Foot-and-Mouth Disease

M. S. SHAHAN AND J. TRAUM.

FOOT-AND-MOUTH disease has become one of the most dreaded of the animal diseases.

Also known as FMD and aphthous fever, it occurs and generally is considered enzootic, or frequently epizootic, in most of the major livestock producing countries of the world, except those in North America, Central America, Australia, and New Zealand.

In Australia it last occurred in 1872.

Africa, Asia, Europe, and South America have not been free from it for decades.

The United States has experienced nine outbreaks of FMD, the first in 1870 and the latest in 1929. In all but two instances, the disease was eradicated and quarantines were removed within a few months. In outbreaks that began in 1914 and 1924, 20 months of ceaseless effort were required before it was considered safe to remove all restrictions from all the involved areas.

The first diagnosis of the disease in Canada was in February 1952. Because of the diligent efforts of Canadian officials, that country was placed on the list of countries free from the infection by the United States Department of Agriculture on March 1, 1953.

The presence of FMD in Mexico was established in 1946, and that country was not declared free until September 1952, after nearly 1 million animals had been slaughtered and 60 million vaccinations had been applied. The infection was again discovered in May 1953, and restrictions against the importation of cattle, swine, sheep, and goats from Mexico into the United States were not lifted again until December 31, 1954.

Susceptibility to natural infection with this acute and highly contagious disease is primarily and almost exclusively limited to cloven-footed animals, domestic and wild. Cattle, hogs, sheep, and goats are most frequently affected in the order mentioned. This order of susceptibility to the disease does not, however, always prevail. Deer were seriously affected in an outbreak in 1924-1925 in California.

Dogs and cats, especially young ones, are slightly susceptible to artificial infection. Rats have been experimentally infected, and rare cases of natural infection of rats have been reported in England. Rabbits have been infected artificially. The European hedgehog may be readily infected experimentally with FMD, and natural infections in hedgehogs have been observed in England. Attempts to infect horses experimentally have failed. Guinea pigs have been used extensively in FMD research since the early 1920's. The disease produced experimentally in this species is a prototype of the disease in cattle, but infection is not acquired by contact.

Research workers at the Pirbright Institute in England discovered that suckling white mice are highly susceptible to intra-abdominal injections of the virus, which causes spastic paralysis and inflammation of the muscles. Embryonated chicken eggs, although generally regarded as nonsusceptible, have nevertheless been reported as susceptible to some strains of the virus passed intermittently through guinea pigs and incubated eggs, or from 1 day-old chicks to incubated eggs.

Generally speaking, all animals of susceptible species in a given herd develop infection in time, but in some circumstances the rate of incidence is considerably less than 100 percent. Inapparent infections have been reported in some outbreaks. Although they show no clinical evidence of the disease at the time, such animals are subsequently immune.

People are rarely infected with FMD even though they are repeatedly exposed to the disease in many countries.

Few authenticated cases of human infection have been reported.

THE DISEASE MANIFESTS ITSELF by the formation of vesicles, or blisters, on the mucous membrane covering the various parts of the mouth, including the tongue, lips, gums, dental pad, and the palate.

Often vesicles also are found on the skin between and above the claws of the feet, and occasionally on the dew claws, muzzle, and nostrils of cattle.

Vesicles may be present on the teats and udder of milking or nursing cows. The lesions may be located at any one, several, or most of these places. The blisters usually rupture within a day or two, leaving a raw, eroded surface. The erosions heal rather rapidly as a rule, but at times they become secondarily infected by various bacteria. The feet and the udder are especially prone to such complications.

In swine the lesions are practically the same as in cattle, but they appear more frequently on the snout or feet.

In sheep, goats, and deer, the lesions are most often observed on the feet, but that may be because the small vesicles in the mouth may be overlooked.

These pathological changes in the tissues produce a series of other symptoms an increase in body temperature, loss of appetite, lassitude, and profuse slobbering. Because chewing may be painful, the animal may eat less or not at all, and therefore loses condition and weight. The animal usually becomes lame. Milk flow usually drops or stops. Abortion, mastitis, and sterility may occur.

