Colorado State Veterinary Equine Researchers Developing New Test for Dangerous Respiratory Infection in Horses

Equine researchers at Colorado State’s James L. Voss Veterinary Teaching Hospital are developing a faster and simpler test to determine a horse’s level of exposure to a vicious, highly contagious bacterial infection known as "strangles."

The test method is an enzyme-linked immunosorbant assay, known as ELISA, which would provide veterinarians with a tool to quickly and easily determine a horse’s risk level by assessing the animal’s blood antibody titer. A low titer translates to a very high-risk level for contracting the disease if exposed. Aware of the risk level, an owner would be better able to make smart decisions for the good health of his animal.

"For example, a horse about to be shipped to a farm where a case of "strangles" has already occurred would have a very good chance of contracting the disease," said Dr. Ann Davidson, one of the equine veterinarians working on the development of the test.

" If an owner is aware that his horse is in the high-risk category, he now has the option of electing not to ship the horse at all or to begin a series of vaccinations to protect it.

Strangles is a highly infectious disease in horses that affects the upper respiratory tract and the lymph nodes surrounding the throat, causing swelling and abscessation. In severe cases, the swelling makes it difficult for the horse to breathe, thus the name "strangles." Streptococcus equi, the bacterial agent that causes the infection, is found worldwide and has been recognized for centuries. It can affect horses of all ages, but younger horses are more susceptible.

"What we hope to achieve with this test is the ability to definitively determine a horse’s risk level so that the owner can more quickly make informed health-related management decisions," explained Dr. Davidson. "One of the challenges in diagnosing strangles lies with horses that are not exhibiting signs of active illness, but are carriers shedding the bacteria. It is often through contact with these shedders that other horses can become infected."

The basis of this rapid immunoassay method, developed specifically for S. equi, relies on whether or not there are antibodies to the bacteria in the blood. If an animal has been exposed to the highly contagious Streptococcus equi bacteria, either through illness or vaccination, the body’s natural response is to form circulating antibodies to fight the bacteria. The test would detect those antibodies and would reflect a high titer. If the horse had not been exposed, the test would reflect a low titer and a higher risk for contracting the disease.

Currently, tests exist only for horses with active signs of disease. The "gold standard" is a laboratory culture of the nasal discharge or abscess material. Another method is a polymerase chain reaction test, or PCR test.

"The advantage to the ELISA test on the horse’s serum is that we can now determine those horses who may not have been exposed and are at risk. Once perfected, we hope the test results will be rapid and readily available, making it a very useful tool for equine veterinarians" said Dr. Josie Traub-Dargatz, another of the test’s researchers. "Horse owners look to veterinary medicine for answers to help them better care for the health of their animals. Our profession is all about helping both animals and people and this test would serve both."

The funding for this research came through a special grant from the United States Department of Agriculture, the Cooperative State Research Education and Extension Services for the Colorado State University Program for Economically Important Infectious Animal Diseases, and the College Equine Research Advisory Council On The Use of Racing Funds for Colorado State University’s College of Veterinary Medicine and Biomedical Sciences.

Dr. Peter Owen and Dr. Mary Meehan in the Department of Microbiology at the Moyne Institute of Preventive Medicine, Trinity College, Dublin, Ireland provided additional assistance.