What is Strangles?
Strangles is one of the most common equine respiratory conditions in the world today, affecting all ages and types of equines. It is fatal in up to 1% of cases, although other types of complications are more common.
A highly contagious disease, strangles is caused by the bacterium Streptococcus equi equi, a strongly host-adapted species that only causes disease in equines. The disease has a variable incubation period of 3-14 days, meaning that it can take up to two weeks from initial exposure for the first symptoms to appear.
Affected animals will appear dull, depressed, lacking in appetite and have an increased temperature. After a few days the lymph nodes around the throat and on the bottom of the lower jaw may swell. In some animals, this can lead to respiratory distress, coughing and difficulty swallowing.
The most recognised clinical sign of strangles is a nasal discharge from one or both nostrils. This may initially be serous (pale yellowy fluid) before turning to pus.
Uninfected horses become infected with strangles via the respiratory tract through contact with the bacteria. Once in the respiratory tract the bacterium sets up an infection in local lymphoid tissue and potentially the guttural pouch (an air chamber just below the ear).
Contact with the bacteria can either be directly from another infected horse, or via an indirect route, i.e. clothes, tack, fence posts, gates, stable doors, shared water troughs etc. It is worth bearing in mind that the bacteria can survive in water for up to several weeks, therefore troughs etc. used by animals subsequently diagnosed with Strangles should be emptied, cleaned and disinfected.
All infected and suspect horses should be isolated immediately and biosecurity measures should be put in place to help prevent spread to other horses.
Whilst clinical signs are a good indicator of strangles infection, diagnosis should be confirmed via culture of bacteria obtained via a nasopharyngeal swab or via pus from the enlarged lymph nodes.
A series of three nasopharyngeal swabs, one week apart can be taken to determine if the bacterium is present in the nasopharynx (the area connecting the throat and the nose). However it is important to bear in mind that even in some cases with a large volume of purulent nasal discharge the swab still may return a false negative - i.e. return a negative result on a horse which actually does have the disease.
The Animal Health Trust has developed a blood test for strangles. This test, called a PCR (polymerase chain reaction) blood test, provides a rapid diagnosis and can identify:
- Horses that are currently affected
- Horses that have been infected with strangles in the last six months and recovered
- Horses that have been previously infected and become carriers.
This test is particularly useful when screening new horses joining a herd and for identifying carriers after an outbreak. However this test does have one important limitation as it requires the presence of S.equi antibodies, which can take up to two weeks to develop in affected horses. Consequently horses in the early stages of infection may not be identified by this test.
Therefore a balanced approach is often required by your veterinarian, potentially using both swabs and PCR blood testing to manage a potential strangles outbreak.
Unfortunately there is no single treatment plan that works for all horses. Most treatment regimes revolve around supportive care, good stable management and ensuring that you have good hygiene measures in place. On occasion antibiotics may be used; however this is case-dependent as antibiotics are not particularly successful at penetrating the abscess capsules that develop.
In some rare cases horses may develop a syndrome called ‘Bastard Strangles.’ Unfortunately in these horses, abscesses develop in other lymph nodes and organs around the body potentially leading to more serious clinical problems.
It is also important to remember that in 10% of infected cases horses will become carriers. These animals will appear healthy, but are in fact persistently infected due to abscess material in the guttural pouches. Over time these carriers may continue to spread and suffer from the disease, particularly during stressful events.