< > Literature Reports of INCUBATION PERIOD for Rabies (with special reference to Raccoons):

Incubation Period

General

• The incubation period for rabies is very variable, generally several weeks but varying from a week to a year or longer, depending on the virus strain, host species and the site of inoculation. In humans, periods from less than 10 days to as long as six years have been recorded, but more than half of cases show an incubation period of one to three months. Bites to the head or neck are generally associated with shorter incubation while bites to the extremities produce longer incubation periods.

In Procyon lotor - Common Raccoon

• The incubation period for rabies in raccoons has been recorded to be as short as 10 days and as long as 107 days. From a study of naturally occurring raccoon rabies in raccoons in Ontario, Canada it was suggested that the most usual incubation period might be about five weeks, but with the possibility of both shorter and longer periods. 

General

• Variable. May be one week to a year or longer. Depends on strain of the virus, host species, site of inoculation and inoculated dose. (B47)
• Variable, usually one to three months but may range from several days to several years. (B209.1.w1)
• Note: There is no real understanding of the long incubation periods which may be observed in natural cases of rabies (six months or longer in wild-caught naturally infected animals and as long as six years in humans). (J128.9.w1)
• In humans: Usually about four to six weeks; periods as short as less than ten days or at the other extreme longer than a six years can occur. (B409.22.w22)
  • Short incubation periods are generally associated with bites to the head or neck, longer ones with bites to the extremities. (B47, B360.29.w29)
  • About 54% of cases have incubation periods of 31 to 90 days, with approximately 30% showing incubation periods of less than 30 days (the shortest recorded being four days) and only about 15% having an incubation period of more than 90 days (including about 1% with an incubation period of over one year). (B360.29.w29)
• In dogs: Similar to humans. (B47)
  • In one experiment, nine to 42 days with one rabies virus isolate and 12 to 69 days with a second isolate, following inoculation into the masseter muscle. (J13.45.w1)
• In cats: Nine days to 12 weeks experimentally, as long as two years for one natural exposure, and a range of two to 12 weeks, median four to six weeks, in one study of natural cases in Maryland, USA. (J4.202.w2)
• In cattle: average one to two months, range 20 to 165 days. (B47) range 20 to 150 day, usual in natural infections about one month. (B362.w7)
• In sheep and goats: variable; 15 to 65 days reported in one outbreak. (B362.w8)
• In pigs: Very variable; 70 days average in one group of affected pigs. (B362.w9)
• Variable, generally several weeks but may vary from less than 10 days to several months. (B209.1.w1)
• In mice following intracerebral inoculation, sometimes as early as six to eight days, and almost always within 17 days, but occasionally as long as 21 days or perhaps even longer. (B416.44.w44)
• Four different rodent species inoculated with a fox rabies virus showed some differences in incubation periods between species. For all species, the longest incubation periods were associated with the smallest doses of virus. (J1.8.w2)
• Experimentally inoculated Sciurus carolinensis - Eastern grey squirrel showed incubation periods ranging from 18 to86 days, with the longest incubation periods being associated with the smallest doses of virus. (J1.8.w2)
• In hedgehogs: longer incubation period for experimental infection of Aetechinus algirus algirus (Atelerix algirus - Algerian hedgehog) with three different virus strains than is seen in guinea pigs or rabbits. (J137.93.w1)
• In ducks three weeks to eleven months. (B13.32.w3)
• In elephants:
  • Incubation periods recorded in elephants (Elephas maximus - Asian Elephant) in Burma include a month for an elephant bitten on the trunk, six weeks for another elephant (site of bite not confirmed), and four weeks for a calf bitten under one eye. (B212.w41)
  • An incubation period of 43 days was recorded for an adult elephant bitten in several places trunk, forelimb, hind legs and tail). (J12.9.w1, P502.1.w1)
• In great apes: a case in a Pan troglodytes - Chimpanzee, signs started about a month after importation from Sierra Leone to the USA, indicating an incubation period of more than a month, since it was considered likely the chimpanzee had been infected in Africa. (J4.162.w2)

