Equine Strangles carriers
Firstly, to understand about strangles carriers, we need to talk about guttural pouches. So what exactly are they?
The guttural pouches are a pair of air pockets found at the back of a horses throat, with each pouch having a lymph node closely associated with it. If abscesses form in these lymph nodes, they commonly burst into the guttural pouch as this is often the path of least resistance compared to breaking through the skin. These pouches connect to the nasal passages of the horse, hence why nasal discharge is seen in a lot of cases of strangles.
For a horse to be clear of infection, all the pus and associated bacteria must drain away. This doesn’t happen all the time, and these horses are known as carriers. They show no clinical signs but still shed the bacteria. Roughly 10% of horses that are infected with equine strangles will become carriers. Carriers can shed bacteria all the time but shedding increases during times of stress e.g travelling, change of management, injury or illness.
This video on the Redwings YouTube page explains how horses become a strangles carrier and what we can do about it.
Understanding the diagnostic tests
When a horse is infected with the equine strangles bacteria, it will produce an immune response. Antibodies specific to the strangles bacteria are produced to help the horse fight off the infection, and it is these antibodies that we detect in the blood test.
For a horse to have antibodies, they must have encountered the strangles bacteria at some stage. Antibodies take two weeks to be produced and can remain in the blood for approximately six months after infection.
With this information in mind, a single negative blood test does not mean your horse is clear of strangles, as it could be that it’s too early in the disease course for antibodies to be produced. This is why, if the first test is negative, another blood test is taken two weeks later (paired sample) to check whether antibodies have been produced at this stage. If antibodies have increased in this period, this is called seroconversion and is a positive result. Two negative tests two weeks apart make strangles very unlikely.
A positive result will need to be followed up by guttural pouch endoscopy as it could mean a few different things:
- The horse is currently infected with strangles
- The horse is a carrier of strangles
- The horse has had strangles in the past 6 months but is no longer infected
Guttural pouch endoscopy
Blood tests are not that good at detecting carrier status, which is why it is important to perform this test after a strangles infection to ensure a horse is not carrying the disease, even if it no longer has clinical signs. We usually wait at least 3-4 weeks after the last horse has shown symptoms.
A long tube with an inbuilt camera is passed into the horse’s nostril right to the back of the throat where the guttural pouches are. We visually inspect the guttural pouches for pus and chondroids (firm balls of pus) and take a sample. Chondroids can be removed using instruments and any liquid pus flushed out. Sometimes the pouches are treated with antibiotics. Once the horse has been treated, a sample will need to be taken again to ensure that no strangles bacteria remains in the pouch. Guttural pouch endoscopy is the most reliable test for ensuring a horse is strangles free.
Swabs can be used to take samples from ruptured abscesses or from the nasal passages, close to the entrance of the guttural pouches. This test is less reliable and at least three negative results at weekly intervals are needed to be considered free from infection.
You can read part 1 of the strangles series here