Serological screening cannot be relied on to prove a horse is not a strangles carrier, a recent report warns.
Asymptomatic carriers are probably the main way that strangles gains access to a previously unaffected yard. Most horses throw off the infection within a few months of an outbreak. But some remain infectious. They carry and excrete the bacteria without showing signs of disease.
These carriers usually retain the infection in the guttural pouches. With time, pus may solidify to form masses known as “chondroids”. These can harbour the bacteria and excrete them intermittently in the nasal secretions - acting as a source of infection for other horses.
The best way of identifying strangles carriers is by culturing a series of swabs from the nasopharynx, or by examining a guttural pouch wash for Streptococcus equi DNA (a PCR test). This can be time consuming and costly.
Blood samples are easier to obtain, and serological testing for Strep equi antibodies has become popular in clinical practice to screen for strangles carriers.
However, recent research suggests that such results should be interpreted with caution and cannot be relied on to be certain a horse is not a strangles carrier.
In a retrospective clinical study, Andy Durham and Jeremy Kemp-Symonds, compared results obtained by serological testing for antibodies against S equi antigens A and C, with the findings of microbiological sampling of the guttural pouches.
The study included 287 horses, that were examined as part of routine surveillance and quarantine procedures on arrival at a welfare charity quarantine unit. Guttural pouch washes were submitted for culture and PCR testing and blood samples were tested for the Strangles A/C ELISA.
(The Strangles A/C ELISA test gives results in terms of optical density (OD). An OD ≥0.5 to either antigen A or C is considered “positive”; OD≤0.2 “negative”; and OD 0.3-0.4 “borderline”.)
The investigators found no significant association between serological status and the presence of S equi in the guttural pouches.
S equi was found in the guttural pouches of nine horses (3.1%). Of those, only one (11%) was seropositive using a cut-off of OD ≥0.5. A further two horses fell in the borderline category, leaving six horses with negative results.
So, in this study, over half of the horses with S equi in the guttural pouches, were negative on the Strangles A/C ELISA test.
Durham and Kemp-Symonds conclude: “It would appear that the only currently reliable means of determining S. equi carrier status is microbiological sampling rather than serological screening.
For more details, see:
Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers.
Durham AE, Kemp Symonds J.
Equine Vet J. (2020)