Hoof and Mouth Disease (HMD) is an acute infectious disease of cloven-hoofed animals. The name "Foot and Mouth Disease" is a misnomer since the animals this disease affects technically do not have "feet." In susceptible populations, there is a high rate of illness when an outbreak occurs but the death loss is minimal. This disease primarily has severe economic implications for the livestock industries. Hoof and Mouth disease is characterized by vesiculation, or blistering of the hooves and oral cavity. Cattle, swine, sheep, and goats are all susceptible. Seven immunologic types, over 60 subtypes and numerous strains have been identified.
The disease is enzootic in many areas of the world, including most of Asia, Africa, and South America. North and Central America, Australia, New Zealand, and Japan are free of infection. It had been largely controlled by vaccination in Europe and the Scandinavian Countries. Until recently the United Kingdom was considered free of the disease. Mexico experienced an extensive outbreak between 1946 and 1953. In 1952, a small focus of infection was identified in the Saskatchewan Province, Canada, but eradication was prompt. The last outbreak of HMD in the United States was in 1929.
The Pan Asia strain of HMD has been isolated as the cause of the disease in Britain. Origins of the Pan Asia strain are thought to be the Indian sub-continent, although some believe it may have come from China. The disease has been contained to sporadic outbreaks in Asia despite its endemic status, until last year when it caused Japan's first outbreak in 90 years and South Korea's first in 60 years. Japan and Korea have eradicated HMD after last year's outbreaks, while Taiwan shows only occasional slight activity. The virus probably entered Britain in food products but it is unknown where these products originated. The disease first appeared in pigs at Burnside Farm in Heddon-on-the-Wall, Northumberland, UK. These pigs may have been fed waste food scraps that supports the theory of food product contamination.
In enzootic areas, the incidence of disease ranges from sporadic to more or less continuous and appears to be related to the efficacy of available vaccines and the proportion of the susceptible animal population immunized. The emergence of a field strain of virus quite different from that in the vaccine being used or the appearance of infection in a naive population gives rise to epizootics that are frequently extensive. Morbidity is generally over 90 percent in susceptible cattle and pigs, and mortality is low or insignificant, except in very young animals.
The most common means of infection is by the inhalation of virus-containing aerosols. Infection can also result from ingestion of relatively large amounts of virus-contaminated materials or entry of the virus into the eye, nose, udder, or uterus. Direct contamination of abraded epithelial surfaces, especially in the oral cavity is an extremely efficient means of introducing the virus. The virus multiplies initially in the pharyngeal region after aerosol exposure, and some virus is also thought to enter the blood stream directly from the pulmonary alveoli and to be circulated to other multiplication sites. Vesicular lesions (blisters) are most frequently observed in the epithelium of the mouth, nares, muzzle, snout, interdigital space, coronary band, teats, and rumen papillae. The virus can also cause degeneration of heart muscle, which may result in sudden death of the host. The virus can be found in all parts of the body during the viremic stage of the infection. Virus is present in extremely large amounts in vesicular material, and essentially all secretions and excretions of diseased animals contain some virus. Such animals also generate infectious aerosols.
Two important aspects of the pathogenesis of HMD lead to control problems. One is the ability of the virus to multiply in the pharyngeal region of vaccinated or even recovered cattle. Another is the ability of the virus to persist in cattle, sheep, and goats, but not in swine, for weeks or even months after all lesions have healed. Much circumstantial evidence shows that the persistently infected animals, referred to as carriers, can transmit virus to other animals and thereby cause new outbreaks of disease, but such transmission has not been shown under controlled laboratory conditions.
Direct losses associated with an outbreak of HMD include the loss of animals that are slaughtered during eradication campaigns and the severe marketing restrictions caused by quarantines that would be imposed on large areas of the country. Indirect losses, predicted to be approximately 10 times the direct losses, result from disruption of the agriculture industry, loss of foreign markets, and restrictions on hunting, fairs, shows, race meets, and sporting events. In those countries where the disease is enzootic, losses result from time and expense of vaccination campaigns, from the production and culling losses during the periodic outbreaks, and from the loss of foreign markets.
