Foot and Mouth Disease

Foot and Mouth Disease

"The Potential for International Travelers to Transmit Foreign Animal Diseases to US Livestock or Poultry."
USDA:APHIS:VS Centers for Epidemiology and Animal Health. August 1998 Foot and mouth disease virus (FMDV), a picornavirus of the genus Apthovirus, has been widely studied for its strong environmental stability. There are multiple sub-types of FMDV (at least 65 divided into 7 antigenic types). Generally, infection with one type fails to provide immunity to infection with other types.

FMDV is very hearty, surviving freezing temperatures. It has been found to be viable in contaminated milk after pasteurization at 72 degrees C for 15 seconds [Which are the standard pasteurization parameters used in the United States--BSE coordinator](Pirtle, EC; Beran, GW (1991) Virus Survival in the Environment. Revue Scientifique et Technique (International Office of Epizoonotics), 10(3), 733-748.)

The virus can survive for extended periods outside the host in protected locations. FMDV has been recovered from cattle stalls 14 days after removal of infected cattle, from urine after 39 days, from soil after 28 days in autumn and after 3 days in summer, and from dry hay at 22 degrees C after 20 weeks storage (Pirtle 1991). The virus is inactivated by sunlight, extremes in pH, and high temperatures.

Human infections do occur, but are extremely rare. The incubation period is from two to six days and the course of disease is short and uncomplicated. Symptoms include fever, vomiting, a sense of heat and dryness in the mouth, and small vesicles on the lips, tongue, and oral mucosa. Vesicles can also occur on the hands or feet. Asymptomatic infections are possible.

Probably the most important route of transmission to humans is ingestion of FMDV infected milk, especially raw milk. Close contact with infected animals can also result in disease transmission, especially in outbreak situations when livestock mortality is high and large amounts of virus are shed into the environment. Groups at risk include herdsmen, veterinarians, technicians, and consumers of raw dairy products.

While clear records of infection with FMDV in humans do exist with confirmed cases in several nations of Eastern and Western Europe, Africa, and South America, the number of confirmed cases on record is less than fifty (Gustafson 1975, Betts 1952, Timoney 1988, USDA:APHIS 1994, Dlugosz 1943). No cases of human infection were recognized during the most recent FMD outbreaks in Mexico and the US, although countless people had close contact with the virus. It has been noted that "the number of credible cases in relation to the number of persons exposed is infinitesimal" (Betts 1952).

Humans can play a role in the transmission of FMDV. The virus can be carried by clinically affected humans for up to approximately 14 days after the onset of the disease. Humans can inhale the virus, trapping it in the respiratory tract for as long as 36 hours (Hyslop 1973). It can then be expelled in the saliva or breath, thus potentially serving as a source of infection to susceptible animals. It is also possible for humans to asymptomatically spread FMDV; however, failed experiments indicate that it is highly unlikely that this mode of transmission would be a source of infection for animals (Callis 1982).

The lack of human cases over the years adds to the general conclusion from human susceptibility studies that humans are not very susceptible to the virus and, at most, play only a minor role in the biological transmission of FMDV (Gustafson 1975). In addition, there is no evidence that spread from human-to-human has occurred, although theoretically it seems possible.

The most important form of transmission of FMDV from humans to animals is mechanical, as the virus can persist on clothing, shoes, or luggage for at least 9 weeks (Cottral 1969). Human travelers are definitely a possible source of infection via this mode of transmission.

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