March 5, 2001
Contact: Stephen M. Apatow
SierraEquest Online: Emerging Infectious Disease Network
Internet: http://www.sierraequest.com Email: sierraequest@smamedia.com
Eastern United States: (203) 668-0282 Western United States: (775) 884-4680
MAJOR RISK FOR LIVESTOCK PRODUCERS/ANIMAL HEALTH WORKERS RELATES TO LACK OF DIRECT EXPERIENCE OR RECOGNITION AND HANDLING OF FOOT AND MOUTH DISEASE
According to the 1994 USDA:APHIS:VS paper "Foot and Mouth Disease - Sources of Outbreaks and Hazard Categorization of Modes of Virus Transmission:"
Foot and Mouth Disease (FMD), a highly contagious viral disease affecting primarily cloven-hoofed animals, continues to be a concern in the United
States even though the last outbreak in North America was eradicated in 1954 (Mexico). The Economic consequences of an unchecked outbreak in the
United States would be great -- estimated in 1979 to be more than $12 billion in the first 15 months.
Livestock animals in this country are highly susceptible to FMD viruses. If an outbreak occurred in the United States, this disease could spread
rapidly to all sections of the country by routine livestock movements unless it was detected early and eradicated immediately. -- USDA:APHIS
Fact Sheet on Foot and Mouth Disease
According to the paper, since the last U.S. outbreak was in 1929, few livestock producers or animal health workers have direct experience in the
recognition and handling of the disease.
CHALLENGES OF FMD IN ECOSYSTEM
Results of Hazard Categorization for Live Animals
Of the 99 animals identified as possible sources of FMDV, 31 were categorized as high hazards, 50 as moderate, and 18 as low. Some of these
animals are discussed below.
WILD BOVIDAE
Although there have been few confirmed primary outbreaks caused by wild game, circumstantial evidence suggests that such animals may be a source of
outbreaks among cattle in countries where wild game and domestic cattle often share grazing land and watering holes. In a large survey conducted
between 1965 and 1969, 1,323 samples of serum were collected randomly from 39 species, primarily in Zimbabwe (formerly Rhodesia). Significant FMDV
antibody titres were found in 16 species, all cloven-hoofed animals. Of particular interest is the African buffalo; of the 116 animals tested, 77
had positive antibody titres. African buffalo often do not show clinical signs and may carry the virus for long periods (5
years). A study in an area in which FMD had not been recorded for 25 years found 14 of 34
buffalos with significant antibody titres. In addition, recent studies involving nucleotide sequencing of the virus support the belief that wild
bovids, particularly African buffalo, may play a role in the transmission of FMDV to cattle.
SUIDAE
Domestic pigs were assigned a high hazard category. They play an important role in the epizootiology of FMD as initiators and amplifiers. They can act
as initiators of FMD by the consumption of infected garbage. Pigs serve as amplifiers of the virus because they can excrete aerosols that contain up
to 3,000 times more virus than produced by an equal number of cattle or sheep. Many outbreaks linked to airborne spread are thought to have
originated from domestic pigs. One characteristic that distinguishes pigs from cattle and sheep is that they appear to harbor the virus only during
clinical stages of the disease and therefore do not act as carriers. Some other species of Suidae have been known to become infected and show
clinical signs of FMD, but it is not known if other such species can carry the virus subclinically. Information is also lacking on transmission of
virus from other Suidae.
CERVIDAE
Most deer, including white-tailed and mule deer like those found in North America, were assigned a high hazard category because of demonstration of
FMDV transmission. At least one white-tailed deer remained a carrier of FMDV for 11 weeks after infection (Table 12). Exotic deer, including red,
sika, and fallow, have been gaining popularity for use on deer farms or game ranches. Such deer have been found to both acquire and transmit FMDV
under natural conditions. There is no information on transmission from other cervids such as moose and elk. Accordingly, those cervids were placed
in the moderate hazard category.
LLAMAS
Llamas were categorized as high hazard animals based on the results of experimental transmission of FMDV under
"natural" conditions, although such experimental work in llamas has yielded somewhat inconsistent results. One
experiment showed that FMDV was transmitted from cattle to llamas, from llamas to swine, and from llamas to other llamas. Responses in the llamas
ranged from generalized infection with mucosal lesions (oral and lingual) and severe lameness to no clinical signs. No virus was isolated from the
oesophageal-pharyngeal (OP) fluid of infected animals after 7 days post-inoculation or post-contact.
In another study, none of 20 llamas exposed to pigs infected with FMDV types A or C developed clinical signs or
yielded any virus from OP fluid or blood. Two of 10 llamas exposed to pigs infected with FMDV type O did
develop mild lesions. After removal of the pigs, additional susceptible llamas and other domestic livestock were placed in contact with the initial
group of 30 exposed llamas; all of the secondary exposure group llamas remained healthy and were
negative for FMDV antibodies. The authors concluded that llamas are resistant to FMDV infection and that they harbor
the virus in their OP region only for a short time.
HUMANS
Humans may acquire FMDV through various routes, the most important of which has probably been ingestion (drinking infected milk). Airborne virus has
also been linked with several human cases, as has direct contact with infected animals. However, the number of documented cases in humans is
relatively small (only in about 40 cases has FMDV been isolated), indicating that humans are quite resistant to FMDV infection and that it is
not a public health problem. Whether or not one considers FMD to be a true zoonotic disease, humans can play a role as carriers or transmitters of
FMDV and were categorized as a high hazard. Virus has been recovered from the nasal passage up to 36 hours after exposure. It is reported that virus
can be isolated from humans with vesicles up to 14 days after the onset of disease. However, the most important form
of transmission from humans to animals is mechanical, because virus can persist on clothing or shoes for at least 9 weeks.
INVERTEBRATES
Although the role of flies and ticks in the epizootiology of FMD is not usually large, it has been demonstrated that ticks and some species of
biting flies can transmit the virus through bites. Ticks, flies, and biting flies were categorized as high hazards, based either on transmission
capability or long carrier status (whether mechanically or biologically). Houseflies can carry FMDV both externally and internally; whether they can
transmit the virus is unknown. It is unlikely that the virus multiplies in the cells of invertebrates. However, experimental transovarial infection of
a portion of a population of Dermacentor ( Anocentor ) ticks has been reported. FMDV has also been transmitted to cattle experimentally by
inoculating them with ground-up earthworms. Given the artificial nature of transmission and given that
infected earthworms are not known to have been associated with any outbreaks, earthworms were placed in the moderate rather than the high
hazard category.
In response to the need for educational resources on Foot and Mouth Disease, SierraEquest Online has constructed an information site which
includes: Photos (FMD in cattle, pigs sheep), OIE: AETIOLOGY - Foot and Mouth Disease,
USDA:APHIS: Fact Sheet: - Foot and Mouth Disease, Centers for Epidemiology & Animal Health (An OIE Collaborating Center), Center for Emerging Issues,
current North American Advisories and more. To access this resource, visit the:
Emerging Infectious Disease Network (EIDNet)
Topic: Foot And Mouth Disease
http://www.sierraequest.com/eidnet/fmd.html
