Horse Health: Veterinary Endoscopes as Carriers of Bacteria

Poorly disinfected endoscopes (“scopes”) can falsely identify carriers of Streptococcus equi subspecies equi, the bacteria that causes strangles in horses. Even following strict disinfecting instructions in a controlled setting, up to 73% of scopes tested positive for S. equi, inadvertently placing blame on healthy horses and prompting unnecessary treatment.^*

Strangles is a highly contagious infectious disease of horses that causes high fevers, nasal discharge, and enlarged submandibular lymph nodes. After recovery, a small percentage of horses (about 10%, but up to 30% in some populations) become carriers for strangles. These outwardly healthy horses have persistent infections in their guttural pouches and continue to shed the bacteria in the environment, putting naïve horses at risk of infection.

To identify carriers after a strangles outbreak, horses are scoped and their guttural pouches either swabbed or washed and tested for S. equi. Those samples can be cultured for live S. equi or undergo quantitative polymerase chain reaction (qPCR) to test for S. equi DNA. Positive horses are then immediately segregated from the herd and treated.

“A false positive diagnosis results in unnecessary treatment, which is both time-consuming and economically draining,” said Catherine Whitehouse, M.S., of Kentucky Equine Research.

Following an outbreak, veterinarians often scope several horses on a single farm. These scopes must be disinfected between every horse.

“Various disinfectant products are available for on-farm use, including accelerated hydrogen products (AHP) or ortho-phthalaldehyde (OPA),” Whitehouse explained.

To determine if these products effectively remove live bacteria and DNA from endoscopes, a laboratory simulation was created. A bacterial slurry of S. equi was used to “contaminate” scopes. Those scopes were then disinfected according to the manufacturer’s instructions with AHP, OPA, or water. Samples were then collected from the presumptively clean scopes and tested by culture and qPCR.

“This study found that all disinfected scopes were culture negative. However, 33% scopes disinfected with AHP were positive by qPCR. Further, 73% and 71% of the scopes disinfected with OPA and water, respectively, were positive for S. equi on qPCR,” Whitehouse said.

This means that even with the high level of manual disinfection used in the study, the qPCR was positive 33-73% of the time, which would likely lead to a misdiagnosis of an S. equi carrier in a clinical situation.

“Veterinarians must be extremely vigilant when cleaning endoscopes, using manufacturer’s recommendations to minimize contamination,” advised Whitehouse.

According to the researchers, new testing assays are being developed that will differentiate between live and inactivated microbes via PCR.

“The ability to distinguish between live and dead organisms would be extremely useful in trying to determine if a horse (culture negative, but qPCR positive) is indeed a true carrier, making residual DNA on scopes no longer a concern,” the researchers wrote.

^*Nadruz, V., L.A. Beard, K.M. Delph-Miller, R. L. Larson, J. Bai, and M.M. Chengappa. 2023. Efficacy of high-level disinfection of endoscopes contaminated with Streptococcus equi subspecies equi with 2 different disinfectants. Journal of Veterinary Internal Medicine. doi:10.1111/jvim.16740.