Virus / Picornaviridae / Foot and Mouth Disease Virus / Detailed Viral Descriptions:
Long page - please wait to load

< > Literature Reports of LIFE CYCLE AND TRANSMISSION for Foot and Mouth Disease Virus:

OVERVIEW

Editorial Summary (Editorial Overview Text Replicated on Overall Disease page - Foot and Mouth Disease virus)

For a disease to remain in a population, each infected animal must infect at least one other animal. If it infects less than this, the number of infected animals will decrease and the disease will die out. If it infects more than this, the number of infected animals will increase. In order for Foot-and-Mouth Disease to be transmitted from one animal to another the virus which causes the disease must a) be produced by, and move out of, an infected animal; b) move from the infected animal to another animal; c) enter a susceptible animal.

SOURCE: THE VIRUS MUST FIRST MOVE OUT OF AN INFECTED ANIMAL. The main ways in which the virus moves out of an infected live animal are in air-borne droplets from the lungs and the fluid-filled vesicles; in the saliva, urine, faeces, semen and milk; secretions from the eyes, nose, prepuce and vagina; and through direct contact with skin or pieces of infected skin that may drop off the animal. Virus may be produced by an infected animal before the damage to the skin and mucous membranes is evident. The virus may also multiply in and thereby be spread by some animals which do not have clinical signs (subclinically affected animals) and may multiply in and possibly be spread from carrier animals.

It may also be transmitted in milk and infected carcasses on meat, hides, and bones which are frequently transported locally, regionally and between continents.

Occasionally, outbreaks are associated with escapes of virus from a laboratory.

SPREAD: THE VIRUS IS SPREAD COMMONLY BY THE MOVEMENT OF INFECTED ANIMALS. THE VIRUS MAY ALSO MOVE INDEPENDENTLY WHILST OUTSIDE THE ANIMAL. Humans, ticks, birds, rodents, dogs and cats may carry and spread the virus if they contact any of the above body substances, and, in the case of humans and probably other vertebrates, for a short time after they have breathed in the infected airborne droplets (in humans up to 28 hours). The wind may move the air-borne droplets containing the virus up to 250km over water and shorter distances over land.

INFECTION: THE VIRUS MUST THEN RE-ENTER A SUSCEPTIBLE ANIMAL in which it can multiply. This can be through inhalation (the lungs), ingestion (the gut system), sexual transmission, conjunctival membranes (eye), inoculation (injection) and damaged skin.

(B47, B58, B73, B207, B210, B213.w1, B216, B217.38.w38, B396, B494.7.w7 - full text provided, B495.3.w3 - full text provided, D34, J3.82.w3, D36.Para.20, D36.Para34, D36.Para37-38, D36.Para39, D36.Para41, D36.Para92, D36.Para94, D36.Appendix II, D36.MapVI, D37.Para128, D37.Para216, D37.MapV, J3.83.w2, J3.89.w1, J3.96.w3, J3.102.w5, J3.108.w3, J3.110.w4, J3.110.w5, J3.111.w3, J3.131.w1, J3.134.w1, J3.148.w3, J3.148.w5, J16.22.w1, J18.41.w1, J18.49.w1, J19.66.w2, J19.68.w3, J19.68.w2, J19.73.w1, J19.74.w1, J19.114.w1, J21.13.w1, J21.16.w1, J21.23.w1, J21.40.w1, J21.41.w1, J21.43.w1, J21.46.w1, J21.69.w1, J35.149.w1, J42.75.w1, J42.84.w1, J42.85.w2, J42.91.w1, J42.118.w1, J62.53.w1, J63.14.w1, J64.7.w2, J64.15.w1, J64.16.w1, J64.21.w23, J64.21.w28, J67.32.w1, J68.B302.w1, J72.41.w1, J75.20.w1, J223.77.w4, J249.91.w2, P5.40S.w2, V.w5, V.w23, W46.Jun01.sib1)

Transmission Mechanisms that have been reported
  • Transmission Mechanisms have not been key-worded for FMD

To Top of Page
Go to general Foot and Mouth Disease Virus page

Sources of Virus

Source Information
Virus Replication
  • Replication takes place in the cytoplasm. The RNA of the virion acts as messenger RNA and is translated into a polyprotein. This polyprotein is then cleaved to yield some eleven individual proteins (B216).
  • Initial virus replication in ruminants following inhalation is in the tissues of the pharynx area. Viraemic spread follows with virus spreading to numerous other tissue types and organs (B216).
  • Epitheliotrophic (J63.14.w1)
  • Virus may be found at high levels in a wide variety of tissues including lymph nodes, adrenals, myocardium, pancreas, thyroid, mammary gland and skin (B210.89.w89)

Sources of virus include:

