Strangles is a highly contagious, infectious disease of the upper respiratory tract and can be serious or rarely even fatal as a result of late diagnosis or complications. Know what to look for and you will have a much better chance of avoiding the unnecessary suffering of your horse and others. Rarely, complications can occur such as bastard (metastatic) strangles, where abscesses can form elsewhere in the body, and purpura haemorrhagica, where the body’s immune system starts attacking the body tissues instead of the infection.
The disease is spread either directly from horse to horse or indirectly via fomites (inanimate objects). Direct infection occurs by bacteria being shed in the discharge from the nose or abscess of an infected horse coming into contact with a non-infected horse. Importantly, disease can also come from horses showing no signs of disease (shedders or carriers). Indirect infection occurs when the bacteria being shed from the horse’s nose or discharge from an abscess is carried to other horses by any object, such as people’s hands/clothing, tack, grooming and yard equipment (fomites). The incubation period is up to 14 days, so there can be up to 21 days between cases showing signs of infection as the abscesses can take up to a week to appear. All horses are susceptible, and even horses that have had strangles previously may not have sufficient immunity to prevent having it again. Recovering hoses can shed the bacteria for 6 weeks. About 10% of horses recovering from the disease will carry the bacteria within their head (in the guttural pouches) and will be healthy horses, but intermittently be a source of bacteria for months to years that can infect other horses. Identifying these carrier horses is key to stopping the outbreak re-occurring on your yard and preventing spread from yard to yard.
How do I know if my horse has strangles?
Fever (above 38.5℃) accompanied with depression and loss of appetite.
Profuse yellow or green nasal discharge from both nostrils (thick mucus).
Swellings on the side of the head and throat which may burst and discharge pus. These abscesses are usually hot and painful and occur some days after the initial fever.
Sore throat – difficulty eating, extending the head.
Atypical strangles (becoming more common, especially in healthy adult horses)
Mild short term fever – may loose appetite for a brief period of time.
Mild clear nasal discharge.
No abscess formation.
These horses are a risk to other horses and should be treated as infectious. However; these clinical signs can be seen with a wide variety of diseases and not just strangles. Ask your veterinary surgeon for advice if you have any concerns.
Isolating affected horses and seeking immediate veterinary attention is essential to stop the spread of the disease.
Diagnosis of an individual horse is based on identifying the bacteria cultured (or a PCR test for bacterial DNA) in a lab from throat swabs or aspirates/swabs from abscesses. However, not all strangles cases will have a positive result even if the horse is infected and spreading that infection to others.
A new blood test (serology) is available that detects antibodies to the bacteria. These antibodies take up to two weeks to form and stay elevated for up to six months after infection, so this test is best for diagnosing exposure, not active infection. A guttural pouch wash using an endoscope is the best way to ascertain if a horse has the disease.
Strict hygiene and disinfection protocols are essential. Creating an isolation area which has a foot dip and specific coveralls for people moving from isolation to the rest of the yard or a different yard is important.
If the diagnosis is positive, either based on clinical signs, culture, PCR or blood test then your vet will discuss treatment options with you, although many horses with a milder case of strangles may not need any treatment other than rest.
Most cases just need intensive nursing care including rest and a dry environment.
Feeding soft palatable feeds is important as the swollen lymph nodes can make it painful to eat, and feverish horses may not have their normal appetite.
Abscesses can be hot-packed to encourage them to burst.
Use of antibiotics in strangles cases remains controversial and should be judged on an individual patient basis by the attending vet.
Recovering horses can and often do shed bacteria for several weeks after the stop of clinical signs.
Tests and Prevention:
Culture and PCR of three throat swabs (nasopharyngeal) 5-7 days apart – any horses returning positive results need guttural pouch lavage via endoscopy.
Culture and PCR of one guttural pouch lavage via endoscopy.
Serology blood test – any horses with positive results need guttural pouch lavage via endoscopy.
Preventing strangles entering a yard involves isolating all new horses for at least three weeks, including horses that have been off the yard and in contact with other horses for any reason, including competitions. Consider performing serology blood tests on all new horses moving on to a yard or keeping a dedicated quarantine section. MSD Animal Health do produce Equilis StrepE which is a licensed vaccine for strangles. However, this vaccine requires two doses placed in the horse’s lip 4 weeks apart for initial immunity, and then a booster every 3 months to maintain immunity. If the booster schedule is not maintained, then the initial two doses must be repeated. Two caveats for this vaccine: the first is that it will not eliminate strangles from a carrier animal or one which has already been exposed to strangles and has not yet become ill. The second is that it is a modified live vaccine, which means that it can very rarely infect other horses with the vaccine strain of strangles. This risk is very low, but it means that vaccinated horses should not come into contact with unvaccinated horses until after the lump that the vaccine produces subsides. Finally, it is currently impossible to tell by a blood test whether a horse has been exposed to strangles or just been vaccinated for strangles, the only way to determine whether the horse is potentially a carrier is to perform a guttural pouch wash and test for the strangles bacteria DNA. We currently recommend a combination approach: keep all new horses in isolation and away from other horses on the yard, and perform paired serology blood tests (one at coming onto the yard, one on leaving isolation) or a guttural pouch wash to confirm that the new horse will not be bringing infection onto the yard. Of course, for this to be of maximum benefit, it is helpful to first be sure there is no strangles carrier on the yard! This is also best done with paired serology tests, but the plan will differ for different yards in different situations, so please phone your Parkside team for the best individualised advice.
Recently Parkside Equine dealt with an outbreak of strangles on a yard in this area after a horse was introduced onto the yard. This horse was a carrier and spread the disease to lots of other horses, some showed clinical signs and others did not. One horse developed purpura haemorrhagic and was referred to the Edinburgh Dick Vet Large Animal Hospital for intensive nursing and treatment and fortunately survived. A testing process was put in place to identify affected horses, until all horses were clear except for the carrier horse. We investigated this horse with our endoscope and found it to have chondroids (lumps of pus) in its guttural pouch which were removed surgically at the Dick Vet.
The yard is now happily free of the disease after all their hard work and perseverance.
Another yard put in place a testing and quarantine regime to ensure they were free of strangles, as they had no symptoms of strangles and no recent arrivals, before bringing some new horses onto the yard. Out of 91 horses and ponies, four tested positive for antibodies to strangles (in spite of not having been off the yard in a couple of years, they had been exposed to new arrivals occasionally, though not recently) and three of those were found to be negative on the guttural pouch wash and DNA test. The fourth was found to have chondroids in one guttural pouch with active strangles bacteria, which were removed via the endoscope at Parkside Vet Group before antibiotic treatment, and that horse has subsequently tested negative on guttural pouch washes for strangles DNA. The one horse carrying chondroids was not actively shedding infection at the time, but if they had become stressed or their immune system compromised, they might have shed strangles bacteria to the entire yard and caused major issues.