23/02/2022
Matches my clinical experience exactly. Getting fed up trying to tell vetting clients that they are wasting their money to gain a false sense of security by taking strangles bloods in lieu of basic biosecurity!
Premovement strangles testing dilemma
Summary- use of an (ELISA/ antibody) blood test is no longer recommended for screening asymptomatic horses for strangles before moving to a new yard.
We have just seen another case where a horse had a negative strangles blood test before moving to our client and then, just after moving, developed a nasal discharge and tested positive for strangles- a total nightmare for the new owner and yard.
One small benefit of the pandemic is that everyone now understands a bit more about testing for infectious diseases, particularly that no test is perfect. The strangles blood test was recently assessed for its ability to detect chronic carriers (horses that have had the disease that remain infectious, but do not show clinical signs) and could only detect 1 in 3 of these. The fact that antibodies do not remain high in chronic carriers may be part of the mechanism why these horses become carriers- antibodies are one of the body’s immune defences to fight off infections that have been met before, and failure to maintain them long enough may prevent horses clearing the infection from their guttural pouches.
If a negative test isn’t very helpful, does a positive result for a horse without signs or recent history of exposure tell us something? Unfortunately, being positive on the antibody blood test is extremely common in the general horse population. The vast majority of these horses, when the “Gold standard” guttural pouch wash is performed, are found not to be infected. These horses may have been exposed many years before and their body has the kept antibodies ready to help fight a new infection if they meet it again.
So, if a negative result does not tell us much, and can give a false sense of security, and a positive result is very commonly a false positive, is there any circumstance we should use the test? And, if we can’t rely on a blood test, how do we prevent carriers causing strangles outbreaks on our yards?
Horses that have been recently infected are likely to have a high and, particularly, increasingly high level of antibodies (if two samples are done). So, if we have a group of horses on a yard where a strangles outbreak has occurred, the test can be used after the end of the outbreak to help to identify horses that have been exposed (even horses that will become carriers usually will have a high value early on) for further testing by guttural pouch washes.
But that doesn’t help us with trying to identify infected horses before moving to a new yard. One option would be to guttural pouch wash every horse before moving but this is expensive and, more importantly, invasive, and so we cannot recommend it routinely. Failing development of a better test, isolation for a minimum of 14 days and ideally 21 (or even 28 days) is the best we can do. Isolation needs to be in a separate air space from other horses and consideration not to transmit disease with objects such as tack and buckets and hands/clothing needs to be made. Taking daily temperatures and looking for any signs of disease are key.
Reference
Durham AE, Kemp-Symonds J. Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers. Equine Vet J. 2021 Jan;53(1):38-43. doi: 10.1111/evj.13276. Epub 2020 May 20. PMID: 32374892.