Infected animals, prior to the development of clinical signs
  • Virus may be found in blood, milk and samples from the pharynx, rectum (swabs) prepuce (swab), vagina (swab) and semen several days prior to the development of clinical signs. (J3.82.w3, J3.83.w2, J3.87.w1).
  • Cattle and sheep may be sources of infection for up to five days prior to the development of lesions allowing diagnosis, and pigs for up to ten days before the development of clinically apparent lesions (J3.82.w3).
  • Large quantities of virus can be released during the pre-clinical period of infection. For example, in saliva up to 10 days before the onset of clinical signs, in semen and milk for four days before clinical signs, in faeces, urine and exhaled reath one to two days before clinical signs. (B495.3.w3 - full text provided)
  • Virus may be excreted in milk and semen for up to four days before disease becomes clinically apparent (J19.114.w1, J35.125.w1, J342.76.w1, D36.Para35)
  • Virus is excreted in air from infected animals, with excretion beginning before the onset of clinical signs (J18.41.w1, J19.68.w5, J42.75.w1, J72.41.w1).
  • Animals exposed to airborne virus release airborne virus up to 22 hours after exposure (from virus trapped on the animal during exposure) (J21.23.w1).
  • Animals exposed to airborne virus release airborne virus again two to seven days after exposure: including from susceptible animals which develop lesions, vaccinated animals which develop lesions and vaccinated or recovered sheep/pigs which do not develop lesions (J21.23.w1).
  • the amounts of virus excreted by different routes may vary with virus strain. (B495.3.w3 - full text provided)
Infected animals, during the period of clinical signs. Animals output virus in:
  • Expired air (J18.41.w1, J19.68.w5, J35.127.w1, J42.75.w1, J72.41.w1, B58, B210). Virus excretion has been demonstrated in exhaled air/respiratory secretions of cattle, sheep, Syncerus caffer - African buffalo, Capreolus capreolus - Roe deer, Dama dama - Fallow deer, Cervus nippon - Sika deer, Cervus elaphus - Red deer, Muntiacus muntjac - Muntjac, Erinaceus europaeus - European hedgehogs (J3.96.w3, J18.49.w1, B209).
  • Different sizes of particles are released on the breath of infected animals, with large and medium sized particles released from the upper respiratory tract and small particles from the lower respiratory tract (J21.29.w1)
  • Epithelium of lesions (e.g. from tongue, feet) (J18.41.w1) (B47, B58, B210)
  • Fluid from vesicles. (B47, B58, B210).
  • In domestic cattle rupture of vesicles in the mouth usually occurs within 24 hours, releasing fluid containing as much as 108 infective units of virus per milliliter. (B58)
  • Milk (J3.87.w1, J18.41.w1, J35.125.w1, B58, B210).
  • Urine (J18.41.w1, B58, B210).
  • Faeces (J18.41.w1, B58, B210).
  • Saliva (B47, B58, B210).
  • Nasal, oesophageal-pharyngeal, lachrymal, preputial and vaginal secretions (B58, B210).
  • Semen (J18.41.w1, J342.76.w1, B58, B210).
    • Trace amounts of virus has been detected in semen of bulls for 42 days after the onset of clinical signs. (J342.76.w1)
    • Boars also can excrete virus in semen during the period of clinical signs. (J342.76.w1)
    • SAT virus has been detected in semen and sheath-wash specimens from Syncerus caffer - African buffalo. (J3.145.w6)
  • Animals infected with FMDV are rarely infective for more than four days after vesicles have ruptured, except for the persistence of virus on their skin or hair. (B207).
  • Virus may persist in mammary tissue for three to seven weeks (B207).
  • Syncerus caffer - African buffalo. During acute infection, excrete in respiratory aerosols, saliva and nasal secretions similar amounts of virus to the rates seen from cattle, but for slightly longer (J64.7.w2).
Animals with subclinical infection (infected but no obvious disease such as visible lesions).
  • Shown able to transmit infection to susceptible animals in contact (J21.46.w1)
  • Animals with sub-clinical infection can be highly contagious despite lack of clinical signs. (J64.21.w28)
Carcasses, prior to disposal
  • Risk until safely disposed of (J21.69.w1, J35.127.w1).
  • Potential source of airborne virus if manhandled or during inefficient initial burning of carcasses (J18.49.w1).
  • Epithelial fragments and secretions from carcasses may continue to be a source of virus for infection, particularly if the environmental temperature is low (J35.127.w1)
  • Airborne virus may persist in areas holding pigs for at least 24 hours after the pigs have been killed and precautions should be taken for at least 48 hours (J35.127.w1).
Carriers
  • Carrier animals might represent a risk of initiating further outbreaks of FMD (J42.118.w1).
  • Ruminants (cattle, sheep and others) may become carriers; pigs do not (J72.41.w1).
  • Ruminants may become FMD carriers [defined as FMDV being present in the oropharynx 28 days or more after the onset of clinical signs]:
    • Following recovery from clinical disease;
    • Following undetected subclinical disease;
    • Following exposure of vaccinated animals to virus.
  • In all these situations, only a percentage of animals become carriers. Not all ruminants (individuals), vaccinated or otherwise, become carriers following contact with FMDV (J42.118.w1).
  • In experimentally infected Aepyceros melampus - Impala and Connochaetes taurinus - Blue wilderbeest, none of the animals became carriers (J1.11.w6).
  • Note: vaccination of animals with properly inactivated vaccines will not in itself produce carriers. (J64.21.w28)

