|Strangles is an infectious disease caused by the bacterium Streptococcus equi (S.equi).
Approx 10% of horses exposed to the strangles bacteria can become ‘carriers’ of the disease and may not show any clinical signs but can act as a reservoir of infection to other horses
The blood test is frequently used pre-movement to assess the risk to the new livery yard of moving a horse to a new yard. Increasingly livery yards are also asking for it to be repeated following a period of isolation at the new yard.
The blood test only shows if the horse has been exposed to the S. equi bacteria within recent history, approximately 6 months. If a positive or borderline result is obtained further testing will be required to determine the horse’s current status.
The test looks at the immune system’s response to the disease in the form of antibodies present in the blood rather than the bacteria itself. These antibodies generally take 2 weeks to form and so if the horse has been exposed to the bacteria within the last 2 weeks the test could show a low antibody response despite having the active disease.
For best practice all horses arriving on a new yard should still undergo a period of quarantine
When interpreting the bloods results it is the higher of two antigen results that is most relevant. Few tests are 100% accurate so the following statements can only be declared in reasonable confidence.
0.0-0.2 – Negative – However a second sample taken 2 weeks after the first would be required to rule out exposure around the time of the first sample.
0.3-0.4 – Borderline Band (‘grey area’) – Require caution in interpretation as they may be consistent with recent exposure to S equi and a rising antibody level. Depending upon your circumstances very often we may recommend:
Repeat this blood sample in 2 weeks – a stable or falling titre gives reasonable confidence that this is a historic exposure to the infection. A rising titre suggests a recent exposure and potentially an active infection or carrier status both of which would require further investigation and isolation.
Perform an endoscopic guttural pouch lavage
0.5 or greater – Positive – highly suggestive of recent exposure to the bacteria and the potential for the horse to be a carrier. We would generally recommend endoscopic guttural lavage (see our factsheet on the subject on our website). Please discuss the result with your yard managers and veterinary surgeon prior to moving the horse.