Tower Equine

Veterinary Surgeons

 

Strangles - January 2013

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About Strangles

Strangles is a bacterial infection that typically causes absesses in the glands around a horse's throat and under the jaw. The signs of a strangles infection can range from lethargy, loss of appetite and a high temperature in the early stages, to swollen glands, burst absesses under the skin and a thick, purulent nasal discharge. Illness can be severe, particularly in foals and youngsters, and in rare cases it is fatal. It is also possible however for infection to be mild with no signs other than a "snotty" nose or a single swollen gland. This means that any similar symptoms should always be checked out, just to be sure that they are not caused by strangles.

Non-Clinical Shedders

Although most horses with strangles are obviously ill, that isn't always the case. Unfortunately there are a small number of horses out there that are infected but show no signs of it at all. They are called silent shedders, or non-clinical shedders. These are usually horses that have had strangles and appeared to get over it, but in fact they are still harbouring some of the bacteria at the back of their throat or in the gutteral pouches, special air-filled sacks that open at the back of the mouth. These horses are still a risk to others because although they are not as infective as horses with full blown strangles, they can still shed live bacteria and potentially infect other horses.

How do horses catch Strangles?

Strangles is usually spread by close "nose-to-nose" contact between horses. The nasal discharge or puss from the absessĀ of a horse with full blown strangles is very infectious. If it gets in to water troughs or on feed buckets, or is spread on hands or tack, it can cause infection.

Strangles is not an airborn infection however - it is not breathed out in to the air in the same way that a cold or flu virus is. This means that unless a horse comes into physical contact with an infected horse or with infective discharge, it will not get strangles. Simply sharing an airspace - being in a different stable in the same barn as an infected horse or being in the same collecting ring for example, does not mean that your horse will catch the disease. This means that an outbreak of strangles is easier to control or contain than a viral disease.

Unfortunately silent shedders complicate the problem because they appear normal and so are allowed to mix closely with other horses, causing further spread of strangles.

How is Strangles diagnosed?

The best way to diagnose an ongoing strangles infection is by nasopharyngeal swab - a swab passed up the nose to the back of the throat. This swab is then sent to the laboratory for bacterial culture. There is a blood test available which is helpful in identifying horses which have been exposed to strangles because it measures their antibody levels to the bacteria. However a higher than normal antibody level does not necessarily mean the horse has strangles because it could have successfully fought off the infection. Repeat blood tests or swabs may be necessary to prove if the horse is clear or infected.

Silent shedders are the hardest to diagnose because they don't show any outward signs of infection. They will usually have high antibody levels on a blood test, but a single nasopharyngeal swab will not always pick up the bacteria. This is because the strangles bacteria hides away in the gutteral pouches - the only sure way of diagnosis is to take a gutteral pouch wash, a fiddly process that requires the vet to pass an endoscope up the horse's nose and into each of the two gutteral pouches.

How do we control a Strangles outbreak?

If a case of strangles is diagnosed on a yard, it is best practice for no horses to enter or leave the yard until the outbreak has been extinguished. This is not always practically possible however and if horses do have to leave they should be isolated and tested at their new premises.

During an outbreak on a yard, all infected horses should be kept separately from those not showing signs of disease. The second group then needs to be tested over several weeks in case they are incubating the infection.

The fact that Strangles is not airborne means that with suitable isolation and good hygiene, including footbaths and separate clothing, it is often possible to limit the spread of the disease within a yard and to keep the number of clinical cases to a minimum. Each outbreak is different so vets will give advice on the most suitable control measures in each instance.

Vaccination

There is a vaccine available for strangles, but there are several drawbacks which limit its usefulness.

Firstly there is the fact that after an initial course of two injections, immunity to strangles lasts only about 3 months, so that a booster shot is needed a minimum of twice yearly and more after if there is an outbreak.

Secondly, the immunity provided by the vaccine is not complete - a fully vaccinated horse can still get strangles, in fact the level of protection could be less than 50%.

Finally, a reduction in the size and severity of a strangles outbreak at a yard is only likely if a significant number of horses on the yard - certainly more than half - are vaccinated. This means there is very little point in vaccinating your horse unless most other owners at the yard follow suit.

This is not to say that vaccination is useless, since it can help limit the scale and severity of a strangles outbreak, but for owners, the cost of the vaccination may be felt to outweigh the benefits.

Avoiding Strangles

How do I prevent my horse getting strangles?

The good news is that strangles is harder to catch than most owners realise. As we have seen, there is no airborn spread of the bacteria so you are very unlikely to catch it at a show for example or out hunting.

The bad news is that the existence of silent shedders means that in a livery yard situation, with shared paddocks, in a yard or with a high rate of horses coming and going, it is not possible to avoid all the risk.

Sensible precautions to take in order to minimize the risk include:-

- isolation of all new arrivals at the yard for a period of two weeks

- prompt isolation and swabbing of any horse that develops a nasal discharge

- avoiding close nose-to-nose contact with horses of unknown strangles risk eg shared travel in a trailer or adjacent stables with bars instead of solid partitions.

Roger Lee MA VetMB CertEM MRCVS