Potential for transmission from carrier animals

  • Carrier animals are defined as animals in which it is possible to detect live virus more than 28 days after infection. However (except for Syncerus caffer - African buffalo) whether the virus is shed from these animals in sufficient quantities to infect susceptible livestock is debated. Experimentally, it has not been possible to prove transmission of virus from carrier animals to susceptible livestock under controlled (biosecure) experimental conditions, although a few field experiments have claimed to show such transmission. Many experiments attempting to transmit infection from carrier livestock to susceptible animals, involving large numbers of animals, have failed to show any transmission. There are a few reports of outbreaks of disease in ruminants after contact with possible carrier cattle, indicating that transmission from carrier cattle is rare, and none associated with carrier small ruminants (J64.21.w28, J249.91.w2). 
  • "Carrier animals are considered to represent a risk of initiating further outbreaks of FMD, although the experimental evidence for this is inconclusive." (J42.118.w1).
  • "... there is considerable anecdotal and field evidence that [carriers] may play a role in new outbreaks of disease." (J42.118.w1).
  • "In South America and Africa, there is strong circumstantial evidence to implicate carrier cattle in the spread of virus." (J16.22.w1)
  • "Detailed scientific evidence for the infectiousness of carriers is weak." (B495.8.w8 - full text provided)
  • N.B. experimental inoculation of susceptible cattle with oesophageal/pharyngeal fluid from carrier animals 16 weeks after infection required the inoculation of up to 4 mL of fluid intralinguodermally to produce clinical FMD. "It may be that in the natural state the amounts of carrier virus transmitted from animal to animal are too small to initiate clinical disease." (J19.66.w2).
  • No transmission from 740 recovered Bos taurus - Domestic cattle to 50 Bos taurus - Domestic cattle penned with them for two months or to 50 Sus domesticus - Domestic pig penned with them for a slightly shorter time (J10.17.w1).
  • Epidemiological evidence of infection occurring on previously uninfected farms following the introduction of recovered Bos taurus - Domestic cattle (length of time between recovery and movement of animals not indicated) (J10.17.w1).
  • After two-and-a-half years of carrier Syncerus caffer - African buffalo (SAT3) being penned with susceptible Bos taurus - Domestic cattle (J3.117.w3).
  • Transmission was not recorded from carrier (SAT1, SAT2, SAT3) Syncerus caffer - African buffalo cows to either their own calves or to susceptible Bos taurus - Domestic cattle housed in an adjacent pen (J62.53.w1).
  • Transfer from carrier Syncerus caffer - African buffalo to in-contact Syncerus caffer - African buffalo but no transfer from Syncerus caffer - African buffalo to in-contact cattle, over a period of two and a half years (J21.16.w1).
  • Transmission of FMD from carrier Syncerus caffer - African buffalo to Bos taurus - Domestic cattle was reported from Zimbabwe (on an island, occurring five months after infection of the buffalo and after the Bos taurus - Domestic cattle had been in contact with the buffalo for the whole period since infection) with nucleotide sequencing showing virus source (J3.134.w1).
  • No evidence of transmission from carrier Bos taurus - Domestic cattle to contiguous vaccinated cattle (J19.68.w1).
  • Viruses from diseased Bos taurus - Domestic cattle and carrier Syncerus caffer - African buffalo from the same area are different, indicating a lack of interspecies transmission (J21.40.w1).
  • Transmission occurred following several months of Syncerus caffer - African buffalo bulls being kept with Bos taurus - Domestic cattle cows (J223.77.w4).
  • There is epidemiological evidence which suggests that carrier Bos taurus - Domestic cattle may have been responsible for outbreaks of FMD in Zimbabwe, with new outbreaks coinciding with the lifting of movement restrictions on recovered animals (J42.118.w1).
  • "investigation of the causative strain by nucleotide sequencing indicated that the virus was persisting in carrier cattle infected during previous outbreaks. The latter had been controlled by vaccination and quarantine of infected farms , and new outbreaks often coincided with the lifting of the quarantine and the movement of recovered animals.... since the recognition of the danger of these carrier animals there have been no further outbreaks in the FMD-free areas [of Zimbabwe]" (J42.118.w1).
  • Data from a carrier virus survey in three localities in Botswana, detected virus (oesophageal/pharyngeal) in 18-23% of cattle in herds in an area with 7 months since the last outbreak, and in an area with 12 months since the last outbreak, in 3% of cattle from one location but 20% of cattle from another location. In the location with 20% virus isolation, isolation of virus was isolated from a 5-6 month old (estimated age) calf, born since the last outbreak of FMD in that area; "The results suggest that transference of carrier virus from animal to animal may occur in the absence of clinical infection." (J19.66.w2)
  • "there is some evidence that in a small number of cases, the virus may pass from carrier animals to susceptible animals; this is suggested by the presence of foot-and-mouth disease antibody in a few susceptible animals after exposure to carriers and by one of them resisting challenge with foot-and-mouth disease virus." (D36.Para41).
  • No success in experimental attempts to transmit FMD virus from carrier Ovis aries - Domestic sheep to susceptible contact animals (B217.38.w38).
  • Some circumstantial evidence that carrier Ovis aries - Domestic sheep may have been responsible for a recrudescence of disease in Denmark in 1983 (B217.38.w38).
  • In experimental vaccination of cattle and subsequent exposure to infection, while some vaccinated animals became carriers (virus or viral antigen detected on probang samples more than 28 days post-infection), transmission of virus to naive sentinel animals did not occur. (J70.23.w1)
  • There is no evidence that vaccinated animals, even if they become carriers, cause infection in susceptible animals: "follow-up outbreaks, caused by the presence of vaccinated carrier animals, have never been observed." (P5.40S.w2).
  • "There have been a number of anecdotal reports of carriers starting new outbreaks of FMD in the field." In Zimbabwe, following an outbreak in 1987, in which cattle were vaccinated and quarantined for 18 months, further outbreaks were recorded, due to the same SAT2 virus strain, shortly after cattle movements were resumed, and again in 1981 after a further period of quarantine. It was suggested that the stress of being moved and mixed resulted in carriers shedding virus and infecting susceptible animals (J64.21.w23).
  • N.B. Detection of carriers is possible on a herd/flock basis even if not reliably on an individual basis due to intermittent excretion.
    • "This [sampling by probang and isolation of virus in cell culture] method of identifying carriers probably has a 50% sensitivity, which can be improved by repeated probang sampling" (J42.118.w1).
    • Use of both viral culture and PCR increases detection rates (J42.118.w1).
      • Note: PCR-positive samples indicate the presence of part of the viral genome, but not necessarily the presence of live virus (J42.118.w1).

Further information regarding the factors affecting the development of carrier status is presented in the Literature Reports: Time Course/ Persistence of Disease in a Susceptible Population page.

Animal products
  • Carcasses of animals killed in disease control operations, following slaughter and prior to disposal (burial or burning).
  • Muscle meat, (inactivated within three days of slaughter as a result of decreasing pH) (D34
    • In meat, the virus may be inactivated by developing acidity in rigor mortis. However, if meat is quick-frozen, preventing acid formation, the virus can survive. It may then be destroyed by resumption of acid formation once the meat is thawed. (B396)
    • Meat which is dry salted or pickled in brine may remain infective (B396).
  • Organ meat e.g. liver.
  • Lymph nodes: virus may be recovered from fresh lymph nodes, and survive for weeks or months in refrigerated lymph nodes.
  • Bone marrow.
  • Hides: virus may and survive for weeks or months in green unsalted hides
  • Milk: virus may survive as long as 18 hours; survival is affected by temperature and pH. The virus is not inactivated by flash pasteurization. The virus is not inactivated by drying of milk to produce milk powder, nor by processing to cheese, butter or casein products. (B396)
  • Semen.
  • Embryos (not if zona pellucida is intact) (J67.32.w1).

(B396, J18.41.w1, J67.32.w1, D34)

An analysis of potential sources of FMD virus and the relative hazards posed by different animals and animal products has been compiled by the US Department of Agriculture Centres for Epidemiology and Animal Health (W46.Jun01.sib1).

