A recent European study found that blood tests (serology) alone did not effectively distinguish asymptomatic strangles carriers from non-carriers.
Symptomless carriers present the biggest risk of introducing strangles to a previously unaffected population.
Most horses throw off the infection, caused by Streptococcus equi, within a few months of an outbreak. But some remain infectious. These carriers usually retain the infection in the guttural pouches at the back of the throat. They carry and excrete the bacteria without showing any signs of disease.
The best way to identify carriers is by culturing a series of swabs from the nasopharynx, or by examining a guttural pouch wash for Streptococcus equi DNA (a qPCR test). Such methods can be time-consuming and costly.
Serological testing for S. equi antibodies has become popular in clinical practice to screen for strangles carriers. But a growing body of research suggests that this method cannot be relied on.
A study by John Pringle and colleagues followed three groups of horses for between six months and two years after strangles outbreaks. Their research has been published in the Journal of Veterinary Internal Medicine.
The horses were differing management regimens; a large breeding farm with recurrent outbreaks of strangles, a closed riding stable with mature horses only, and a boarding stable for convalescent horses of varying breeds and ages.
Carriers were defined as horses from which S. equi could be cultured, or S. equi DNA could be found on a qPCR (real-time quantitative polymerase chain reaction) test, in samples from the nasopharynx or guttural pouches.
The researchers compared the clinical appearance, guttural pouch endoscopy, and inflammatory markers between carriers and non-carriers and found no real difference between the two groups. Neither did serology distinguish carriers from non-carriers.
“Of particular concern however was that two of the culture-positive carriers 14 months after outbreak A, and the culture-positive mare associated with outbreak A, tested seronegative,” they report, “suggesting lack of persistence of seropositivity despite carriage of viable S. equi.”
They conclude: “Silent carriers of S. equi do not differ clinically or on markers of inflammation to their noncarrier herd‐mates. Moreover, serology alone will not distinguish carriers in comingled horses.”
The research was carried out by John Pringle and Miia Riihimäki, both of the Department of Clinical Sciences, at the Swedish University of Agricultural Sciences, Uppsala; Monica Venner and Lisa Tscheschlok from the Equine Veterinary Clinic, Destedt, Germany, and Andrew S Waller of the Animal Health Trust, Suffolk, UK.
Markers of long term silent carriers of Streptococcus equi ssp. equi in horses
John Pringle, Monica Venner, Lisa Tscheschlok, Andrew S. Waller, Miia Riihimäki.
Journal of Veterinary Internal Medicine. https://doi.org/10.1111/jvim.15939