Control of strangles outbreaks by isolation of guttural pouch carriers identified using PCR and culture of Streptococcus equi

Previous use of repeated nasopharyngeal swabbing and culture of Streptococc us equi showed that healthy carriers developed in more than 50% of 'strangl es' outbreaks. The guttural pouches were the only detectable site of S. equ i colonisation on endoscopic examination of horses during one of these outb reaks and S. equi was sometimes not detected by culture of nasopharyngeal s wabs from carriers for up to 2 or 3 months before nasal shedding resumed sp oradically. A more sensitive way of detecting S. equi on swabs from establi shed guttural pouch carriers was therefore required. Conveniently selected 'strangles' outbreaks were investigated in detail using endoscopy, in order to develop and assess a suitable polymerase chain reaction (PCR) test. We report here 3 protracted 'strangles' outbreaks on different kinds of establ ishments in which between 29 and 52% of sampled horses were infected as det ected by culture and/or PCR. Of the infected horses, between 9 and 44% were identified as carrying S. equi after clinical signs had disappeared and th e predominant site of carriage was the guttural pouch. Prolonged carriage o f S. equi, which lasted up to 8 months, did not cease spontaneously before treatment was initiated to eliminate the infections. The detection and isol ation of the carriers, in conjunction with strict hygiene measures, apparen tly resulted in the control of the outbreaks and allowed the premises to re turn to normal activity. Comparing PCR and culture, many more swabs were fo und to be positive using PCR (56 vs. 30% of 61 swabs). Similar results were obtained for guttural pouch samples from 12 established carriers (PCR 76% and culture 59%). These results from repeated samples from relatively few a nimals need confirming using more long-term carriers. PCR can also detect d ead organisms and is, therefore, liable to yield false positive results. De spite this drawback, it is argued that PCR provides a potentially useful ad junct to culture of nasopharyngeal swabs in the detection of asymptomatic c arriers of S. equi following outbreaks of 'strangles'.