Laboratories working with live virus
  • Laboratories working with live FMDV must comply with Containment Group 4 regulations. (B396)
    • Some outbreaks in Europe have been associated with accidental escape of FMDV from laboratories, or incomplete inactivation of vaccine virus. (B396, J9.293.w1, J80.61.w1)
Factors affecting virus production
Virus strain
  • The amount of virus excreted varies with the strain of FMD virus (B495.3.w3 - full text provided, J3.152.w4, J19.68.w5).
Host species
  • Pigs exhale much more virus than do ruminants such as cattle, sheep, deer and goats (B495.3.w3 - full text provided, J3.96.w3, J19.68.w5, J21.33.w1, J21.69.w1, J72.41.w1).
  • The disease in Sus scrofa - Wild boar is similar to that in domestic pigs and excretion of virus is probably in similar quantities (J64.15.w1).
  • Infected deer exhaled virus in quantities similar to those exhaled by cattle and sheep, although much less than the quantity exhaled by pigs. Levels of aerosol virus present were lower for fallow deer and red deer than for roe, muntjac or sika deer (J3.96.w3).
  • Aepyceros melampus - Impala shed much less virus than do Bos taurus - Domestic cattle (J64.7.w2, J66.67.w1)
    • However, transmission from impala to cattle and to buffalo has been demonstrated. (J66.67.w1)
  • In acutely infected Syncerus caffer - African buffalo virus may be detected in blood, nasal secretions, saliva, preputial secretions, faeces and air samples collected from the immediate vicinity of infected buffalo. Virus is generally detected at lower quantities and less regularly than for Bos taurus - Domestic cattle infected with the same virus type. Virus may be detected in nasal secretions and saliva of buffalo for up to four weeks after infection (unlike cattle). Virus transmission from buffalo to cattle occurred only when there was direct physical contact between the two species while the buffalo were in the acute stage of infection (e.g. resting and grooming together), not if the animals were in the same pen but avoiding one another, nor to Aepyceros melampus - Impala in an adjacent pen despite sharing water troughs and hay racks. (J62.53.w2).
Herd size
  • Larger herds may pose a greater risk for spread within a premises and for transmission by direct, indirect means or airborne spread (J35.134.w2).
Severity of infection - less virus produced with subclinical infection.
  • Decreased virus production in vaccinated animals, even if not fully protected.
Stage of infection
  • Virus output is maximal in the early acute stages of the disease (J21.23.w1, J3.148.w5, J72.41.w1)
Length of time of clinical signs/time to slaughter of infected animals
  • Total amount of virus produced is affected by the number of days an infected animal is alive and producing virus (J21.69.w1).
Immune status
  • Animals exposed to airborne virus release airborne virus up to 22 hours after exposure (from virus trapped on the animal during exposure) (J21.23.w1).
  • Animals exposed to airborne virus release airborne virus again two to seven days after exposure: including from susceptible animals which develop lesions, vaccinated animals which develop lesions and vaccinated or recovered sheep/pigs which do not develop lesions.(J21.23.w1).
  • "Vaccination of animals before exposure resulted in less or no virus being detected" in air from animals exposed to virus (J21.23.w1).
  • Suggested: control movement of (vaccinated) animals for two weeks following contact with infection.
  • Also: exposure to animals showing signs of FMD, followed by contact with susceptible animals within 22 hours, may lead to transmission. This is true for animals and people (i.e. acting as mechanical vectors -Editor's comment V.v5).
  • (J21.23.w1).
  • In vaccinated cattle exposed to virus, reduced transmission of virus to susceptible in-contact animals (J21.46.w1)
  • In emergency-vaccinated sheep exposed to virus. Reduced local replication of virus in oesophageal/pharyngeal area and thereby reduced transmission to susceptible in-contact sheep and reduced carrier status (J70.17.w4).

Potential transmission of virus by cattle infected after vaccination

  • Cattle vaccinated 3 weeks before exposure to infection with homologous strain resisted clinical infection; 4/18 developed subclinical infection, but none transmitted to in-contact susceptible cattle; some vaccinated animals challenged at high dose were found to be carrying virus (oesophageal/pharyngeal fluid) at 21 days after exposure.
  • Cattle vaccinated two weeks before exposure to infection with homologous strain transmitted infection but not disease.
  • Cattle vaccinated 4-7 days before exposure to infection with homologous strain transmitted disease to in-contact susceptible cattle; disease took 16-21 days to appear. All vaccinated animals were carrying (O/P) virus at 21 days after exposure. Some vaccinated cattle then developed signs of disease.
  • Shown: reduced chance of transmission of disease from vaccinated animals after exposure. Suggestive that only low excretion of virus from vaccinated animals after exposure even at two weeks.
  • Suggested:
    • Vaccinate all susceptible animals within an emergency vaccination zone.
    • Keep vaccinated and unvaccinated animals (at junction of vaccinated zone with unvaccinated) separate for at least three weeks.
    • Vaccines producing a faster response may be useful.
    • Revaccination may be useful to boost immunity.
  • (J21.46.w1)
  • There is no evidence that vaccinated animals, even if they become carriers, cause infection in susceptible animals.
    • "follow-up outbreaks, caused by the presence of vaccinated carrier animals, have never been observed." (P5.40S.w2).

