Strangles testing
All infectious diseases may be investigated by either looking for the organism
itself, or looking for an immune response against it.
Diagnostic testing for strangles can be used to help answer 2 different questions
and it is important to be clear exactly which question(s) needs answering:
- Has this horse been infected recently with strangles?
- either in response to a clinical concern or perhaps as screening
of a known outbreak, or
- Is this horse a chronic carrier of strangles?
- perhaps as a check before entering a new yard.
The best choice of test differs between these 2 circumstances Understanding the
stages and progression of infection and where the organism is likely to be found
and when, is crucial to selection of the right test.
Detection of acute cases/exposed in-contacts:
o nasopharyngeal washes (qPCR)
o paired serology
Detection of chronic carriers
o guttural pouch lavage (qPCR)
o (serologic tests have no value in screening for chronic
carriers)
- DETECTION OF THE STRANGLES ORGANISM ITSELF
Both qPCR and culture should ideally be performed. Although qPCR is far more
sensitive overall, occasional culture+/qPCR- cases are seen which justifies using
both lab methods.
Decide on the best sampling method based on where the infection is most likely
to be:
Acute cases are best approached with nasopharyngeal washes.
Chronic cases are best approached with guttural pouch lavage.
a) Initially acute strangles infection will be a diffuse nasopharnygeal mucosal
infection. Diagnosis will be best achieved by sampling a large surface area of
nasopharyngeal mucosa.
b) Infection will then localise in lymph nodes.
c) Lymph nodes may rupture either externally or internally (retropharyngeal) into
the guttural pouch(es).
d) If horse fails to clear infection then empyema and chondroid formation follows
and horse becomes a chronic carrier.
Q1. Has this horse been infected recently with strangles?
Acutely infected strangles cases are unlikely to have guttural pouch infection
(caused later by rupture of retropharyngeal lymph nodes into the guttural
pouches). Therefore, guttural pouch lavage is not an appropriate test for these.
Nasopharyngeal Lavage
The advantage of this technique is that it samples a much larger surface area of
the nasopharyngeal mucosa than other methods such as nasal or
a b c d
nasopharyngeal swabbing, and therefore has a significantly higher diagnostic
rate.
Equipment needed: • a 40-50 cm sterile tube or dog urinary catheter • 50-60 mL warm sterile saline • Sterile universal container
Technique: • Sedate horse • Insert tube/catheter into nasopharynx (typically about 40 cm) • Ensure head hangs down and squirt in sterile saline • Collect fluid as it exits nostrils into the sterile universal container • Submit washing for PCR/culture
Q2. Is this horse a chronic carrier of strangles ?
Cases of strangles should generally become free from infection between 6-10
weeks following initial exposure to strangles. Infection persisting longer than this
(i.e. 6 weeks to lifelong) are regarded as chronic carriers, many of which show no
clinical signs of such.
For selection of appropriate tests, it is important to remember that when Strep
equi persists in a horse for the medium to long term (>6 weeks), it does so almost
invariably within the guttural pouches. Therefore, any test which does not directly
sample from the guttural pouches carries a significant risk that it will not be
detected (even when repeated several times). There is no acceptable substitute
for guttural pouch lavage when trying to identify strangles carriers.
Guttural pouch lavage
Unlike all alternative tests, when PCR, culture and cytology are performed on
bilateral guttural pouch lavages, it is highly unlikely that a strangles carrier will be
missed. The technique requires an endoscope but is remarkably easy to perform
as long as the details of the procedure are followed exactly as below.
Equipment needed:
Sedation
Endoscope
Endoscopic guidewire
2 x endoscopic catheters
2 x 30 mL syringes of sterile saline
Technique
Sedate horse
Insert endoscope via ventral meatus (push scope ventrally during
insertion)
Approach ipsilateral guttural pouch ostium and insert the guidewire
through the dorsal aspect of the ostium and into the guttural pouch
Orientate the endoscope so that the guidewire is on the pharyngeal
luminal side of the endoscope, thus opening the ostium
Pass the endoscope forwards into the guttural pouch as the wire is slowly
withdrawn
Examine the pouch carefully paying particular attention to the floor of
the medial compartment (which overlies the retropharyngeal lymph
nodes)
Remove guidewire and pass an endoscope catheter
Hold the horse’s head reasonably elevated and inject 20-30 mL sterile
saline into the pouch
Aspirate the fluid from the pool that collects in the medial compartment
Divide the sample between a plain universal container (PCR & culture)
and EDTA tube (cytology)
Other tests
Direct swabbing/aspiration from abscesses
Although one would imagine that direct sampling from an abscess should be a
highly reliable sample, evidence indicates as few as 20% of samples collected from
strangles abscesses are culture positive and perhaps only 80% PCR positive.
Nasal/nasopharyngeal Swabs
Swabs collected from the nose or nasopharynx are fundamentally compromised.
In suspected acute cases these swabs sample an appropriate area, but only a very
small part of it and nasopharyngeal washes are diagnostically superior. In
suspected chronic carriers, nasopharyngeal swabs are fundamentally
compromised by the fact that they do not sample from the infected site (the
guttural pouches). Numerous instances are seen where repeated negative
nasopharyngeal swabs are collected from horses with guttural pouch infections.
Therefore, although successful isolation of Strep equi from nasopharyngeal swabs
is very meaningful, negative nasopharyngeal swabs, even when repeated several
times, do not rule out the possibility of Strep equi infection and should not be
relied upon to establish freedom from infection.
- SEROLOGIC TESTING
As with all serologic tests, strangles serology simply indicates exposure to Strep
equi at some time in the past. Although strangles serology was once promoted as
a screening test for strangles carriers, several studies have now established that
carrier status cannot be implied from serology. When Strep equi becomes
sequestrated within the equine guttural pouches, immunity wanes at the same
rate as in horses that have fully cleared the infection. In fact, in one large study
the vast majority of established strangles carriers were seronegative.
Thus, the only value of serologic testing for strangles is when looking for a rising
titre in paired samples which is indicative of recent exposure.