09/07/2025
With so many yards now requesting pre-movement strangles testing, we thought we would outline some of the facts around this testing:
Strangles: Pre-Movement Testing
Up to 10% of horses infected with Strangles will go on to become Carrier animals; those who harbour the infective bacteria, Strep. Equi equi, in their Guttural Pouches. It is these horses who are the primary source of Strangles infection between yards as they appear outwardly healthy. As such testing all horses prior to moving yards is a valuable tool to try and limit the spread of Strangles.
There are a range of tests available for the detection of Strep. Equi equi bacteria in carrier animals, but each have their pro’s and con’s.
🩸Blood Samples
Blood samples are by far the easiest and least invasive method for detecting Strangles, but the test has limitations which must be understood when interpreting results.
Firstly, the blood sample does NOT look for the bacteria itself, instead it looks for the response generated by horse’s immune system to infection. After an infection the body ‘remembers’ the infection and maintains a supply of antibody in the blood ready for re-infection. It is this process which is the basis of vaccination for a wide range of diseases in horses and people, and as you’ll know over time this antibody supply decreases.
Unfortunately, the bacteria present in Carrier animals are highly adept at ‘hiding’ from the immune system, preventing re-stimulation and leading to decreasing antibody levels over time. Thus, carrier horses sampled several months/years after their infection may give false negative results.
To mitigate for this the Strangles blood test has been developed to have a very high specificity (increasing the likelihood of a positive result in an infected animal). This commonly leads to tests being less sensitive (increasing the likelihood of a negative horse showing a positive result, a false positive).
The test looks for horse antibodies against two antigens (specific proteins on the surface of the Strangles bacteria). These are called Antigen A and Antigen C. There are widely published reference ranges for each of these antigens meaning results can be easily interpreted. Unfortunately, different laboratories run slightly different tests so it is not possible to directly compare results obtained via different labs. This is important in cases of repeated or follow up tests where results MUST be comparable.
There are three possible results from a Strangles blood test;
• Positive - the tested horse shows a high level of antibody indicating recent infection or exposure to the Strep equi bacteria.
• Negative - the tested horse has a very low level of antibody indicating no recent exposure.
• Equivocal - the tested horse has a mid-range antibody level, potentially indicating decreasing antibody from an old infection, or increasing antibody level from a recent infection.
Positive and Negative results are fairly self-explanatory; the horse either shows evidence of infection and requires further investigation, or is negative at which point is deemed to be safe to move.
It is important to note that a negative result, whilst providing a degree of security that the horse is free from infection, could be a false negative result.
It is the equivocal results which cause much more confusion as it cannot be determined from a single blood sample which scenario (recent or very old exposure) is the cause. In cases of equivocal results owners have 2 choices:
• Wait 14 days and repeat the blood test. If antibody levels have not risen over this time then recent infection is highly unlikely (must not be in contact with other horses in this time between tests).
• Guttural Pouch sampling for Strep. Equi equi bacteria.
Despite its shortcomings the ELISA blood sample is still the mainstay of pre-movement testing due to its low cost and minimal invasiveness; but owners need to be aware of the potential drawbacks of this test.
💦Guttural Pouch Lavage Sampling
Here sterile saline is flushed into the Guttural Pouch and withdrawn into a sample pot.
The sample is then submitted for PCR DNA analysis (where the DNA of the strangles bacteria is detected) or culture (where bacteria is grown).
PCR
PCR is highly sensitive, making it a much more reliable test for detecting carrier animals, but it cannot distinguish between DNA from live bacteria and DNA from dead bacteria. This means there is a possibility of PCR giving false positive results, but a very low chance of false negative results.
🧫Culture Testing
Culture, where the samples submitted are placed on petri-dishes in incubators to try and grow bacteria, has largely been superseded by PCR DNA analysis. This is because culture is not very sensitive and can result is a large number of false negatives (where infected horses give negative results).
Fortunately, when performing Guttural Pouch lavage your vet will be able to see the inside of the pouch and may be able to observe pus or chondroids (dried balls of pus) present in the pouch (The absence of seeing these does not rule out infection).
🦠Nasopharyngeal Swabs
These are rarely performed for pre-movement testing as they are poorly sensitive for bacteria unless the horse is actively infected and shedding; in which cases the clinical signs of disease are normally quite obvious.
Here, a swab is passed up the nose into the back of the throat, where it is rotated several times to encourage swallowing.
False negative results are very common due to:
• Early onset disease and no bacterial shedding yet occurring
• Intermittent shedding from the Guttural Pouch
Once taken the swab tip can be submitted for PCR analysis or culture in the same way as the Guttural Pouch Lavage sample.
‼️Take Home Message
Strangles is an endemic disease, present in the UK at all times and future outbreaks are inevitable. Pre-movement testing is a very helpful, but not foolproof, means to reduce the risk of spreading Strangles between yards.
Each pre-movement test has its own list of advantages and shortcomings, and this should be carefully considered before deciding which test/tests to use in each individual case.
Your vet will be able to advise you of the most suitable test, and its subsequent results, and help you navigate this sometimes confusing topic.
Even with testing, having a good yard quarantine programme for new arrivals is VITAL; allowing time for internal parasite (worm) detection and treatment, and for any other viral/bacterial infections to present themselves. As such we recommend a minimum 14-21 day quarantine programme for all new arrivals prior to their introduction to the main herd.
