Research initiated at the Equine Infectious Disease Laboratory (EIDL) at Texas A&M University aimed at exploring the genomic components of strangles has now led to an improved assay at the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL).
Strangles is caused by the bacterium Streptococcus equi subspecies equi (S. equi) and is a highly contagious disease that affects the upper respiratory system and lymph nodes of horses. Though it maybe suspected due to clinical signs, diagnostic detection is the only definitive method of identifying strangles. The ancestor of S. equi, Streptococcus equi subsp. zooepidemicus (S. zoo), is considered a commensal organism and may cause pneumonia, endometriosis, and abortion in horses. Together, both organisms contribute to high morbidity and variable mortality among horses.
As a graduate research assistant at the EIDL, Ellen Ruth Alexander Morris, PhD, began working on the project in early 2020 with Dr. Noah Cohen, Director of the EIDL.
“There was no initial plan to develop a new assay,” Alexander Morris said. “The idea evolved from another ongoing project, where we were examining the genome-wide differences between 50 strains of S. equi and 50 strains of S. zoo using whole genome sequencing.”
Their evaluation of the different strains led to the development of additional primers, which are DNA sequences that can be used to detect both S. equi and S. zoo.
Current detection methods
Classically, strangles was detected by culturing S. equi and S. zoo; a slow and less sensitive method. Over time, multiple tests using polymerase chain reaction (PCR) technology have been developed to detect S. equiand S. zoo individually.
Although quicker and more sensitive than bacterial culture, PCR testing for strangles is still somewhat limited. PCR testing uses primers to target specific DNA sequences to determine an organism’s presence. Because of the genetic similarities between S. equi and S. zoo, some tests may not be able to differentiate between the two organisms and lead to false positives. Conversely, there are instances where the typically targeted sequence of S. equi has been truncated or deleted and therefore testing leads to a false negative. Other PCR tests, such as the one previously offered at TVMDL, cannot differentiate coinfection with S. equiand S. zoo due to the organisms’ multiple genetic similarities.
Improved PCR testing
The new primers were designed from S. equi and S. zoo strains collected from clinical samples of Texas horses and using publicly available S. equi and S. zoo strains from across the world. TVMDL’s molecular diagnostics section performed validation testing using the new primers and determined they could be used to detect and differentiate between S. equi and S. zoo. This assay also includes an internal control that serves as a monitor for PCR efficiency and sample inhibition. Following validation, TVMDL can now use these primers for routine diagnostic testing.
“Our hope is these new PCR targets will aid in the diagnosis of strangles, identify cases of concurrent infection of S. equi and S. zoo, or improve differentiation between the two organisms,” Alexander Morris, who is now a postdoctoral research assistant at TVMDL, said.
TVMDL now offers an improved version of Streptococcus equi Multiplex (rtPCR) at the College Station and Canyon laboratories. For more information on this test, or other equine testing options, visit tvmdl.tamu.edu or call the College Station or Canyon laboratories. To learn more about EIDL, visit their website at vetmed.tamu.edu/eidl.
