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).