Recent research underscores the difficulty in identifying “silent carriers” of strangles—horses who can continue to infect herd mates months after an outbreak seems to have subsided.

Caused by Streptococcus equi bacteria, strangles is characterized by a high fever, depression, appetite loss and enlarged lymph nodes between the jaw bones. Thick, yellow pus may drain from the infected horse’s nostrils, and abscessed nodes in the throat may burst and drain. In severe cases, large abscesses in the head and neck obstruct the airways causing the horse to struggle to breathe, hence the disease’s descriptive name.

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Strangles can spread rapidly through a herd as S. equi bacteria are shed in mucus and airborne droplets. Isolating sick horses is a key part of controlling an outbreak, but some infected horses do not show signs of illness, making them difficult to identify. These horses, known as “silent carriers,” can continue to introduce infection to susceptible members of the herd for months or even years.

Looking for a way to detect silent carriers, researchers at the Swedish University of Agricultural Sciences in Uppsala, the Equine Veterinary Clinic in Destedt, Germany, and the Animal Health Trust in Suffolk, England joined forces to study more than 100 horses involved in three different strangles outbreaks.

Six to 10 months after each outbreak began, the researchers examined all the horses on the affected farms, looking for fever, swollen lymph nodes, nasal discharge or other residual effects of strangles. The researchers also performed nasopharyngeal lavages as well as guttural pouch endoscopy and lavage on the horses, and used polymerase chain reaction (PCR) to test the fluids for the presence of S. equi. Finally, researchers collected blood samples and analyzed them for signs of inflammation and antibodies signalling exposure to S. equi.

The PCR test results showed that 15 of 97 horses in the first outbreak were harboring S. equi six months later. From the second outbreak, 14 of 38 horses were carriers 10 months later. From the third outbreak, four of seven horses carried S. equi bacteria nine months later. However the researchers found no significant association between clinical signs and the carrier status of the horse—meaning those still potentially shedding S. equi appeared healthy. Likewise, no significant differences in the number of antibodies and inflammation markers were found in blood samples from carrier versus noncarrier horses.

The researchers note that in the face of recurring outbreaks, identification of silent carriers would require that all horses in a herd undergo nasopharyngeal lavage or guttural pouch lavage and subsequent PCR testing for the presence of S. equi.

Reference: “Markers of long term silent carriers of Streptococcus equi ssp. equi in horses,” Journal of Veterinary Internal Medicine, October 2020

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