Strangles testing and prevention

Strangles is a respiratory infection caused by the bacterium Streptococcus equi equi. It can cause a range of clinical signs, from none through to serious disease. Most horses recover fully and fatalities are rare, but it can be very unpleasant for severely affected horses, and can lead to financial loss and significant disruption for owners. 

The course of the disease can be complicated, not least because a small proportion of horses affected with strangles go on to become long-term asymptomatic carriers of the disease. This is because abscesses in the back of the throat can discharge into the guttural pouches - air-filled sacs above the throat - where pus can gradually solidify, developing into firm balls called chondroids, which can persist indefinitely. These carrier horses can infect other horses without showing signs of disease themselves.

To prevent this, yards have developed protocols designed to minimise the risk of horses arriving while infected with the strangles bacteria. These protocols are based primarily around isolation and blood sampling for the antibody of the bacteria that causes strangles. 

While these protocols prevent some infected horses entering yards, a recent study has shown that blood samples are less effective than was originally suggested for detecting long term carriers. In the largest study conducted on this issue, the majority of carriers went undetected. However, blood testing is known to be very effective at detecting whether horses have been exposed to the strangles bacteria in the previous few months, so can still play an important role in disease control.

Recent research conclusions

• Horses exposed to the bacteria recently, that is from 10 days to a few months ago, probably WILL test positive for antibodies in their blood. This is regardless of whether they have exhibited clinical disease, and whether they successfully cleared the disease or became carriers
• Horses exposed to the bacteria more than a few months ago probably WILL NOT test positive on the antibody blood test. This is regardless of whether they became carriers or not
• The only reliable way to detect a long-term carrier is to perform a guttural pouch wash, by passing an endoscope up the nasal passages into the guttural pouches and sampling fluid, to test for the presence of bacterial DNA using a PCR test. 

Strangles prevention strategy

Every yard, whether private, professional or livery, must consider its own individual situation and decide what biosecurity measures best fit its needs. Yard managers may wish to consider the potential for effective isolation, the cost of testing procedures, the risk profile of horses entering the yard and the consequences of a strangles outbreak. 

We would suggest that all yard managers:

• Implement a strict isolation protocol for 14 days minimum for all new entrants to yard
• Consider the risk profile of horses entering the yard. Horses most likely to pose a risk are those from an unknown background, especially if they have travelled with or were stabled with other horses from unknown backgrounds. This is even more important if horses have arrived from outside the UK
• Choose the tests which strike the right balance between disease prevention, practicality and cost for their own yard:

If using guttural pouch scoping, decide whether to scope all horses or only the higher-risk horses

If using blood sampling, bear in mind that this will probably only detect horses which were exposed to the strangles bacteria between 10 days and a few months ago. It will then be necessary to scope the horses who test positive to see if they pose an infection risk to other horses

If you do not test, recognise that there is a risk of unknowingly admitting an infectious horse, even if it is showing no clinical signs of disease. 

Test costs

• Blood sampling approximately £70-£80 (excluding visit fees)
• Guttural pouch wash approximately £300-320 (excluding visit fees).

For information on how to interpret blood sample tests, follow the link below.

Vaccination

A new vaccine against Stretococcus equi equi was launched in 2022. This vaccine, called Strangvac, is unrelated to the previous strangles vaccines and can reduce the severity of disease and reduce the spread of strangles on a yard. Please have a look at our information sheet below on this for more details.

Questions?

Strangles prevention is a complicated issue, and our vets are happy to discuss individual situations with our clients. Please contact us by email on EQH@ed.ac.uk and one of the practice vets will call you back at a mutually convenient time. 

Useful links

Strangles vaccine

Interpreting strangles test results