To Top of Page
Go to general Foot and Mouth Disease Virus page

Methods of spread of virus between host animals

Source Information
Spread of virus from one animal/premises to another
  • "The most common mechanism of spread is by the movement of infected animals. The next most likely mechanism is by the movement of contaminated animal products, e.g., meat, milk, semen, skins, etc.... Disease may also be spread on fomites, vehicles or by people... windborne spread can occur over considerable distances if climatic and some other special circumstances are favourable." (J72.41.w1).
  • Note: much spread of FMD, for example during the 2001 outbreak in the UK may be labelled as "local spread", but without the mechanism of "local spread" being defined. (3.148.w5)
On/in infected animals
  • Principle means of disease spread in FMD-endemic countries (J16.22.w1).
  • This is the most common mechanism of spread within a herd or flock, with direct transmission of virus in exhaled droplets and droplet nuclei from infected animals to susceptible animals (J3.148.w5).
  • Transmission at farm boundaries may occur when animals are in contact (nose-to-nose) or at larger distances due to airborne spread. (B495.3.w3 - full text provided)
In animal products (J16.22.w1)
  • Meat (including organ meat, lymph nodes) or bones, e.g. fed as swill to pigs, or bones fed to farm dogs (D36.Para15-16).
  • Milk: by ingestion (infective dose for pigs 10(5.) ID50 by ingestion. Milk may contain up to 10(5.5) ID50/ml, therefore less than 1ml would be required. If milk contained 10(2.3)/ml, 0.5 litres would be required. Calves require larger dose by ingestion, but infection by this means would also be possible. (J18.41.w1)
  • Increased probability of at least one animal becoming infected if larger numbers are each fed a given volume of milk than if only few animals are exposed to this source (J64.16.w1).
  • Calves may be infected by feeding on infected milk or dairy residues from infected cows, including cows which are not yet showing clinical signs. (J63.14.w1).
  • Infective milk has been shown as a cause of spread of foot-and-mouth disease (J3.87.w1, D36.Para94)
  • Spillage of infected milk could result in the production of aerosols which could be inhaled, as well as milk which could be ingested or spilt onto people and transferred (J18.41.w1). Release of milk as aerosols from bulk tankers, unless fitted with an appropriate filter on outlet valves, may also occur (J63.14.w1).
  • Infected milk may contaminate milk lorries, storage tanks etc. and therefore contaminate later consignments of milk (D36.Appendix II).
  • Infection may be disseminated in meat, hides, bones, milk (B58)
  • Infection may be disseminated in urine or faeces, including slurry, dried faeces and contaminated soil (J72.41.w1).
  • Slurry may be a source of airborne virus, particularly if disposed of by spraying (J18.41.w1, J18.49.w1, D37.Para85).
  • In semen (D36.Para19).
  • In materials produced from glands of infected animals (D36.Para.20).
In carrier animals
  • Carrier animals are considered by some scientists to represent a significant risk of initiating further outbreaks of FMD (J42.118.w1).
  • Other scientists believe the risk to be exaggerated and to actually be extremely low. (64.21.w28, J249.91.w2),
Carriage by humans
  • Humans are recognised to be one of the principle disseminators of FMD. They are extremely mobile, widely distributed and their movements are difficult to control (J75.20.w1).
  • Humans may act as mechanical vectors (including virus carried on hair and in respiratory tract). (B58)
  • People who might transport disease include family farmers working several farms together, "relief" stockmen or milkers, large-animal veterinarians, and "animal dealers, hoof trimmers, sheep shearers, bulk milk collectors, feed deliverers, milking parlour engineers, dairy hygiene inspectors, Defra and trading standards officers, and nutrition advisors." (B495.3.w3 - full text provided)
  • Stockmen may be very likely to spread FMD if their fingernails become contaminated and they "nose-restrain" cattle, as cattle are particularly sensitive to virus entering the upper respiratory tract and its mucosal surfaces (J72.41.w1).
  • Virus was recovered from air expelled while coughing, sneezing, talking and breathing as well as from the nose, throat and saliva, and for up to 28 hours; also shown can be transmitted from one person to another (on one of four occasions after talking for a few minutes) (J19.68.w3, J21.23.w1).
  • Virus has been transferred successfully from infected to non-infected animals via air expelled from humans while coughing, sneezing, snorting and breathing (J3.89.w1).
  • Precautionary period for humans who may be considered potential transmitters of FMD virus following contact with infected animals is generally taken as three to five days (V.w5, V.w23).
  • FMDV may survive at least 11 weeks on boot leather and 14 weeks on rubber boots (D36.Appendix II).
  • Humans may also transport contaminated material such as soil or faeces, for example on their shoes, and these may remain infective for considerable periods of time (J72.41.w1, J63.14.w1).
  • Risks of transport by casual visitors such as tourists crossing farmland are poorly understood. (B495.3.w3 - full text provided)
  • Showering (not just hand washing) and changing outer clothing is recommended to prevent transmission of FMDV by people after handling infected animals (J3.153.w6).
    • FMDV was recovered from a nasal swab of one of five humans immediately after examining post mortem pigs with FMD, but not from any humans at 12 to 84 hours after contact with infected pigs (J3.153.w6).
    • Transmission of FMD to sheep, but not to pigs (which are considered less susceptible than sheep to infection), occurred after people who had been in contact with infected pigs (carrying out typical investigative procedures of handling, examination, blood sampling etc.) washed their hands and changed their outer clothing and handled susceptible animals in a similar manner (J3.153.w6).
    • However, no transmission occurred to either sheep or pigs from people who had showered and changed their outer clothing after handling infected animals. This study suggested that humans were unlikely to transmit the virus from infected to susceptible animals provided they had showered, removing all visible organic contamination from their bodies, and had dressed in clean outer wear (J3.153.w6).
On/in other species
  • Birds, rats and other wildlife species and domestic species such as cats and dogs as well as invertebrates such as flies, ticks all may act as mechanical vectors (J3.102.w5, J18.41.w1, J42.81.w1, B58, B495.3.w3 - full text provided, D36.Para.34, D36.Para37-38).
  • Wild animals which are susceptible to FMD also may transport the virus between premises. (B495.3.w3 - full text provided)
On/in non-living fomites such as vehicles, fodder, clothes etc.
  • Infection may be disseminated via vehicles, bedding, buildings, utensils and other fomites including packing materials (J3.102.w5, B58, B495.3.w3 - full text provided)
  • Air inside a vehicle in which infected animals have travelled may contain sufficient virus to infect the next load of animals by inhalation. Virus would be present on the walls and floor as well as in the air in the container. (J18.41.w1).
    • Example: In the 2001 outbreak of FMD in the UK, the virus was introduced into mid-Wales when a lorry, used the previous day to transport infected [but not diagnosed] sheep, was then used to transport other sheep to a Welsh market. (J3.149.w8)
