Scrapie in Sheep

C. R. OMER.

THE DISEASE of sheep known as scrapie is caused by a filterable virus. It represents a chronic neurosis characterized by symptoms of intensive and progressive pruritis, or itch, progressive debility, and locomotor incoordination. The course of the disease may extend over several months. Almost invariably it ends in death.

Scrapie was found in New Zealand and Australia in 1952 in animals imported from Great Britain.

The first case of scrapie in the United States was reported in Michigan in 1947. The disease appeared in California and Ohio in 1952. Since that time scrapie has been diagnosed in Illinois, Indiana, New York, Connecticut, Oregon, Texas, and Tennessee.

All breeds of sheep are susceptible. It has recently occurred only in the Suffolk or Cheviot breeds or their crosses. Most of the affected sheep found in the United States have been imported from Canada or are the progeny of such importations an indication that infection possibly has been carried by sheep imported into Canada from English flocks that may have been either infected or exposed.

At one time it was thought that the disease followed only certain blood lines of sheep. An experiment was carried out in Scotland, starting in November 1932, by Dr. J. Russell Greig. Healthy sheep were placed in a pasture that had contained scrapie-infected sheep, without coming into direct or indirect contact with the infected animals. This exchange was made twice weekly between healthy and diseased sheep.

In 1936-3 years and 3 months later the first symptoms appeared in the healthy group. Seven cases showed typical symptoms of scrapie between February 1936 and May 1938, when the experiment was concluded. This exposure afforded experimental evidence that scrapie is an infective disease, the period of incubation is prolonged, and the causal agent can be transmitted through the medium of pasture.

The exact method of elimination of the virus from the infected animal is not known.

THE SYMPTOMS of the disease are slow to develop. Only an experienced sheepman may be able to recognize the earliest symptoms. The sheep are nervous, apprehensive, and more excitable than usual. Slight tremors in the head and neck are observed. The animal becomes tense with excitement when it is moved. Rapid muscular tremors, affecting particularly the thighs and flanks, are manifest. The head and neck are carried high and somewhat stiffly. The facial expression is staring and fixed. The ears frequently assume unnatural positions. Grinding of the teeth is commonly noticed. The wool loses its luster and becomes dry.

The outstanding symptom is that the animal rubs against fixed objects. At first the owner may suspect that the animal is infected with scabies, but a close examination of the skin discloses no evidence of scabies. The scratching usually begins in the region of the rump, particularly around the tail head, and gradually extends over the sides, shoulders, and neck. When the sheep is rubbed by the hand over the loin or rump, it will usually show a definite "scratch reflex" nibbling movements of the lips and sometimes vigorous wagging of the tail.

The itch becomes more intense, as the disease progresses and the animal spends a great deal of time rubbing. One can see evidence of this rubbing on fences and sides of buildings where pieces of wool hang. The skin of the animal will show no evidence of any diseased condition, except possibly abrasions from the scratching. Some sheep do not rub their wool but bite the skin on their legs and pull wool with their mouths.

The temperature of the infected animal remains normal throughout the course of the disease. Thirst is a frequent symptom. There is no diarrhea. The. bleating is weak and tremulous.

The animal's appetite remains good. The sheep gradually loses weight, becomes weak, and walks with an unsteady or staggering gait. It may fall after running a short distance, particularly if it tries to jump over objects on the ground. Eventually it will be unable to stand. It will then lie on its side and make running movements of the feet. It will continue to eat in this position if feed is within its reach.

The course of the disease varies in duration, from 6 weeks to 6 months before the animal dies. Instances are known in which the animal dies within 14- days after the first symptoms are noticed.

THE NUMBER OF ANIMALS actually infected in a flock may vary from 4 to 20 percent, although it is possible that 50 percent of the flock may be affected, or there may be only an occasional case over a period of several years. Interviews with farmers in whose flocks the infection was diagnosed brought out that some of them had lost a few sheep without knowing that such a disease existed. Scrapie is a new disease in this part of the world.

As the disease has an extremely long incubation period, only seldom does one find an animal under 18 months that shows the symptoms. Animals as old as 11 years have been infected. Most of the cases in the United States have been in animals about 3 years old. It is possible that some animals may harbor the infective agent and never show symptoms of the disease themselves.

Scrapie is different from parasitic skin diseases commonly found in sheep.

In scabies, the mites can be located and the scabs they form can be seen with the naked eye, and there is no incoordination of gait. Ticks and lice also can be seen on the skin and in the wool. Dipping the animals in special preparations for the elimination of these parasites will stop their rubbing.

Scrapie must also be differentiated from listeriosis, a bacterial disease of short duration. Animals that have listeriosis draw their heads to one side, circle in one direction, and push against fixed objects.

No specific postmortem lesions are present in the animals infected with scrapie, except those that microscopic examination of sections of the brain stem and adjoining spinal cord will reveal. Evidence of vacuolar degeneration of nerve cells characteristic of this disease is seen then.

WE KNOW OF NO TREATMENT for sheep infected with scrapie.

The most effective method of eradication has been the slaughter of all infected and exposed animals. This method has been recommended in England and has been actually carried out in infected flocks in New Zealand, Australia, Canada, and the United States.

Thorough cleaning and disinfecting of premises after slaughter is important. Federal indemnity is available for partial payment of sheep destroyed under this program in the United States. Several States also have made funds available for indemnity payments.

A program has been developed in all States whereby flocks containing sheep that have been imported or purchased from flocks that may have been exposed to scrapie are inspected at 6-month intervals by State or Federal veterinarians. In this way the flock owner learns about the symptoms of this disease and which officials to notify if he suspects scrapie in his flock.

C. R. OMER is veterinarian in charge, Animal Disease Eradication Branch, Agricultural Research Service, the United States Department of Agriculture, Albany, N. Y.