Strangles blood test
For the last four years, scientists at the Animal Health Trust, in Newmarket, have been analysing the genetic structure of Streptococcus equi, the bacterium which causes equine strangles. Their aim was to identify the antigens that are produced specifically by this bacterium.
With funding from The Horse Trust, two antigens that are unique to S. equi, and which are targeted by the equine immune system following exposure to strangles, have been identified. On the basis of these two specific antigens, AHT bacteriologists have developed a new diagnostic test which identifies horses that have recently been exposed to strangles with a sensitivity of 91.5% and a specificity of 90.2%. Brigadier Paul Jepson, Chief Executive of The Horse Trust, said “We are delighted that our funding for this project has been so fruitful. This represents major progress in our battle to reduce the suffering caused by this common and debilitating disease. It will be particularly valuable to organisations which have a transient population of horses, with regular new arrivals for which medical histories are not always available”.
The test is an ideal method of distinguishing those horses which have been exposed to S. equi during a strangles outbreak or to screen horses prior to movement, competition or sales. The use of this new blood test, which will be available from 1st March 2008, should lead to a significant reduction in the number of new strangles outbreaks.
As with all serological tests of this type, horses will take approximately 2 weeks after first exposure to give a positive result. However, a negative result from a sample taken more than 2 weeks after potential exposure does indicate, with 95% confidence, that the horse concerned is unlikely to have been exposed in the recent past.
If the test result is positive for either or both antigens, the horse is likely to have been in contact with strangles in the recent past. This does not necessarily mean that it is currently infected with Strangles and could indicate any of the following:
- Exposure to strangles and incubation of the disease
- Acute phase strangles, in which case the horse may show clinical signs such as nasal discharge or abscessation of the lymph nodes of the head or neck
- Infection with strangles in the previous 6 months, with or without clinical signs, followed by full recovery
- Infection with strangles in the past, with or without clinical signs, resulting in immunity to the disease in the face of recent exposure
- Past infection with strangles which has resulted in the horse becoming a carrier
This new diagnostic test will identify carriers with 90.9% sensitivity and 82.6% specificity. Once identified, carriers can be treated using guttural pouch endoscopy.
The test requires a tube of clotted blood (red top) and will cost £25 per sample. Please click here for full details of all diagnostic services available at the Animal Health Trust.
Please click here for advice on the appropriate use of the strangles blood test
Further information on the strangles research program at the Animal Health Trust can be found here.
For details of the ‘Breaking the strangles hold’ campaign and how you can make a difference please click here.
Receiver operating characteristic curve for antigen A
(the perfect assay has an area under the ROC curve of 1.0)
Receiver operating characteristic curve for antigen B
(the perfect assay has an area under the ROC curve of 1.0)
Summary of sensitivity, specificity, negative predictive value (NPV) and positive predictive value (PPV) estimates for the strangles blood test and exposure/carrier outcomes
Notes:
- Antigen A threshold for positivity set at 0.5
- Antigen B threshold for positivity set at 1.0
- Sensitivity = % of true positives that test positive (100-sensitivity = % false negative)
- Specificity = % of true negatives that test negative (100-specificity = % false positive)
- NPV = predictive value of a negative test = % of test negatives that are truly negative
- PV = predictive value of a positive test = % of test positives that are truly positive
Please click here to view our press release 'Major Breakthrough In Battle Against Strangles'.