In water
  • May be contaminated by e.g. shreds of infected epithelium.
  • Virus may survive in such conditions for as long as 67 days (shorter time in summer) (J63.14.w1).
  • Concentration of 10 (1.5) ID50 per ml required for infection by ingestion of contaminated water (J18.41.w1).
  • May survive in water sufficiently long to allow spread to a neighbouring property in a stream (D36.Para39).
Airborne
  • May occur on occasion both locally and over longer distances (J42.75.w1).
  • Airborne transmission is one method of short-distance spread (e.g. within a farm). ()
  • In the UK in 2001, airborne transmission from the original infected pig farm is thought to have been responsible for infection of sheep and cattle on several farms 1-9 km away; the cold, damp conditions during February 2001 would have favoured survival of the virus in the environment during airborne dispersal. (J3.152.w, J3.153.w8)
  • "It is apparent that the amount of virus released by a species of animal is only one of the factors influencing the degree of airborne spread after an initial outbreak. The number of animals involved, the period before the disease is reported as well as the topography of area, the livestock density and the climatic conditions are all important" (J19.68.w5).
  • "Spread by this means is not a common event as it requires the simultaneous occurrence of certain climatic and epidemiological conditions." (J19.124.w2).
  • Airborne transmission over considerable distances may occur in suitable conditions, particularly if high relative humidity (greater than 60%). (J72.41.w1, B47, B58, B207). Transmission as far as 250km/156 miles has been recorded (B207).
  • Outbreaks spreading downwind from an initial incident in a farm with affected pigs near Oswestry, Shropshire, in 1967 (J19.68.w2).
  • Modelling of airborne spread agreed with observed spread in outbreaks in Hampshire in 1967 and Northumberland in 1966 (J3.108.w3).
  • Outbreaks on Jersey and the Isle of Wight in 1981 were apparently due to virus windborne from infected pigs in Brittany (J3.110.w5)
  • Windborne virus commonly implicated in spread of FMD to Denmark from northern Germany and to Norway and Sweden from Denmark (J3.110.w4).
  • Modelling of potential airborne spread agreed with observed spread for outbreaks in Denmark, Sweden and the Channel Islands (J3.110.w4).
  • Possible airborne spread from Jordan to Israel reported and modelled using a "short-range" model; source suggested to be wild boar and while outbreaks occurred in sheep and in Gazella gazella - Mountain gazelle. (J73.44.w1).
  • Airborne FMD Virus can travel considerable distances in the right atmospheric conditions - low turbulence, gently moving wind, and longer distances over water, where turbulence is generally reduced, than over land. (B495.3.w3 - full text provided)
  • Potential for airborne spread of virus from carcass pyres (D37.Para128); initial results of studies carried out during the 2001 FMD epidemic in the UK indicate that FMD virus is not likely to be dispersed from pyres (J3.148.w3)
  • In the UK in 2001, airborne transmission might have occurred early in the epidemic, from pigs. additionally, it has been suggested that the virus may have travelled e.g. along valley floors in hilly areas. (B495.3.w3 - full text provided)
Factors affecting virus spread
Changes in movement of infected animals
  • Effective control of animal movements is necessary in any scheme to control FMD (J16.22.w1)
  • Movement of animals which are subclinically infected or are in the early stages of disease and not yet showing clinical signs may play a large role in disease transmission (J3.131.w1, B217.38.w38).
    • This is thought to have been very important in the 2001 outbreak in the UK, particularly with multiole long-distance movements of sheep. (B494.7.w7 - full text provided, B495.3.w3 - full text provided, J3.149.w8)
Changes in movement of animal products
  • Control of the movement of animal products is important in the control of FMD (D37.Para216).
  • It was suggested in the Northumberland Report that it would be safest, if slurry must be disposed of, to deposit it in bulk on an area of land within the property (e.g. a waste area), as seepage over the from such an area is less likely to be hazardous than aerosols from slurry spreading or the creation of a large area from which infected material may be transported on the feet of birds or other wildlife. (D37.Para85).
Changes in movement of carrier animals
  • The potential for transmission from carrier animals occurs only if those animals are placed in contact with susceptible animals.
Changes in movement of people
  • Reduced movement of potentially contaminated people from one premises to another decreases risk of virus spread.
Changes in movement of living and non-living fomites
  • Restriction of movement of vehicles etc. from areas of infection and into unaffected areas/premises is recognised as important for the control of FMD. (D37.Para216).
Effect of weather/environmental conditions
  • From 1967-68 epidemic: cases near Winchester, spread showed clear association with wind direction during wet weather (J19.68.w2)
  • FMD is relatively resistant to sunlight and to "Open Air Factor"; this may affect the likelihood of virus spread. (J19.74.w1)
  • "several factors, including environmental conditions, appeared to influence the rate of recovery of virus" in experiments regarding airborne transmission (J42.75.w1).
  • Wind direction is important in determining the area exposed to airborne spread. The spread of FMD from Nantmawr over the Cheshire Plain in the first three weeks of the 1967-68 epidemic in the UK was clearly downwind, based on the prevailing wind direction (J19.68.w2, D36.Para92, D36.MapVI, D37.MapV).
  • "long-distance spread is dependent on wind direction and speed and is favoured by low temperature, high humidity, and overcast skies" (B216).
  • Long-distance airborne spread is more likely in temperate areas than tropical areas (B216).
  • Airborne spread is affected by survival of virus, deposition of infective virus, characteristics of the virus plume, effect of topography on the virus plume (J3.108.w3)
  • Precipitation (rain) does not appear to affect the airborne transmission of FMDV in itself; however it may occur in association with factors likely to increase virus survival, in particular high relative humidity (J3.108.w3).
  • Changes in environmental conditions, particularly relative humidity, affect virus survival.#
  • Movement of airborne virus is affected by wind strength and direction,
  • Movement over the sea is affected by air and sea temperatures as well as wind speed: less upward dispersion of virus, and therefore greater horizontal distance of spread in quantities sufficient for infection at ground level, when air temperature is greater than sea temperature and winds are light (J3.110.w4, J3.111.w3).
  • Risk of airborne spread of FMD from continental Europe to England is generally low, as the ideal conditions for transport of virus for such a distance, even over the sea, do not occur very frequently (J3.111.w3).
  • Conditions suitable for virus survival and long-distance spread occur most often in autumn and winter
Removal of virus from the environment
  • Carcass burning/burial; rapid disposal of animals is recognise to be important for control of FMD outbreaks (J21.69.w1).
  • Disinfection with e.g. acid/alkali.
Use of models of airborne spread