In Procyon lotor - Common Raccoon

Raccoon rabies variant

• Long incubation periods have been recorded for raccoons with rabies: 39 and 79 days after capture for two naturally infected raccoons from Florida, and 107 days following inoculation. (B358.4.w4)
• In raccoons inoculated with a rabies strain isolated from the salivary gland of a raccoon in Florida in 1961, the incubation period ranged from 18 to 107 days. Incubation periods following inoculation with different doses of virus were as follows: 10^4.0 MICLD₅₀, 18 and 4 days; 10^3.9 MICLD_50, 31 and 39 days; 10^3.8 MICLD₅₀, 107 days; 10^3.5 MICLD₅₀, 27 - 66 days. (B358.4.w4)
  • One of the raccoons developed rabies 107 days after inoculation which was 15 days after the initiation of injections of cortisone; it is possible that this represented reactivation of infection. However, even longer incubation periods (137 and 177 days) have been recorded following challenge of Mephitis mephitis - Striped skunk with skunk-variant rabies virus. (B358.4.w4, J13.27.w1)
    • It has been suggested that long incubation periods in semihibernating animals may serve as an overwintering mechanism for rabies. (J13.27.w1)
• In raccoons experimentally inoculated intramuscularly with a suspension from the salivary gland of a naturally infected raccoon from Pennsylvania, at 10^4.2 MICLD₅₀, the incubation period ranged from 18 to 65 days. (B360.16.w16)
• In raccoons experimentally inoculated in the masseter muscle with 0.3 mL of a raccoon rabies isolate from a naturally infected raccoon from Pennsylvania (titre 10^5.8 MICLD₅₀ per mL), the incubation period averaged 20.6 days +/-2.7 days, with all succumbing within 25 days. (J212.8.w1)
• A study of naturally occurring raccoon rabies in the united counties of Leeds and Grenville, Ontario, Canada (where raccoon rabies first entered Canada), where there was intense surveillance for the disease, including trapping of raccoons in 5 km areas around detected cases, looked at 61 cases from 13 July 1999 to 25 May 2001. From analysis of clustering of cases in time and space, it was suggested that the modal incubation period was about five to six weeks, although at short distances (less than 1 km) there was also a significant cluster of cases at three weeks. High pair counts at intervals of less than two weeks were mainly at short distances (less than 3 km) and were considered likely to reflect variation in incubation period for raccoons infected by the same individual. There was also a long tail on the distribution, extending to 19 weeks, and it was commented that long incubation periods increased the risk of an infected individual moving a long distance before transmitting the disease. Recognised limitations of the study included a lack of knowledge of the proportion of rabid raccoons detected. Additionally, problems with the methodology were acknowledged. (J67.56.w1)

Other rabies variants

• In raccoons inoculated with a strain of rabies from a fox salivary gland, isolated in Alabama, the mean incubation period in raccoons was about three weeks, which was considerably shorter than the incubation period for foxes or skunks given the lower lethal dose. (B358.4.w4)
  • Intramuscular inoculation with fox salivary gland suspension from an Alabama rabid fox produced incubation periods of 10-13 days (median 12 days) for raccoons inoculated with 10^2.2 MLD₅₀, 17-42 days (median 26 days) for raccoons inoculated with 10^3.2 MLD₅₀, and 13-35 days (median 19 days) for raccoons inoculated with 10^5.2 MLD₅₀. (B358.4.w4, B360.16.w16, P21.64.w1)
• In a raccoon injected intramuscularly in the right hind leg with 0.2 ml of a skunk rabies isolate, the first signs were seen after 18 days. (J196.58.w1)
• A study comparing survival of raccoons with and without oral vaccination found that in three trials raccoons (unvaccinated or vaccinated) which succumbed to rabies did so within 11 to 41 days. It was further noted that in other experiments, following exposure with similar virulent virus concentrations, no incubation periods over two months had been observed. (J1.25.w4)
• In a raccoon experimentally inoculated intramuscularly in the masseter muscle with 15,000 mouse lethal dose 50 (MLD50) of saliva from a coyote infected with bat rabies from Mexican free-tailed bats Tadarida brasiliensis mexicana) (the coyote had been infected by inoculation with saliva from a coyote which had been infected by a on-bite route following exposure to a cave of the bats), furious rabies developed after an incubation period of 29 days. (J13.27.w3)
• In a raccoon experimentally inoculated intramuscularly in the masseter muscle with 4,000 mouse lethal dose 50 (MLD50) of saliva from rabid Mexican free-tailed bats Tadarida brasiliensis mexicana, furious rabies developed after an incubation period of 29 days. (J13.27.w4)
• In raccoons experimentally inoculated with a rabies strain from a dog (MD 5951) and inoculated in the cervical musculature with 10^5.5 MICLD₅₀ as part of a vaccine trial, clinical signs developed (or death without clinical signs occurred) 10 to 21 days after inoculation. (J135.83.w1)
• Five non-vaccinated raccoons injected intramuscularly with 0.5 ml of 1 x 10^5.5 MICLD₅₀/mL street rabies (MD5951 strain, originally isolated from a dog) died within 10 to 26 days of inoculation. (J1.26.w8)
• In raccoons experimentally inoculated by intramuscular inoculation, into the masseter muscles, of 1 x 105.5 MICLD₅₀/ml street rabies virus (MD5951), as part of a vaccination trial, animals which succumbed to rabies died at 19 to 25 days post infection. (J1.28.w12)
• Raccoons inoculated intramuscularly with 10^6.0 street rabies (MD5951 strain, originally isolated from a dog) died or were euthanased in extremis within 10 to 21 days of inoculation. (J13.50.w2)
• Following inoculation of raccoons with 10^3.2 MICLD₅₀ New York City/Georgia canine strain rabies virus (NYC/GA) from fox salivary glands (using a known lethal dose for raccoons), into the masseter muscle, the incubation period was 11 to 21 days. (J1.28.w8)
• In five raccoons experimentally inoculated in the masseter muscle with a canine rabies virus isolate (MD 5951, at 0.3 ml of culture with 10^6.9 MICLD₅₀ per mL), the incubation period averaged 14.2 days (+/- 2.2 days (SD)) and all succumbed or required euthanasia within 17 days. (J212.8.w1)