased. (J4.213.w3, J91.57.w1, J270.10S4.w1)

Cycles of infection
  • In the mid-Atlantic outbreak, in the 1980s it was noted that after several years, a previously affected area could develop a new epizootic wave. (J270.10S4.w1)
  • In Maryland, following the initial epizootic the disease has remained enzootic in the state with minor upsurges of rabies at intervals of three to four years as the population recovered after each outbreak. (J4.201.w2)
  • In Virginia, from 1984 to 1989, in five counties in northern Virginia in which rabies incidence had been high throughout the epidemic, the percentage positive showed a two- or three-year cyclic peak, while this was not evident in the absolute percentage positive. Suggested reasons for these cycles were increases in susceptible populations due to new raccoons being born, or due to movement of raccoons. (J1.28.w10)

Raccoons have periods of greater and lesser contact with one another during the year, which may affect disease transmission.

  • During the mating season there is contact between males and females for mating, and conflict between males for access to females. Males may mate with more than one female and females may mate with more than one male during oestrus. (B285.w2, B402.9.w8, B403.27.w27, J101.98.w1, J334.57.w1)
  • During much of the summer, raccoons may have little contact with one another. However:
    • Males may remain in groups. (J334.55.w1)
    • Raccoons may interact at shared resources such as garbage dumps. (J1.38.w1)
    • Females and their offspring generally remain together after weaning until the fall or the following spring, and such relationships may be resumed after the mating season. (B402.8.w7, B403.27.w27, B405.w1, J332.79.w1)
    • In areas of high population density, daytime resting sites such as a den tree or a brushpile may be shared. (J30.76.w1, J334.55.w1)
  • During the winter, raccoons may become relatively inactive, not truly hibernating, but remaining in dens for prolonged periods. (B180.w1, B402.6.w5, B403.27.w27, B405.w1, J59.16.w2) However, raccoons may have contact with one another while sharing dens, (B402.6.w5, P62.13.w1) and, during active periods, at communal feeding sites. (B406.38.w38, J331.148.w1, J332.37.w1)
  • While raccoons may be less social during summer and autumn, opportunities for interaction could occur where raccoons have to pass one another, for example along the banks of water courses, or in suburban and urban habitats, where they may follow paths along buildings, hedges and walls, and in drain culverts. (J332.28.w1, J332.58.w1)
  • In harsh environmental conditions, raccoons may use remaining clumped food resources for longer periods than usual, providing a longer period during which contacts may occur. (J364.4.w1, P103.1983.w1)
  • In urban and suburban habitats, as well as in some natural habitats, clumped food resources, at which raccoons may interact, may be present year round. (J364.4.w1, P103.1983.w1)
  • Further information on social interactions is provided in:
  • It has been suggested that raccoon rabies will continue to be transmitted (and therefore persist in a population) if the period of the year during which raccoon are not interacting with one another is sufficiently short to be bridged by the incubation period of rabies in raccoons. (J364.4.w1, P103.1983.w1)
Endemic rabies

"In the enzootic state, a slow, continuous circulation of the virus within the maintaining host population must occur by subclinical, mild and chronic, or lethal but prolonged infections. The proportion of infected animals of a particular host population that must possess this perpetuating type of infection to maintain an enzootic focus may vary according to the host species, biocenose, or virus infectivity and pathogenicity." (J1.6.w2)

Possible mechanisms for maintenance of rabies in the raccoon population after an epizootic, allowing continued enzootic rabies, include survival and seroconversion of some infected raccoons, which are then protected by their immunity, or low herd immunity combined with a long latency period for infections, allowing continued low-level transmission. (J91.57.w1)

  • Herd immunity, if present, may hamper the development of epizootic levels of rabies. (J100.148.w1)
  • It is not known what proportion of raccoons develop natural immunity. (J270.10S4.w1)
  • A high population turnover would decrease the effect of development of immunity and a low population turnover would increase its effect. (J4.213.w4)
  • Note: different raccoon populations show different rates of population turnover, as indicated by the proportion of juveniles in the population; population age structure can also vary between years in a given population. Further information is provided in: Common Raccoon Procyon lotor - Life Stages (Literature Reports) - Longevity/Mortality

Information relating to development of immunity to rabies in raccoons is mixed:

  • Mathematical modelling applied to data from the mid-Atlantic and northeastern US states from the mid-1970s onward, fit best for a population in which the development of immunity was rare, with less than 5% of susceptible raccoons becoming immune when contacting the virus. (J135.97.w3)
  • A number of studies have shown antibodies to rabies in 3.0% to 35% of the raccoon populations studied. (J1.28.w9, J13.23.w1, J63.5.w1, J100.123.w1, J100.148.w1, J270.10S4.w1)

It is not known what role subclinical, mild and chronic or prolonged but lethal infections may play in the maintenance of endemic raccoon rabies.

  • Possible mechanisms for chronic, latent infections to be established include:
    • Low quantities of virus in the inoculum (i.e. in the saliva entering a bite wound);
    • Low virulence components in the virus gene pool;
    • Transmission by routes other than biting, such as direct or indirect contact between urine from an infected raccoon and the nose or tongue of a susceptible raccoon. (B358.4.w4)
  • It has been suggested that long incubation periods may assist in the maintenance of rabies over winter in "semihibernating species." (J13.27.w1)
  • Long incubation periods (at 79 and 107 days post capture) have been observed for raccoons developing rabies, but it is not known with what frequency such long incubation periods occur. (B358.4.w4)
  • A study looking at temporal associations between detected cases of rabies in raccoons in Ontario, Canada, suggested that the modal incubation period might be about five to six weeks, although some case clusters suggested the possibility of shorter (three weeks) incubation periods and the data were also consistent with some individuals showing much longer incubation periods, up to 19 weeks. (J67.56.w1)
  • It is possible than chronic latent infections occur and are then reactivated after a long period, reactivation being associated with physiological changes during breeding activity and stress. (B358.4.w4)
  • Further information on the incubation period is provided in Rabies - Incubation Period (Disease Reports)
Associated techniques linked from Wildpro

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Authors & Referees

Authors Debra Bourne MA VetMB PhD MRCVS (V.w5)
Referee Dr Robert G. McLean (V.w42), Rick Rossatte (V.w95)

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