Epidemiological studies, laboratory findings and meteorological knowledge in combination indicate that in order for the potential of airborne spread to occur over a long distance to be maximised, it is necessary to have a high output of virus from infected animals, a low dispersal and high survival of airborne virus and a large number of susceptible livestock exposed to the airborne virus for many hours (J3.110.w4, J3.110.w5).

Models for prediction of airborne spread may be used to indicate:
  • Amount of virus likely to be liberated from infected animals, which is affected by:
    • Virus strain (J18.49.w1)
    • Species infected (virus source): greatest for pigs (J3.148.w5, J18.49.w1), J3.110.w4.
    • Number of affected animals (i.e. number releasing virus) at any one time (J3.148.w5, J3.110.w4).
    • Stage of infection (affects amount of virus produced)
    • Number of days for which animals shed virus (time to slaughter and disposal) (direct effect on number of days may act as source, also increased likelihood with time that more animals in an infected herd/flock will be infected and producing virus, until acute stage of disease is completed)
  • Whether airborne spread is likely (depending on environmental factors affecting virus survival (e.g. relative humidity) and atmospheric factors affecting spread of a virus plume at ground level).
    • Higher survival if relative humidity over 60% (J3.108.w3). "For example, given a relative humidity above 60% RH and a wind speed of 10 metres per second, virus could survive during the time taken to travel 100km" (J18.49.w1).
    • Virus will be carried upwards and will be considerably diluted in conditions of strong thermal activity, but in the absence of thermal (conductive) activity, such as in times of temperature inversion, dilution will be minimal (J3.110.w5, J18.49.w1).
    • Less dilution of virus due to turbulence at night than during the day, due to reduced atmospheric turbulence at night (J3.108.w3).
    • Higher concentrations in the virus plume are maintained under low windspeeds (J3.110.w5). (J3.108.w3)
    • Conditions are most likely to be favourable for distant airborne spread in autumn and winter (J18.49.w1)
    • Lowest dilutions of virus and therefore highest risk of infection over the sea for the period March to August (J3.111.w3).
  • Likely direction of spread (depending on wind direction and topographical features). (J3.110.w4, J19.124.w2)
  • Likely distance of spread of virus (depends on wind speed, topographical features, other meteorological factors affecting virus dispersal and deposition). (J3.110.w4)
  • Longest distances of spread are most likely over the sea (J3.110.w4, J3.110.w5)Likelihood of a given dose of virus reaching susceptible animals (depends on initial amount of virus released at source and dispersal of virus plume).
  • Likelihood of susceptible animals becoming infected depends on:
    • dose required for infection by respiratory route (varies with virus strain and host species).(J18.49.w1)
    • amount of air inhaled (varies with on species and individual animal size). (J3.108.w3)
    • number of animals in a herd/flock (greater "sampling of air" by larger herds).(J21.13.w1).
  • See: Use of Models of Airborne Spread for Foot-and-Mouth Disease

To Top of Page
Go to general Foot and Mouth Disease Virus page

Routes of Infection - entry of the virus into a new host body

Source Information
Routes of transmission of virus into susceptible animals
  • Experimental infection by direct contact, indirect contact, virus in food, virus in water, virus in aerosol, direct instillation into the lungs and intravenous inoculation. (J42.91.w1).
  • "An aerosol dose containing very little virus is capable of producing disease in cattle, but the dose of virus required to infect by the alimentary is several-thousand-fold higher" (B58)
Inhalation
  • The most common route of infection for ruminants is inhalation of airborne virus (infection via the respiratory tract). Infection by ingestion requires much greater quantities of virus (B216, J19.114.w1).
  • "Cattle and sheep can be infected by inhaling doses in the range 10 to 25 infectious units." (J72.41.w1)
  • A review of many studies showed that doses as low as 18 infectious units (IU) in cattle and 8 IU in sheep have been found to cause infection giving rise to lesions, during experimental transmission of virus from affected pigs, given to the recipient animal through a mask. In general, for different methods of intranasal infection, the lowest doses were required by cattle, followed by sheep, while higher doses were required by pigs. (J35.X.w1)
  • Sheep are highly susceptible to respiratory-route infection (B217.38.w38).
  • Virus transmission from experimentally-generated aerosol to cattle confirmed by experiment (J42.75.w1).
  • Impala are highly susceptible to aerosol infection; unpublished data indicates they may become infected from as little as one cell culture infective dose of virus. (J66.67.w1)
  • Low quantities of virus in natural aerosols from infected pigs are sufficient to infect sheep
    • Infection best detected by viraemia and seroconversion
    • About 27% of sheep known to be infected did not develop vesicles.
    • Mean time to onset of viraemia 2.5 days, to pyrexia 3.8 days, to appearance of vesicles 4.7 days.
    • (J21.41.w1)
  • Low quantities of virus in natural aerosols from infected pigs are sufficient to infect cattle.
    • Infection with low doses may not result in the development of detectable vesicular lesions, i.e. may result in subclinical infection: hypothesised that virus replication may be "restricted to sites in or associated with the respiratory tract". J21.43.w1
    • Similar doses required to infect calves and sheep. Longer exposure times likely to be required for exposure to natural virus plumes to result in infective dose.
    • "the respiratory route is the most susceptible natural portal of entry by which cattle can be infected by FMD virus. The only other route which approaches the respiratory tract in sensitivity is artificial, that is, intradermal inoculation of the tongue".
    • J21.43.w1
  • Pigs are relatively resistant to infection by natural aerosols of FMDV (J3.148.w4); by the intranasal route of infection (J35.X.w1).
    • Attempted transmission from experimentally infected pigs to other pigs in the same airspace, using a strain of virus isolated from animals during the 2001 UK epidemic (isolate UKG 34/2001) definitely failed to infect 9/10 pigs and subclinical infection of the tenth pig was thought to be associated with a brief direct contact with one of the infected pigs (J3.148.w4).
    • Pigs would be unlikely to become infected due to airborne virus associated with as many as 1000 infected pigs more than 0.2km upwind or 100 pigs more than 0.1km upwind (J3.148.w5).
    • A review showed that the lowest dose to cause infection in pigs was 22 infetious units (IU), compared with 8 IU in sheep and 18 in cattle, but 125 IU were required to give rise to lesions in pigs, and often pigs failed to become infected in intranasal infection experiments (J35.X.w1). 
Ingestion - may be a major route for pigs.
  • Initial infection of pigs may occur when infected meat is ingested. (B207)
  • Pigs may be infected by about log103.9 ID50 orally (J72.41.w1)
  • Ruminants require more than log105.8 ID50 (almost one million) infectious units orally (J72.41.w1)
Through mucous membranes - if rubbed directly into mucous membranes of the mouth in cattle (B47).
  • Probably depends on traumatic injury to the membranes (B216).
  • May occur if cattle are "nose-restrained" (J72.41.w1).
  • May occur through abrasions or cuts in the mouth (J72.41.w1).
Damaged skin
  • Sheep are highly susceptible to infection via virus entering breaks in epithelium, for example through foot-rot lesions (B217.38.w38).
Venereal transmission (including by artificial insemination)(B216):
  • Virus may be found in vaginal secretions of cows (J63.14.w1).
  • Virus may be found in the semen of bulls prior to the appearance of clinical signs, as well as during the clinical disease and for a period afterwards (J63.14.w1, J342.76.w1).
  • Virus may be found in the semen of infected boars before and during disease, but transmission to sows by insemination does not appear to occur (J342.76.w1).
  • Transmission to heifers by artificial insemination has been demonstrated (J18.41.w1).
  • Transmission of SAT virus from Syncerus caffer - African buffalo to domestic cattle may be associated with sexual activity, and may be venereal. Transmission has been reported when buffalo bulls were maintained with female cattle, but not when steers were maintained together. In two experiments in which transmission occurred after long periods (five and eleven moths), transmission in both experiments occured in December, i.e. early in the breeding season. (J3.145.w6)
Eye - conjunctiva
  • By virus in aerosol droplets (J68.B302.w1).
Inoculation
  • Deliberate injection, as occurs for transmission and vaccination experiments.
Iatrogenic 

