The diagnostic tests equine veterinarians use are evolving to improve how we manage infectious diseases in horses

Diagnostic tests are an invaluable part of equine disease detection. Through diagnostics, veterinarians can obtain pertinent information about a horse’s condition that allows for a quick, targeted response to restore health. These tests also help practitioners monitor disease progression and response to therapy and adjust treatment accordingly. Advances in diagnostics for equine infectious diseases have led to increased specificity and sensitivity, giving the veterinarian and the horse owner an upper hand in disease management and control.

Common Equine Infectious Diseases

Horses in the United States are at risk of contracting a variety of infectious diseases. Diagnostic tests are available for some of the more prevalent and serious diseases, including the following:

• Anaplasmosis, a tick-borne disease veterinarians in the Northeast test for in cases of fever without obvious cause.
• Contagious equine metritis, a venereal disease that causes subfertility in mares and chronic infections in stallions.
• Eastern and Western equine encephalomyelitis, two potentially fatal mosquito- borne diseases.
• Equine coronavirus, which causes fever and sometimes intestinal signs.
• Equine herpesvirus, which can cause respiratory disease in young horses, abortion in broodmares, and neurologic disease.
• Equine infectious anemia, a viral disease that attacks the immune system.
• Equine influenza, a viral respiratory disease causing fever, cough, loss of appetite, and nasal discharge.
• Equine protozoal myeloencephalitis, a neurologic disease caused by protozoan parasites Sarcocystis neurona and Neospora hughesi.
• Leptospirosis, a bacterial infection that can result in abortion, chronic uveitis (moon blindness), and/or kidney failure.
• Lyme disease, a tick-borne bacterial condition that can cause lameness, arthritis, neurologic disease, and uveitis.
• Pigeon fever, which is characterized by severe inflammation and tissue damage.
• Potomac horse fever, a gastrointestinal disease causing high fever, colitis (inflammation of the large intestine), diarrhea, and dehydration.
• Rabies, a fatal zoonotic (can spread between animals and humans) viral infection that affects the neurologic system and salivary glands.
• Rhodococcus equi, a bacterial pneumonia in foals.
• Rotavirus, a common cause of foal diarrhea.
• Salmonella, a bacterial cause of gastrointestinal disease.
• Strangles, an infectious respiratory disease caused by Streptococcus equi.
• Vesicular stomatitis virus, which can cause blisters and sores in the mouth and on the tongue, muzzle, teats, or coronary bands of affected livestock.
• West Nile virus, a mosquito-borne neurologic disease that can be fatal.

Find a list of equine infectious diseases and information about each on the American Association of Equine Practitioners website (aaep.org/guidelines/infectious-disease-control/using-guidelines), TheHorse.com/149646 (a visual guide), or TheHorse.com/topics/infectious-diseases.

Types of Testing for Horses

Veterinarians rely on diagnostic tests to gather information about a patient they can use to make a diagnosis. Available diagnostics for equine infectious diseases include clinical chemistry (analyzing body fluids), hematology (complete blood count, or CBC), serology (blood serum tests such as enzyme-linked immunosorbent assay, or ELISA), immunologic tests (e.g., immunoassays, which detect proteins made in response to infection), microbiology (bacterial and fungal culture and viral isolation), molecular-based testing (such as polymerase chain reaction, or PCR), and virus isolation.

All of these diagnostic tests require a specimen obtained from the horse, such as blood, a nasal or nasopharyngeal swab, feces, or other bodily fluid. Blood is the most common sample veterinarians collect for laboratory testing.

Most diagnostic tests for infectious diseases detect either the causative organism or antibodies against that organism.

Clinical chemistry provides information about various organ systems’ functions; practitioners commonly use these tests to monitor treatment response and disease progression, particularly when managing a horse with an infectious disease. It provides information on the concentrations or activities of substances (ions, molecules, or complexes, and whether they’re rising or falling) in body fluids. Therefore, tests might require whole blood, plasma, serum, urine, cerebrospinal fluid, or other body fluids. Veterinarians compare the test results to normal reference ­values—ranges expected for healthy horses. (Find a visual guide to clinical chemistry and CBC at TheHorse.com/116893.)

“A classic example of using clinical chemistry and CBC as a diagnostic aid would be for a horse presented for chronic weight loss or a horse with vague clinical signs that do not point to a particular disease,” says Terry Hensley, MS, DVM, assistant agency director for diagnostic services at the Texas A&M ­Veterinary Medical Diagnostic Laboratory, in College Station. “And nowadays, many veterinarians have the capability to run chemistries and CBCs in the clinic, which allows for quick test results and possibly an early diagnosis.”

