Update on Selected Equine Diseases

One is fairly new in horses, others have been around for a long time, and a few are expanding their range. Read on for information on five diseases that affect horses.

Equine coronavirus. Recognized in 2010 as a pathogen affecting horses, equine coronavirus (ECoV) is an RNA virus that causes diseases of the respiratory and digestive tract. Other species-specific coronaviruses affect humans, cattle, dogs, and a variety of other mammals and birds. Clinical signs of ECoV in adult horses include loss of appetite, lethargy, fever, diarrhea, and colic.  Infection is most common in mature horses, with only one-fourth to one-half showing signs of illness. It is often self-limiting, with horses recovering within one to four days after signs are seen. An outbreak at a farm or stable usually lasts only three or four weeks.

Equine protozoal myeloencephalitis. EPM is a neurologic disease caused by two protozoa, Sarcocystis neurona and Neospora hughesi. S. neurona is passed to horses through opossum droppings. Horses are an intermediate host. EPM can be passed from mares to foals through transplacental transmission of N. hughesi. Prevention involves management steps such as reducing exposure of horses to opossum feces, discouraging wildlife in areas where horses are kept, avoiding feeding horses on the ground, and minimizing stress in horses. Ponazuril, diclazuril, and sulfonamide/pyrimethamine are used to treat affected horses. No protective vaccine exists.

Lyme disease. With its sometimes vague and shifting signs, this illness is difficult to diagnose and treat in horses. Research has found that some horses that were pronounced disease-free based on clinical findings or bloodwork were actually positive, and some horses that showed clinical signs and positive serology did not have the disease. Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to horses by a bite from an infected tick. Clinical signs may include weight loss, depression, fever, hypersensitivity to touch, neck and back pain, joint inflammation, facial paralysis, and unusual, dangerous, or aggressive behavior. Signs may come and go, and may also have other causes. The most effective treatment is antimicrobial therapy.

Equine influenza virus. Significantly more cases of this serious and highly contagious respiratory disease have been reported across the U.S. in the last several years. Since 2013, EIV outbreaks have been reported in Oregon, California, Idaho, Nevada, Montana, Utah, Arizona, Texas, Oklahoma, and Florida. Also, what was once considered to be a disease of mostly young racehorses has been seen in an increasing number of mature horses, even among those that have been previously vaccinated against EIV. Any horse that is stressed by events such as competitions, sales, or transport can be considered somewhat more susceptible to infection. Gatherings of horses offer an opportunity for the virus to spread, and winter management is sometimes a factor if horses are housed inside in close quarters. Keeping horses current on vaccination against EIV is an important, if not completely effective, way of preventing infection.

Pigeon fever. Named for the swollen abscesses it produces in a horse’s chest area, pigeon fever is caused by Corynebacterium pseudotuberculosis bacteria and has nothing to do with pigeons. It is most common in western states but has been increasingly reported in the midwestern and eastern regions of the country. C. pseudotuberculosis is transmitted to horses by flies that carry the soil-borne organism to open abrasions or wounds. Diagnosis is by physical examination, ultrasound, culture of material from the abscesses, clinicopathologic data, and serology. Treatment involves draining the abscesses and using antimicrobial therapy. Complications may include ulcerative lymphangitis and internal infection. No vaccine is available, so prevention steps include fly control, regular manure disposal, and proper care of wounds.