Mostly the mortality in adult animals is less than 5 percent but is somewhat higher in calves and other young animals. In some outbreaks, however, mortality has reached 50 percent of the animals affected. In such instances, sudden deaths may follow severe inflammation and degeneration of the heart muscle, with or without the usual symptoms.

THE INCUBATION PERIOD following natural exposure varies from 2 days to a week but may be longer, depending on the particular strain of virus and the nature and extent of exposure to infection. Experimentally inoculated animals commonly develop lesions within 24 to 48 hours, but the lesions may be delayed a week or more.

In recovered animals there is residual damage, which may not be obvious for some time but which affects the general well-being and productivity of the animals. Recovered animals, especially cows, frequently are destroyed or sent to market at a sacrifice rather than retained in the hope that they will regain normal productivity.

THE VIRUS of FMD is one of the smallest viruses that cause disease in animals. Its size has been estimated to be between 8 and 12 millimicrons--that is, 3 million to 2 million measure 1 inch.

By field and laboratory observations and tests, at least six types of FMD virus have been found. These are designated as A, O, C, SAT 1, SAT-2, and SAT-3. The last three types have thus far been reported only from Africa. The others have been found in various parts of the world.

Each of the six types produces a disease picture that cannot be differentiated by the symptoms and lesions from the disease produced by the other five types. An animal that has recovered from an attack of FMD is immune for several months to 2 years to another exposure of the same type of virus, but it can be infected readily with any of the other five types. The six types thus are immunologically distinct, as well as serologically different, as reflected in tests with serum of such animals.

The virus is present in the fluid and coverings of the vesicles. It may be found in the blood and in various organs, tissues, secretions, and excretions, and therefore in the urine and milk in the initial, feverish stage of the infection.

The conditions under which mate-rials harboring the virus are found in and outside living animals or in the slaughtered animals determine its viability and infectivity.

In the living animal, the virus in the vesicle coverings and fluid, in other parts involved in the vesicles, and in most of the body tissues and organs usually loses its infectivity within 5 to 7 days after the lesions appear. Certain parts, such as the skin, hair, and loose parts and crevices in the hoofs, however, sometimes carry infective virus for some time.

It is also generally believed that the virus loses its activity in a short time within the body and that animals in the later stages of the disease are relatively less dangerous.

Many investigators and livestock sanitary officials believe nevertheless that virus may remain alive on parts of the animal not reached by the circulating blood, that virus carriers exist, and that such animals may harbor the virus for a long time after recovery. The percentage of carriers, however, is believed to be very small.

The field evidence presented to support this view is strong. Instances are reported in which the disease occurred in clean herds shortly after the addition of animals that had previously been affected, other possible sources of infect ion having been excluded. Animals have been held responsible for causing outbreaks of the disease more than a year after they recovered from it.

In the slaughtered animal, the formation of acid in the normal process of rigor mortis--stiffening of muscles after death in muscle tissue rapidly inactivates the virus that may be in the muscles and other parts reached by the acid. Quick freezing, however, suspends acid formation, and such muscle may retain its infectivity until thawed. Lymph nodes, liver, kidney, bone marrow, rumen, other organs, and residual blood are not affected by the changes attending rigor mortis and, depending on circumstances, may retain infective virus for weeks. They, like the muscle tissue that has not undergone rigormortis, may retain infective virus indefinitely when frozen.

Varying conditions affect viability of the virus outside the animal body. Virus exposed to direct sunlight is readily destroyed. But virus in tissue fragments or even on contaminated materials, such as hair, feed, and stable equipment, may remain infective for weeks under average stable and farm conditions.

That the virus does not always perish rapidly outside the animal body is strongly indicated by the fact that FMD sometimes has appeared on premises during the gradual restocking that usually is started under the eradication policy in this country 30 to 90 days after slaughter of affected and contact animals and disinfection of the premises. In one instance in California the virus persisted on such premises for 345 days. In the control and eradication of the outbreaks of FMD it is advisable, therefore, to regard the virus as relatively resistant to physical and other influences.

The viability of FMD virus in milk and milk products depends in general on the degree and rate of acid formation and on the temperatures involved in the processing of the various products. The virus is destroyed by heat, and standard pasteurization temperatures generally are considered adequate for its inactivation.