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Accuracy of diagnostic tests for detection of strangles carriers

Horse Respiratory SystemStrangles, infection of the upper respiratory tract with Strep equi var. equi, is common in the equine population. A subclinical ‘carrier’ status exists in which residual bacteria persist, usually in the guttural pouch, for an extended period (up to several months or even years) with the absence of any clinical signs. Such horses are recognized as pivotal in the spread of the disease acting as potential sources of infection for naive animals and ensuring persistence of infection between outbreaks. Detection of these carrier animals is therefore desirable and testing for strangles is a common prerequisite before moving to new equine premises. Identification of carrier status can be performed by identification of Strep equi via culture and/or PCR (detection of DNA) (ideally from the guttural pouch) or measurement of antibodies in the blood. Antibody testing (serology) is often used as a ‘screening test’ to detect horses that should have guttural pouch endoscopy. A comparison of the diagnostic accuracy of serology and PCR at detection of Strep equi has recently been performed1.

Retrospective data from 287 horses arriving at a welfare charity quarantine unit that underwent microbiological sampling of guttural pouches and serological testing were reviewed. In total, nine (3.1%) horses were found to be guttural pouch carriers on microbiology and 35 (12.2%) horses were positive on serology. Of the 35 horses positive on serology, one (2.9%) had evidence of S. equi in a guttural pouch lavage. Of the 252 horses with negative serology, eight (3.2%) had evidence of S. equi in guttural pouch samples. One (11%) of the nine horses found to be guttural pouch carriers was positive on serology at the standard cut‐off of ≥0.5, but lowering the cut‐off to ≥0.3 identified two additional guttural pouch carriers. There was no association between serological (antibody) status and guttural pouch carriage of S. equi. Therefore, negative serology to Strep equi should not be relied upon when determining if a horse is a chronic carrier.

To explain these results, it is useful to consider several points. Early in disease, positive bacterial identification may be made prior to antibody production (i.e. PCR positive/serology negative). Secondly, the duration of antibody response will in most cases extend beyond the period in which the bacteria is present (i.e. PCR negative/serology positive). Strep equi may also be able to evade the immune response (therefore test negative for antibodies) if deep in solid material such as chondroids in the guttural pouch.

1. Durham, AE, Kemp-Symonds, J. Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers. Equine Vet J. 2020; 00: 1– 6. Available HERE

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