Serum amyloid A (SAA) testing is performed on serum. It’s a stallside diagnostic test that came available in the past decade and is useful for monitoring inflammation in the horse. Melanie Ramirez, DVM, a veterinarian at Weems and Stephens Equine Hospital, in Aubrey, Texas, says SAA has greatly enhanced diagnostics in the clinical setting.

“SAA is an acute phase protein that is produced by the liver in response to an inflammatory stimulus,” she says. “­Measuring SAA is a useful tool for monitoring the course of inflammation in a horse. Its levels rise and fall more quickly than CBC and fibrinogen (another inflammatory biomarker), allowing for close monitoring of (treatment) progress.”

Ramirez adds that SAA testing paired with other diagnostics—such as hematology and clinical chemistry—has been tremendously useful in her practice to diagnose horses showing signs of respiratory disease and fever of unknown origin.

Sensitivity and Specificity

Scientists use sensitivity (true positive) and specificity (true negative) to evaluate diagnostic test accuracy. Sensitivity is the probability of getting a positive test result in an individual with the disease, Hensley explains. It is the potential that a test will recognize that the horse has the infection. Specificity is the opposite: It is the probability of a negative test result in an individual without the infection or the potential that a test will recognize that a horse does not have the infection. 

“It is important to note that no diagnostic test is 100% accurate,” Hensley says. “There is always the possibility of a false positive or false negative being reported. Test results should be reviewed in conjunction with the clinical signs being observed in the horse.”

He points out that improvements in diagnostics are typically small but always advancing to improve sensitivity and specificity and, therefore, diagnostic accuracy. For example, researchers have shown that SAA testing is more accurate for detecting inflammation than traditional testing (Belgrave et al., 2013). Advancements in technologies such as these enhance diagnostic efficacy, allow veterinarians to make diagnoses more quickly, and are more economical than many conventional techniques.

The development of PCR has also completely revolutionized diagnostics through the detection of RNA and DNA. In the past couple of decades, conventional PCR has advanced to real-time quantitative PCR (qPCR), which has become the molecular tool of choice for diagnostics.

“More qPCR diagnostic tests continue to be developed, and we may soon see this technology available in the clinical setting on a limited basis,” says Hensley. “We are also seeing more multiplex qPCR tests that can detect multiple targets in one test, making them more efficient and accurate.

“Improved sensitivity, specificity, and reliability have been achieved in qPCR over conventional PCR,” he continues. “Use of qPCR in diagnostics in horses has led to advancements in orthopedics, detection of disease, and overall identification of horse DNA, such as in meat products (in a USDA Food Safety and Inspection Service regulatory setting) and archaeological discoveries.”

Again, not all diagnostic testing can be performed at a veterinary clinic. Some diseases must be diagnosed through more advanced testing at a diagnostic laboratory that serves animal health professionals, researchers, and animal owners. Also, results from tests submitted to diagnostic labs aren’t always available that same day. It might take several days to complete a bacterial culture, for instance. 

“Diagnostic laboratories generally provide a multitude of cutting-edge diagnostic techniques, while maintaining classical diagnostic modalities,” explains Hensley. “However, the equine industry has seen better baseline testing in-house for chemistries and hematology, although in some cases, confirmation at a diagnostic laboratory may be necessary.”

An example of a cutting-edge technique diagnostic laboratories are using is advanced testing for Lyme disease in horses. Animal Health Diagnostic Center researchers at Cornell University developed the Equine Lyme Multiplex Assay to identify antibodies against the causative bacterium, Borrelia burgdorferi.

Linda Mittel, DVM, MSPH, senior extension associate in Cornell’s Department of Population Medicine and Diagnostic Sciences, says the Multiplex Assay allows for earlier detection—as soon as three weeks after infection instead of five to six—and is more accurate than conventional Lyme ELISAs (Wagner et al., 2013).

“In humans, early infection stages (of Lyme disease) have a better prognosis and increased success rates after antibiotic treatment than chronic infection stages,” she says. “Similarly, the Multiplex Assay allows for a better definition of the horse’s current infection status and may assist in determining treatment options and prognosis.”

Take-Home Message

Equine infectious disease control depends upon appropriate sample collection and submission, along with accurate diagnostic testing. Advancements in testing serve to improve how we manage disease in horses. Although no test is 100% accurate, diagnostic testing is paramount in managing horse health.

Having your veterinarian examine your horse in a timely manner so he or she can collect the appropriate samples for testing is key to diagnosing disease early and initiating the appropriate treatment.