Inoculation with contaminated vaccines, contact with contaminated veterinary instruments (B216).

Factors affecting transmission into potential hosts
Species
  • Infective dose by a given route varies depending on host species (J18.41.w1, J3.148.w5).
  • "compared with sheep and cattle, pigs are relatively resistant to infection by airborne FMD virus" (J3.148.w3).
  • Volume of air breathed by cattle is greater than that breathed by sheep or pigs (higher respiratory exchange rate), therefore greater likelihood of exposure of cattle to a given dose over any set time period (J19.124.w2, J68.B302.w1).

Infective dose for the species by different routes

  • Different routes of infection require different doses of virus in order for infection to occur, e.g. a lower dose is required to infect cattle by inhalation than by ingestion (J18.41.w1).
Immune status of potential hosts
  • Higher dose required to infect animals with immunity (e.g. following vaccination) (J21.46.w1, (J18.49.w1).
Invasiveness of virus strain
  • Amount of virus required to initiate infection following exposure of a given host type varies considerably depending on the virus strain involved (J18.41.w1).
Herd size
  • Incidence of infection noted to increase with increasing herd size for cattle during the 1967-68 FMD epidemic in the UK (J21.13.w1).
  • "Air sampling" by large herds will be greater than that by small herds; this could explain the difference in incidence (J21.13.w1).
  • Modelling of airborne transmission using a probabilistic approach and assuming transmission in the form of droplet nuclei indicates that the size of a herd is very important in the transmission of infection (J3.148.w2).
    • Use of this model for data from actual disease outbreaks (e.g. FMD outbreak in Hampshire 1967) fitted the observed data of herds affected and was able to explain transmission to farms despite the estimated mean infectious dose per animal reaching those farms being calculated as less than that required to cause infection (J3.148.w2).
  • Exposure of large numbers of animals could result in infection despite less than a "minimal dose" of airborne virus (J3.148.w5).
Infective Dose
  • The amount of infective virus in a substance is often expressed at the ID50 per unit weight or volume of the substance (e.g. per gram or per mL). (J64.16.w1)
    • The ID50 is the dilution of material which would infect 50% of the individuals exposed at that dilution. (J64.16.w1)
  • An alternative way of measuring the quantity of virus is to look at the number of plaque forming units (PFUs) per unit weight or volume of the substance, measured by placing different dilutions of the virus-containing material onto cell culture monolayers. The number of plaque forming units equates to the number of virus particles, as each infective virus particle may produce one plaque (J64.16.w1, B73).
  • The measurement of PFUs tends to give less variable results than than the ID50, which is generally based on tests done on limited numbers of animals, but may still give different results depending on the cell culture used (J64.16.w1).
  • A third method uses the minimum infective dose (MID), which is calculated for different species and different exposure routes (J64.16.w1).
    • However, this may not be entirely applicable for quantifying risks of biological contamination (J64.16.w1).
  • It may be assumed that "each infectious unit has a non-zero probability of independently infecting an animal". That is, infection may arise from a single pathogen (e.g. a single virus particle) entering a cell and replicating in it, with the numerous virus particles produced then invading other cells (J64.16.w1).
  • Although a higher concentration of virus is more likely to contact, adhere to and invade a susceptible cell, the exposure of large numbers of cells to low virus concentrations may also result in infection of at least one cell (J64.16.w1).
  • As only a single infective particle is required, in theory, to set up an infection, infection may occur despite exposure of the animal to less than the "minimum infective dose" (J64.16.w1).
  • The probability of infection, based on the number of plaque forming units in a unit quantity of material may be calculated mathematically and is dependant on not only the concentration of infectious particles (PFU per litre) but also on the number if animals exposed to the material:
    • "In other words, when sufficient numbers of susceptible animals are exposed to products which have low levels of contamination (and even if all individual animals receive less than the so-called MID [minimum infective dose]) there is still a chance of infecting one animal from the group." (J64.16.w1)
  • For a highly contagious disease, such as FMD, infection in even one animal is likely to lead to infection of other in-contact animals and establishment of infection in the group of animals. (J64.16.w1)
  • Note: The probability of infection on exposure to low levels of pathogen, even below the theoretical "minimum infectious dose" must be considered when calculating risks of transmission of FMD by potentially contaminated material, including calculation of the risk of airborne transmission.
    • A probabilistic approach has been used in the risk assessment of airborne transmission of virus based on the fact that infectious droplet nuclei, containing virus particles, are randomly distributed in the atmosphere and some animals may inhale none while other inhale may one or more and become infected. In this model, it was clear that the number of animals exposed (i.e. the size of the herd/flock) was an important factor and could lead to infection despite a low mean dose per animal (lower than the theoretical minimum infectious dose) reaching the herd/flock. (J3.148.w2).

To Top of Page
Go to general Foot and Mouth Disease Virus page