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

Abstract

Summary Previous use of repeated nasopharyngeal swabbing and culture of Streptococcus equi showed that healthy carriers developed in more than 50% of ‘strangles’ outbreaks. The guttural pouches were the only detectable site of S. equi colonisation on endoscopic examination of horses during one of these outbreaks and S. equi was sometimes not detected by culture of nasopharyngeal swabs from carriers for up to 2 or 3 months before nasal shedding resumed sporadically. A more sensitive way of detecting S. equi on swabs from established 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 establishments in which between 29 and 52% of sampled horses were infected as detected 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 the predominant site of carriage was the guttural pouch. Prolonged carriage of S. equi, which lasted up to 8 months, did not cease spontaneously before treatment was initiated to eliminate the infections. The detection and isolation of the carriers, in conjunction with strict hygiene measures, apparently resulted in the control of the outbreaks and allowed the premises to return to normal activity. Comparing PCR and culture, many more swabs were found 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 animals need confirming using more long‐term carriers. PCR can also detect dead organisms and is, therefore, liable to yield false positive results. Despite this drawback, it is argued that PCR provides a potentially useful adjunct to culture of nasopharyngeal swabs in the detection of asymptomatic carriers of S. equi following outbreaks of ‘strangles’.

Use of a Reverse Thermodynamic Gel to Manage Chronic Shedding in Equine Strangles

“…Management of outbreaks is complicated by the development of asymptomatic carriers with the causative organism (Streptococcus equi var equi) being retained within the guttural pouch 1,2 . Treatment of carriers relies upon removal of physical infection and locally administered antimicrobials.…”

Thermophilic helicase-dependent DNA amplification using the IsoAmp™ SE experimental kit for rapid detection of Streptococcus equi subspecies equi in clinical samples

“…Some animals harbor S. equi in the guttural pouch for months after clinical recovery and may become long-term carriers that intermittently shed the organism. 8,14,15 There are over 9.5 million horses in the United States, some of which are frequently moved long distances between racetracks and other competitive events and for breeding, sales, and training. Thus, there is potential for transmission of infectious agents including S. equi.…”

“…Moreover, identification of atypical S. equi requires additional biochemical tests. 20 Polymerase chain reaction (PCR) assay based on seM, the gene for the antiphagocytic M protein of S. equi, 24 is more rapid, sensitive, and specific than culture, 11,14,23 but is technically complex, requires costly instrumentation, and reagents are not widely available. Also, shipment of samples to qualified laboratories delays decisions concerning movement and transportation of horses and increases the cost of testing.…”

Development and Application of Loop-Mediated Isothermal Amplification Methods Targeting the <i>seM</i> Gene for Detection of <i>Streptococcus equi</i> subsp. <i>equi</i>

“…S. equi infects the horse's cranial lymph glands and is highly communicable to other horses. Outbreaks of strangles have been reported in many countries and can cause scheduled horse racing meets to be cancelled [4,[6][7][8]11]. S. equi produce an M-like protein, SeM, and the seM gene has been proposed as a target gene to distinguish S. equi from different strains of streptococci including Streptococcus equi subsp.…”

“…However, the diagnostic procedure conventionally used as a culture examination for S. equi requires significant time before the result is obtained. The alternative PCR method that detects the S. equi gene to confirm the diagnosis genetically can be applied clinically [1,4,11]; however, significant time is required to conduct the PCR method, although the disease can be diagnosed more promptly with PCR than by culture. The development of a rapid diagnostic method is highly desirable to alleviate these delays.…”