Initial signs are dullness, inappetance, decreased milk production, and fever, followed by an increased salivation, lameness, and serous nasal discharge. Smacking of the lips, drooling, and increased nasal discharge follows the appearance of early vesicles or blisters on the epithelium. As the blisters increase in size and number, soreness of the hooves leads to treading and kicking, and lesions become readily apparent. Vesicles may appear on the lips, gums, dental pad, tongue, nares, muzzle, interdigital spaces, and teats. These rupture in time and leave eroded areas. Lesions on the hard palate are seldom seen as vesicles but appear first as erosions. All food is refused if the mouth is severely involved, and severe hoof involvement may lead to reluctance to rise or refusal to move. The nostrils may become occluded with a muco-purulent discharge. Lactating cows may develop vesicles on the teats. Abortions have been observed in early pregnancy and calves with few visible lesions may die suddenly.
Early signs include fever, inappetance, and reluctance to move. Hoof involvement is usually severe, with pale areas of early vesiculation around the coronary band and in the interdigital space. Blisters may be seen on the snout and often reach considerable size before rupturing. Vesicles also appear in the mouth but are usually much smaller and shorter lived than those of cattle. Hoof vesicles rupture and leave raw areas around the top of the hoof. Such lesions are extremely painful and frequently cause an aberrant gait. Heavy animals may refuse to rise. Extension of these hoof lesions may lead to the sloughing of the hoof. Pregnant sows may abort, and nursing piglets may die suddenly.
Sheep and Goats
In general, a less severe picture is observed than that seen in cattle. Again, early signs are dullness and fever. Blisters may develop on the lips, gums, dental pad, or tongue but soon rupture and leave shallow erosions. Hoof lesions, consisting of vesiculation of the coronet or interdigital space, result in lameness that may be the first sign detected. Abortions or sudden death of nursing lambs may be the first evidence of disease in breeding flocks.
Oral lesions in all species tend to heal quickly unless secondary bacterial infection supervenes. Hoof lesions take longer to heal, and hoof rot may develop and lead to extended lameness and sometimes abnormalities of the hoof. Mastitis, post-abortion retained placenta, and uterine infection are also seen.
During an autopsy, animals with blisterlike lesions in the mouth or nares and on the muzzle, snout, hooves, or teats should be suspected of having HMD. Depending upon the stage of disease, the lesions may also resemble erosions or ulcers. The sudden death of young animals or numerous abortions should lead to careful examination of adults. The differential diagnosis of suspicious conditions should include:
- Cattle -Vesicular stomatitis, bluetongue, infectious bovine rhinotracheitis, malignant catarrhal fever, bovine virus diarrhea/mucosal disease complex, mycotic stomatitis, and rinderpest.
- Swine -Vesicular stomatitis and swine vesicular disease
- Sheep - Bluetongue, sheep pox, contagious ecthyma (Orf), and ergot poisoning.
- Goats - Goat pox and contagious ecthyma (Orf).
Laboratory diagnosis is required if one is to be certain of the disease involved. The various vesicular diseases, especially in swine, are clinically indistinguishable. Hoof and Mouth disease is a reportable disease in the United States and if HMD is suspected, federal or state animal health officials should be notified immediately.
There is no known treatment for HMD. The United States subscribes to the eradication program that requires that all infected or exposed susceptible animals are killed and the carcasses buried or burned. Animal holding areas are then cleaned and disinfected and, after an appropriate waiting period, repopulated with a few susceptible, naive animals to determine the efficacy of the disinfecting procedure. All of the above procedures are performed under the direction of the U.S. Department of Agriculture. The Emergency Programs Staff of the Animal and of the Plant Health Inspection Service is specially trained to handle such situations.
Vaccines to control HMD are used in many parts of the world. Because vaccinated cattle can be infected even when exposed to homologous virus strains, HMD is seldom, if ever, eradicated by vaccination alone. However, vaccinating a large proportion of the susceptible population can keep the incidence of disease very low. The large number of antigenic variants that have appeared from time to time attest to the ability of the virus to adapt, and control officials must remain vigilant lest a strain of virus emerge against which the vaccine strains do not afford protection. For these reasons, HMD is considered to be enzootic in countries using vaccine.
Hoof and Mouth disease is widespread throughout the world, and modern rapid transportation of people and animals poses a real threat for its introduction into the United States. People can visit infected premises and be back on a US farm on the same day, perhaps bringing the virus with them. Literally tons of animal products are confiscated at ports of entry each year in an attempt to keep out foreign animal diseases. The USDA in under continuous pressure to allow the importation of animals and animal products from countries where HMD is enzootic. When such importation is allowed, it is done in the safest way possible. There is always the chance of illegal entry to circumvent the stringent requirements. Because of these potentially dangerous situations, the veterinary practitioner, in concert with the livestock operators in the US, must be constantly alert. Early diagnosis is the best defense and all suspicious conditions should be immediately reported to the state of animal health officials.
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