The Horse

MAR 2019

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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YOUR GUIDE TO THE 2018 AAEP CONVENTION A30 March 2019 The Horse | AAEP Wrap-Up TheHorse.com/AAEP2018 ALEXANDRA BECKSTETT STACEY OKE, DVM, MSC Strangles: Why the Gold Standard for Detecting Active Infection Changed S trangles, an infectious respiratory disease caused by Streptococcus equi subspecies equi bacteria, spreads with ease. Thus, it's important that veterinarians diagnose it quickly and accurately to prevent outbreaks. They have two methods of doing so: culture and the more reliable qPCR test, which detects bacterial and viral DNA. Nicola Pusterla, DVM, PhD, Dipl. ACVIM, pro- fessor of medicine and epidemiology at the University of California, Davis (UC Davis), School of Veterinary Medicine, re- viewed these methods and their efficacy. In recent years, he said, qPCR has supplanted traditional S. equi culture as the gold-standard diagnostic test for strangles. While culture is convenient and cost-effective, it's time-consuming and can produce false negatives. The qPCR test is much faster, with better sensitivity and specificity (ability to accurately iden- tify results). The results, however, can be difficult to interpret, said Pusterla, as vet- erinarians often can't determine whether it has detected a dead— indicating past exposure or infection—or live organism. So he conducted a study of 30 horses with suspected S. equi infection to deter- mine how to best interpret qPCR results. Pusterla collected 85 biological samples via nasopharyngeal swabs and guttural pouch (an internal sac that represents an outgrowth of the eustachian tube and acts to equalize pressure within the ear) lavage. He used three molecular methods to determine organism viability: absolute quantitation of the target gene (basically, determining its numbers), detection of messenger RNA, and absolute quantita- tion after enrichment (incubation in a culture broth to grow bacteria). Based on his results, Pusterla said any of these qPCR approaches could be ef- fective in diagnosing true strangles cases, but if absolute quantitation produces positives, consider those S. equi organ- isms to be viable. This can help the veterinarian deter- mine whether a horse has an active infec- tion and take the appropriate biosecurity steps to prevent disease spread, he said. It can also confirm whether it's safe to remove a horse from quarantine. Diagnosing ECoV: Take Fecal Samples Equine coronavirus (ECoV) is a relatively recently recognized infectious disease of adult horses. Its clinical signs of fever, lethargy, anorexia, colic, and diarrhea, however, can point to other diseases. This presents a diagnostic chal- lenge for veterinarians. "Equine coronavirus makes liars out of horses," Pusterla said, "particularly horses with fever, but no respiratory signs (nasal discharge, coughing) and no gas- trointestinal signs (diarrhea, colic). What sample do you collect (on sick horses to confirm an ECoV infection)?" To find out, he studied 277 horses from across the United States with acute fever and at least one other ECoV infection clinical sign, but no colic or diarrhea. Pusterla used qPCR testing to look for viral DNA in feces and nasal swabs from each horse. He also looked at other fac- tors, such as age, breed, use, and history. He found that: ■ 20 (7.2%) horses tested positive on feces for ECoV, four of which tested positive on nasal secretions, as well; ■ Of these 20, 9% also tested positive for other respiratory pathogens (equine herpesvirus, influenza, etc.), but there were no statistically significant associa- tions between concurrent infections; ■ Draft horses were overrepresented in the study and six times more likely to test positive for ECoV than were other breeds; ■ Performance horses and those on farms with large equine populations were more likely to have ECoV (five times and 2.7 times more, respectively). "This goes back to the highly conta- gious nature of ECoV," said Pusterla (horses contract it by inadvertently ingesting virus shed in the feces of an infected animal); and ■ Positive horses did not frequently cough or have nasal discharge. Pusterla said ECoV's main signs are subtle—anorexia, lethargy, and fever. And while the virus doesn't consistently induce gastrointestinal signs such as colic or diarrhea, affected horses' feces are the most reliable sample to test. "Nasal detection is low and likely represents environmental contamination (breathing in ECoV particles) through fecal shedding," he said. So if you see a horse in "standby mode" (lethargic, feverish), test his feces for ECoV, particularly if he shows no evi- dence of nasal discharge, Pusterla said. BIANCA MCCARTY Diagnosing Disease

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