Foreign Diseases

By Edwin I. Pilchard.

Horse owners of the United States enjoy the freedom of this country's $12 billion equine industry from over 13 highly destructive diseases affecting horses in many other parts of the world.

Among the most important of these diseases are Venezuelan equine encephalomyelitis, dourine, surra, African horsesickness, glanders, and contagious equine metritis.

Cooperative efforts by industry and government succeeded in eliminating glanders from the United States by 1934 and dourine by 1942.

A sudden incursion of Venezuelan equine encephalomyelitis (VEE) into Texas in 1971 was promptly eradicated by a well-coordinated program of vaccination and aerial spraying of thousands of acres of land with pesticide to destroy the vector mosquitoes.

The virus causing VEE attacks the central nervous system, causing sleepy or depressed behavior, difficulty in walking, and sometimes a braced stance with feet placed widely apart in an apparent effort to avoid falling.

As the disease advances, swallowing becomes difficult. Most unvaccinated horses then rapidly lose consciousness and die.

Humans are susceptible to VEE and sometimes may develop fever, stiff neck, severe headache, and other signs, after being bitten by mosquitoes carrying the virus. Most humans recover.

Dourine and surra are caused by different members of the same group of microscopic parasites, Trypanosoma equiperdum and T. evansi, and they differ widely in their effects on infected animals.

Dourine is transmitted by infected stallions to susceptible mares during breeding, and is seen as soft swelling of the underside of the body and the legs, sometimes lameness, and abortion. In contrast, surra is transmitted principally by tsetse flies to horses, cattle, and camels, resulting in fever and anemia.

Perhaps the most dramatic and destructive of the foreign diseases of horses is African horsesickness. Spread by biting midges, the virus of this disease first produces restlessness, then colic, then a rapidly fatal pneumonia in which foamy fluid fills the lungs and airways. Horses that survive a few days may show soft swellings of the head above the eyes, underside of the chest and abdomen, and lameness. Also, pregnant mares may abort.

Caused by the bacteria Pseudomonas mallei, glanders produces coughing, severe irritation of the inside of the nose often resulting in nosebleed, and skin ulcers. Another foreign disease affecting the nose is schistosomiasis, caused by the microscopic parasite, Schistosoma nasalis.

Contagious equine metritis appeared for the first time in the United Kingdom and France in 1977, and has since spread to several other countries. It entered the United States but was quickly contained and has been eliminated, largely through diligent industry action and compliance with government requirements to prevent spread, or reintroduction from abroad.

This disease can prevent pregnancy in mares. Bacteria causing contagious equine metritis irritate tissues of the infected mare's reproductive tract, producing an outpouring of thick, white or yellowish fluid from the vagina a few days after breeding by a contaminated stallion.

Besides Venezuelan equine encephalomyelitis mentioned earlier, at least three other foreign diseases can attack the equine nervous system.

Borne disease can produce loss of muscular control of the legs and body, and paralysis of the tongue in horses, sheep, and goats. It is caused by a virus.

Japanese encephalitis in horses may be seen as a fever accompanied by violent behavior and death. In areas where vector mosquitoes transmit the virus, it can also cause sickness in swine and humans.

Equine encephalosis, caused by a virus that has been reported only from Africa, has been described as a rapidly fatal disease characterized by fever, excited behavior, and incoordination. In any discussion of diseases of the nervous system, it is well to remind the reader that rabies virus can cause signs like those just described, and that contact should be avoided with the sick animal, its saliva, or other body fluids.

To protect the $12 billion American equine industry, a large assortment of regulations exist to assure that horses brought into the country are free of disease. Anyone transporting horses, other animals and animal products into the United States should be aware of these rules well ahead of time, so all necessary health testing and certification can be completed.

Other Diseases

Several other foreign horse diseases should be mentioned because of their potential to spread to new geographic areas, and lack of an effective treatment. One of these is Getah disease, a mosquito-transmitted viral disease reported from Japan in recent years. Infected horses develop soft swellings of the lower legs that interfere with locomotion. A vaccine has been developed to prevent Getah.

Melioidosis, described in the Foreign Diseases chapter of Section IV Sheep and Goats, can produce a variety of signs in horses by damaging the internal organs and lymph system.

Vesicular stomatitis occurs in the United States from time to time, and some strains or types of the virus that causes it are foreign. Vesicular stomatitis gets its name from the appearance of blisters (vesicles) and raw ulcers in the mouth (stoma) of infected horses, swine, cattle and humans. More is said about it in the Foreign Diseases chapter of Section V Swine.

Preventing foreign disease outbreaks in horses and other livestock and poultry requires cooperation by all who transport horses or other animals or animal products from affected countries into the United States.

Edwin I. Pilchard is a Principal Staff Officer, Veterinary Services, Animal and Plant Health Inspection Service.