What is strangles?
Strangles is an infection of the upper respiratory tract with a bacterium called Streptococcus equi equi.
Strangles is diagnosed either directly by detection of S. equi itself or indirectly by detection of rising levels of antibody against S. equi in blood samples.
Direct detection of S. equi is either by laboratory isolation or by qPCR detection of its DNA from nasopharyngeal swabs, abscess contents and/or guttural pouch washes.
Shedding of S. equi into the nasopharynx often occurs intermittently, so repeated swabbing is recommended to confirm negative results or alternatively a single guttural pouch examination.
Horses entering new premises should be quarantined for 3-4 weeks in case they are incubating the disease. With strangles, infected horses may or may not have clinical signs or they may be subclinical carriers. All new arrivals should be examined for signs of illness (high temperatures, dullness, not eating, nasal discharge, swollen or abscessed lymph glands around the head or neck). Any horses with such signs should be immediately isolated and veterinary advice sought.
Routine use of the strangles ELISA blood test during isolation can identify previously infected and potentially infectious horses quickly. Ideally samples should be taken at the beginning of isolation and after three weeks of isolation to check for rising antibody levels (seroconversion) indicating an immune response after exposure to S. equi. If any of the quarantined horses are ELISA blood test positive on either the first or second test then further guttural pouch endoscopy and sampling with screening for S.equi by qPCR is required.
The carrier state (continued presence of S.equi in the absence of clinical signs) may be diagnosed or excluded by sequential nasopharyngeal swabs (minimum of 3) or, preferably, endoscopic examination (‘Scoping’) of the guttural pouches and submission of guttural pouch washes for testing by qPCR.
A series of three nasopharyngeal swabs, usually collected one week apart, will result in detection, by positive qPCR, on at least one of the swabs in >90% of carrier horses. As the sensitivity of S. equi detection for identifying guttural pouch carriers on three nasopharyngeal swabs is broadly equivalent to testing bilateral guttural pouch samples, the latter approach is the recommended sampling protocol for determining infectious status in seropositive, asymptomatic horses
Although carriers only shed S. equi intermittently, over 90% of carriers maintain specific antibodies in their blood. These antibodies can be detected by a blood ELISA test, which may provide a useful tool to help identify some, but not all, carrier animals. Recent investigations of the blood ELISA in detecting chronic carriers have highlighted that negative blood ELISA results, either as single or paired samples, do not guarantee absence of a carrier state. It is therefore preferable for all potential carriers irrespective of their serological status, especially those of high or unknown risk status, to be examined by gutteral pouch endoscopy and sampling with screening for S.equi by qPCR.
A small but important proportion of horses that have recovered from strangles become persistently infected (most commonly in their guttural pouches) with S. equi for months or even years. These ‘carriers’ may have no obvious clinical signs of disease but can intermittently shed S. equi, which can then infect naive horses. These subclinical carriers are probably the most important factor in persistence of infection on premises between outbreaks and can initiate new outbreaks following their inadvertent movement to new premises making guttural pouch examination prior to movement all the more important.
Info from the HBLB international codes of practice 2022
British Horse Society Strangles information leaflet