Equine Influenza

E. R. DOLL.

DISEASES described as equine influenza are known also as pinkeye, shipping fever, stockyard fever, catarrhal fever, and epizootic cellulitis. The names have been applied to a complex group of contagious respiratory and systemic infections of horses, mules, and asses.

Two different filterable viruses are known to be involved in this disease complex. The viral infections often are complicated by superimposed or secondary bacterial infection. Streptococcic infection is the most frequent bacterial complication.

The most serious cost of respiratory diseases is the loss of work by draft animals and the interruption of training, racing, or showing of pleasure horses. Abortion is the main economic loss in breeding establishments.

One virus has been known for 30 years as the equine abortion virus. In sucklings and weanlings and in susceptible older horses, it causes a short period of fever, which is accompanied by symptoms of a mild respiratory catarrh.

The disease may become severe when it is complicated by bacterial infection. On breeding farms the disease is commonly referred to as colds. The term "influenza" is often used at racetracks and training stables. Pregnant mares seldom show symptoms but may abort 3 weeks to 3 months after infection by the virus.

The second virus causes high fever, symptoms of severe generalized illness, and respiratory and intestinal disturbance. The virus causes severe losses from abortion when pregnant mares are infected. The specific lesion it causes is a necrosis of the small arteries (arteritis). Other lesions and the symptoms result mainly from damage to the arteries.

The disease caused by either virus and the bacterial complications associated with infection by each have been described as equine influenza.

RESPIRATORY INFECTIONS occur frequently in horses and mules that pass through sales barns, remount depots, stockyards, fair stables, training stables, racetracks, and other establishments where many animals from separate sources are brought together.

Respiratory infections spread rapidly in large groups of horses and may become highly virulent and cause severe illness or death. Respiratory disease occurs sporadically on farms. Often it is mild and usually is introduced by a newly acquired horse. Farm horses that are stabled and pastured separately may escape the disease.

The signs of the different virus infections and the bacterial infections that follow as complications often are similar. Diagnosis depends on careful consideration of signs and lesions and laboratory tests.

Abortion virus infection can be distinguished from infection by the virus causing arteritis by complement-fixation tests or serum neutralization tests. Serum samples should be tested in pairs, one taken during the acute febrile stage and the other 2 to 3 weeks later.

Fever (103 106 F.), watery discharges from the nostrils and eyes, inflamed eyes and nostrils, depression, weakness, and rapid breathing are early signs of infection by the virus causing arteritis of horses.

Difficult breathing is common. Exhalation is accompanied by a heave line and in more severe cases a pronounced abdominal lift.

Swelling of the legs, abdomen, mammary glands, scrotum, and sheath due to edema (accumulation of fluid) is a frequent symptom.

The term "pinkeye" refers to the swelling and congestion of the inner membrane of the eyelids, which may be so severely swollen that the eye is closed completely.

Severely affected animals are depressed, lose sensation to commands and physical pain, eat and drink erratically, and may finally refuse feed altogether.

Some animals are restless, move about without purpose, and frequently shift weight from one foot to another with cracking noises in the joints.

In critical cases the respiratory rate increases, the nostrils are flared, and breathing is accompanied by sounds that indicate edema of the lungs and accumulation of fluid in the chest and abdomen.

Some horses develop colic and profuse diarrhea and suffer great loss of weight. The horse becomes prostrate. Death occurs within 4 to 8 days after the onset of fever in acute cases.

Some horses have only a fever and slightly reduced activity and appetite. They recover promptly and the disease is not apparent to a casual observer.

In mild or moderate cases, without complications, the temperature returns to normal in 3 to 6 days and complete recovery follows promptly. The fever persists 7 to 9 days in severe cases and is followed by gradual recovery, which may take a week or longer. The mortality in uncomplicated cases is usually less than 1 percent, but may be as high as 5 percent. In the severe intestinal form it may be 10 percent or higher.

A second rise of temperature, often fluctuating, usually signals the development of complicating bacterial infections. They may affect the lungs, causing pneumonia; the intestinal tract, causing profuse fetid diarrhea; or the lymphatic system, causing abscess formation.

Other complications are inflammation, of the nervous system, with severe depression, paralysis, twitching, or excitability; inflammation of the kidneys, resulting in frequent, scanty urination; and degeneration of the heart muscle, with serious changes in the character and rate of the pulse and prolonged or permanent weakness. Severely affected eyes may develop a keratitis, corneal ulcers, and cataracts. Laminitis and swelling of tendon sheaths may occur.

RESPIRATORY DISEASES are spread mostly by direct contact between infected and susceptible horses.

The viruses may be spread by objects with which the sick come in contact mangers, troughs, feed, water, salt boxes, pails, cars, trucks, utensils, brushes, and brooms. They may be airborne in infected stables. Apparently healthy carriers of the viruses probably occur. The virus apparently dies in a short time in unoccupied stables.

Newly acquired animals should be held in rigid quarantine in an isolated stable, with separate attendants, for 15 to 30 days. The same practice should be used for horses being returned to a farm from fairs, shows, or racing circuits. The viral diseases are contagious, and sanitary precautions are not likely to prevent their spread through an infected stable. Strict isolation of an infected stable and adequate sanitary practices often prevent spread to other units on a farm.

Horses that are to be shipped may be protected to some degree against complicating bacterial infections (which follow viral respiratory diseases) by repeated injection of bacterins.

No specific vaccines or antiserums are available for the viral diseases. Temperatures should be taken twice daily of horses that are in transit and after arrival at fairgrounds or racetracks, so that isolation and treatment can be started promptly.

At the first sign of illness, usually a fever, the animal should have absolute rest in a clean, well-bedded, well-ventilated stall. It should have access to good hay and clean, fresh water. It should be fed lightly with grain and given light exercise after the temperature returns to normal. Exercise is important for reducing and preventing swelling of the legs.

Affected horses should not be returned to work until a normal physiological state is established. In mildly and moderately affected horses with uncomplicated recovery, a good rule would be 3 weeks after the temperature has returned to normal. Severely affected horses and those that develop complications may need more time.

Medicinal treatment should be prescribed by a veterinarian. Early treatment, at the first appearance of fever, may include the use of one or several antibacterial drugs to prevent development of complicating bacterial infections. Often no other treatment is necessary. Severe cases may require medication for regulating the bowels, Cardiac and respiratory stimulants may be necessary. Prostrate animals require artificial feeding and administration of fluids, salts, and nutrients intravenously.

E. R. DOLL, a native of Missouri and graduate of the University of Missouri and Michigan State University, since 1943 has conducted research on antibiotic therapy for bacterial diseases of foals, breeding problems of mares, hemolytic disease of newborn foals and pigs, virus abortion of mares, and viral respiratory infections of horses at the Kentucky Agricultural Experiment Station.