11 November 2019

Blood testing for strangles

Strangles is the most commonly diagnosed infectious disease in horses worldwide. But, in the UK, we have a blood test that helps identify horses that may be sick or carrying the disease and could infect other horses.

When and whether to take a blood test can cause some confusion for horse owners and yard managers so we have asked our Senior Veterinary Surgeon and strangles expert, Nic de Brauwere, to help.

See our short film below to hear some frequently asked questions about blood tests answered by Nic.

In answer to the question “How long does the blood test last for?” this is his response. We hope this will help horse owners and yard managers better understand the limitations of relying on results of tests taken in the past when trying to clarify an individual horse’s disease status.

How long does a strangles blood test last for?

It tells you a horse’s immune status as measured by its strangles antibodies up until the date the blood is sampled, unless the horse is held in isolation from that time onwards in which case it is valid for as long as the quarantine is maintained.

As mentioned in our film, the level of antibodies varies in any one horse due to a number of factors, including the immune health of the horse and the time since it was exposed to strangles and the amount of strangles they were exposed to. Current knowledge suggests that horses can have high strangles antibodies in their blood up to eight months after having strangles. The test can also identify carriers, which is the main reason why screening is done.

To help identify recently infected horses that have not yet shown symptoms - and might never show obvious signs of strangles - sampling should be carried out at least 10 days after a horse goes into quarantine because this allows time for the immune system to react. If the horse is not isolated, then there is a chance the horse could contract strangles after being tested but before being moved.

Once a horse has been tested, the blood result is likely to be useful information for a period of months, even up to a year, if the following criteria are met:

Chances of exposure to strangles is low, for example, the horse stays in a stable herd with minimal movement of animals on and off site, especially if not attending events where it meets other untested horses.
No signs of disease have been detected since the test. Be careful here, though, because not seeing full blown strangles symptoms does not guarantee that your horse hasn’t been exposed to and contracted strangles. With the number of mild cases now recognised through better testing, it is possible for a horse to contract strangles, display few symptoms and yet become a carrier that can infect other horses. So even with reasonable monitoring it is possible to miss this situation. Daily temperature checks could mitigate this to a significant degree.
If the horse carries out a normal amount of mixing through attending shows, but biosecurity practised by the owner, at the event and at the home yard are good, for example, through testing new arrivals to a yard or segregating resident areas from event/show areas on a yard, then health monitoring should mean that further blood tests are not needed. The PASS scheme recognises that every yard operates differently and so they have three membership levels that are awarded according to the frequency of blood testing offered. Annual testing on yards with high traffic is advisable, as well as extra tests following unusual situations, for example, unknown horses straying onto your yard.
Making sure that anytime a horse moves yards, a test is taken as the stress involved can compromise a horse’s immunity. The horse will also meet a new herd of horses and enter into an environment with an unknown strangles history. If the new yard requires screening, then the new horse should be tested as a matter of course. The more often a horse moves, the higher its stress levels and the greater the risk of meeting a horse with strangles, or a carrier, along the way. This means that an older blood test is not valid when a horse moves onto a new yard, though previous negative results can be very useful additional information for the new yard to know about. It can, for example, show that the owner has been consistently careful about strangles.

Remember, if in doubt speak to your vet. They can help you find the best strangles screening option for you, whether that is when moving yards or as part of a pre-purchase examination.

You can find more information about strangles prevention for yards and horse owners by clicking here.

If you’re a yard manager and you already require screening for strangles at your business, please consider joining our campaign by taking the Stamp Out Strangles Pledge. Pledgers are offered support in ensuring your liveries comply with hygiene standards and yard agreements, and helps us to promote better biosecurity as the new norm that other yards should aspire to.

To read more about strangles, here’s some helpful links below:

What is Strangles? A helpful introduction to the disease

Premium Assured Strangles Scheme (PASS) - https://www.sruc.ac.uk/info/120473/premium_assured_strangles_scheme

Surveillance of Equine Strangles project - https://www.aht.org.uk/disease-surveillance/surveillance-equine-strangles