Texas Equine Veterinary Association

www.texasequineva.com | 24 News from Texas A&M University School of Veterinary Medicine & Biomedical Sciences About Diagnostic Testing for Strangles Noah Cohen, VMD, MPH, PhD, DACVIM-LA Director of the Equine Infectious Disease Laboratory Professor & Associate Department Head for Research and Graduate Studies Texas A&M University School of Veterinary Medicine Dr. Noah Cohen is an equine veterinarian with advanced training in equine internal medicine and infectious disease epidemiology. He is the Glenn Blodgett Chair of Equine Research at the Texas A&M School of Veterinary Medicine & Biomedical Sciences. His laboratory focuses on developing improved methods for diagnosis, treatment, and prevention of equine infectious diseases. Strangles is a common disease of horses worldwide caused by Streptococcus equi subspecies equi (SEE). Because it is highly contagious, SEE can result in outbreaks that cause widespread suffering for affected horses and can be expensive to control because of costs for treatment and biosecurity and because of lost revenues for facilities. us, rapid and accurate identification of infected horses is essential for controlling and preventing strangles. e standard method of detection of SEE has been microbiologic culture. is method has the advantage of allowing for antimicrobial susceptibility testing and also allows for strains to be curated for molecular epidemiological studies. Microbiologic culture, however, has disadvantages. It can take days to isolate and identify the bacterium, and speciation with biochemical testing is imperfect. Moreover, SEE can be outcompeted/overgrown by other bacteria, including its close relative S. equi subspecies zooepidemicus (SEZ), which limits sensitivity of the method. Because colonies of SEE and SEZ appear similar and because SEZ is a common commensal of the upper airway of horses, co-infections of SEE and SEZ occur. us, if the wrong colony is selected from a culture plate for further testing to identify the strain, an SEE infection or co-infection might be missed. e limitations of microbiologic culture led researchers to develop PCR tests for SEE. Various PCR protocols are used at veterinary diagnostic laboratories in the United States. Each of the targeted genes, however, have important limitations. For example, some diagnostic laboratories target the gene encoding the M protein of SEE (SeM), but this test fails to detect some strains of SEE. Moreover, some strains of SEZ can test positive for SEE depending on the primers (and probe) used for this target. Although PCR assays targeting multiple genes of SEE have been reported to perform better than those for individual genes, these assays are not completely accurate, and have increased complexity and cost which limits their widespread adaptation by diagnostic laboratories. Moreover, none of these tests permit simultaneous detection of SEE and SEZ. e Equine Infectious Disease Laboratory (EIDL) at Texas A&M University has developed novel primers and probes for detection of SEE and SEZ. ese primers and probes were developed by sequencing strains of SEE and SEZ recovered from horses in Texas. ey were further evaluated by demonstrating that they aligned in silico correctly with over 1,000 isolates of SEE (over 700) and SEZ (over 300) from various countries in 3 continents. Scientists from the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) partnered with the EIDL to further develop this test as a duplex (i.e., simultaneous detection of SEE and SEZ in a single assay) quantitative PCR assay for use in a diagnostic laboratory setting. e TVMDL assay meets guidelines of the American Association of Veterinary Laboratory Diagnosticians for

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