Hoof and Mouth Disease
by Dr. Richard Wallace
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
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
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
- 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.