Diagnosis of strangles
Diagnosis of acute disease
The diagnosis of strangles relies on a thorough understanding of an animal’s history, with particular respect to onset, management structures, and possible exposure, including the history of travel, or new arrivals to the farm (Boyle et al., 2018). Clinical signs can be variable and non-specific; indeed, not all animals develop clinical signs (Duran and Goehring, 2021, Boyle et al., 2018). Nevertheless, they form a vital part of any clinical diagnosis, especially during an outbreak where the testing of all affected individuals may not be necessary (Rendle et al., 2021). Many diagnostic modalities can aid in the diagnosis of strangles and its complications, including radiography, ultrasonography, and endoscopy, as well as clinical pathology (Boyle et al., 2018, Duffee et al., 2015, Van de Kolk and Kroeze, 2013).
Pathogen identification historically relied on the culture of S. equi due to its low cost and wide availability (Waller, 2014). However, sensitivity can be as low as 30-40% (Lindahl et al., 2013, Boyle et al., 2012, Pusterla et al., 2021), and other beta-haemolytic Streptococci such as S. zooepidemicus can complicate interpretation (Boyle et al., 2018). Low levels of bacterial shedding, the presence of host-produced growth inhibitors, sample site and poor sampling technique can also lead to false negative results (Pusterla et al., 2021).
Advances in polymerase chain reaction (PCR) (Webb et al., 2013, Noll et al., 2020, Willis et al., 2021) and loop-mediated isothermal amplification (LAMP) assays (Boyle et al., 2018), with their shorter turnaround times, have improved the sensitivity and specificity of the detection of S. equi and these assays are now regarded as the gold standard (Boyle et al., 2018). PCR and LAMP assays detect DNA of live and dead bacteria indiscriminately; although efforts to determine the physiological state and viability of S. equi using molecular approaches show promise (Pusterla et al., 2018). Despite the potential for false positive results, all positive PCR cases should be taken seriously, even if they are culture negative (Rendle et al., 2021, Boyle et al., 2018, Waller, 2014, Pusterla et al., 2018). Identification of animals with clinical signs consistent with strangles, regardless of the PCR result, should result in strict movement restrictions and biosecurity protocols (Willis et al., 2021, Rendle et al., 2021).
Advances in diagnostics and surveillance are interlinked: techniques such as quantitative PCR (Webb et al., 2013), nested PCR (Noll et al., 2020), and LAMP assays (Boyle et al., 2021) are rapid and possess high sensitivities and specificities These technologies allow for the creation of clinically valuable surveillance schemes (McGlennon, 2019), with both laboratory and veterinary contributors. Point-of-care assays have limitations in detection threshold, but have the potential to reduce diagnostic turnaround times and provide a simpler option to caregivers (Slovis et al., 2020). This would allow for the screening of high-risk animals, reducing diagnostic guesswork, and ensuring well-timed enaction of biosecurity measures.
The successful identification of S. equi , whether through bacterial culture or molecular methods, is dependent on the stage of infection (Rendle et al., 2021) and the sampling site and technique used (Boyle et al., 2017). A single negative test result does not equate to the absence of infection (Boyle et al., 2018). S. equi is only present transiently on the nasal mucosa and is often undetectable until the lymphoid abscesses rupture, which typically occurs 1-4 weeks after infection (Rendle et al., 2021). Consequently, nasal swabs and washes will often yield negative results in the initial stages of infection (Boyle et al., 2018). Using quantitative PCR, it was found that nasopharyngeal lavage was the optimal sampling technique with the highest sensitivity, followed by nasopharyngeal swabbing and then nasal swabbing (Lindahl et al., 2013).
Diagnosis of persistent infection
Carriers of S. equi do not differ clinically and cannot be diagnosed on the basis of inflammatory markers, including white blood cell counts and serum amyloid A (Pringle et al., 2020b, Christoffersen et al., 2010, Davidson et al., 2008); therefore, carrier status has little impact on systemic inflammation. There is conflicting evidence on the utility of endoscopy scoring since many carriers have grossly normal guttural pouches (Pringle et al., 2020b, Riihimäki et al., 2016); however, Boyle et al. (2017) found distinct differences are visible in many carriers.
Endoscopically guided guttural pouch lavage followed by quantitative PCR is recommended for the detection of persistent infections (Boyle et al., 2018). This technique provides visualisation of the guttural pouch, allowing identification of chondroids, inflammation, or empyema; although, contamination of equipment can result in false positive results (Svonni et al., 2020). LAMP assays have been demonstrated to be comparable to PCR for this purpose (Boyle et al., 2017). Guttural pouch lavage has been validated as superior to a single nasopharyngeal swab or lavage (Boyle et al., 2017). However, nasopharyngeal lavage on three separate occasions has been demonstrated to predict freedom from persistent infection (Pringle et al., 2022, Sweeney et al., 2005), with repeated testing mitigating the possibility of false negatives. Serological testing is unable to identify carrier animals (Durham and Kemp-Symonds, 2021) and does not replace these other more invasive, expensive and time-consuming methods of detection. Guttural pouch lavage combined with quantitative PCR is considered the best, albeit imperfect, method for carrier detection (Svonni et al., 2020, Boyle et al., 2018, Rendle et al., 2021).