Strangles is a highly contagious, bacterial, respiratory infection caused by Streptococcus equi, which affects horses, ponies and donkeys of all ages. It is always present at a low level within the UK horse population but new cases arise every year especially when the weather becomes colder. A vaccination is now available in the UK to help control and prevent the disease.
The aim of this information sheet is to inform you of the clinical signs of strangles and how the bacteria spread between susceptible horses. It also contains information on how to control the infection and hopefully prevent it entering your yard.
Infected horses quickly develop a high temperature (up to 41°C) causing them to become depressed and go off their food. In the following days the lymph nodes around the throat enlarge due to abscesses forming with in them. These can result in respiratory obstruction and difficulty swallowing, hence the name strangles. Although the name sounds concerning, complete respiratory obstruction does not occur. The abscesses in the lymph nodes may rupture of their own accord or may need to be surgically opened by your vet. Infected horses often develop a nasal discharge which may start clear but becomes thick with pus and often copious in amount. They frequently also develop a cough. In very rare cases, the bacteria may affect other lymph nodes in the body and cause abscesses to form through out the horse’s body. This is caused bastard strangles and is usually fatal. Infected horses can develop varying severity of clinical signs and a nasal discharge without glandular swelling is sometimes all that is seen. Younger horses (under the age of 5 years) tend to develop more severe clinical signs.
The diagnosis is relatively straightforward in horses that develop the classical signs and is confirmed by taking a swab from the back of the horse’s nasal cavity (nasopharynx) or by directly swabbing a draining abscess. The more challenging cases are those which only develop a nasal discharge without the classic lymph node enlargement, as this can resemble other, less serious bacterial respiratory diseases. We recommend isolation of any horse that develops a thick nasal discharge and taking nasopharyngeal swabs for culture to check for the strangles bacteria. An unusual form of strangles is recognised which produces flu-like symptoms and often becomes persistent in a yard. Reaching an early diagnosis in these cases is very difficult as the clinical signs demonstrated are non-specific and consequently a large number of horses may be affected before a diagnosis of strangles is reached. Recently a new blood test has become available to identify if new horses entering a yard have been exposed to the infection in the past or to monitor long standing infections. We do not recommend use of the blood test as a diagnostic tool in acute outbreaks.
Once strangles has got into a stable yard it spreads quickly between horses either by direct contact or indirectly on equipment or personnel. The bacteria are shed in nasal discharge and in pus draining from open abscesses. The bacteria can survive in the environment for long periods (up to 8 weeks on tack and in the wood of stables) and can survive in water troughs for at least 4 weeks. Good hygiene is therefore essential in controlling the disease. The bacteria infect the lymph nodes of a horse via aspiration into their respiratory tract. Clinical signs develop with 3-14 days after infection.
Treatment of the individual patient is primarily supportive nursing care. Your vet will administer non-steroidal anti-inflammatory medication (such as Finadyne™ or Bute) to control their increased body temperature. Antibiotics are not always used as they cannot easily penetrate the abscess capsule and may slow down the horse’s recovery by preventing the abscesses from draining. The decision to give antibiotics depends on the stage of the infection and will be decided by the attending vet. Applying a hot compress to the enlarged lymph nodes may be advised to encourage the abscesses to rupture, and after they have burst flushing the cavity with antiseptic will be required until they have healed. It will take several weeks for them to totally heal up.
The most important aspect to limit the spread of disease is by maintaining strict hygiene and isolation of all infected horses. Early detection of disease by closely monitoring the body temperatures of in contact animals and immediately segregating any suspected cases will significantly reduce the number of horses that come down with clinical disease. The disinfectant, Virkon™ is effective at killing the bacteria and foot dips and the use of gloves when handling infected horses is paramount. Any equipment belonging to the horse should not be shared and should be thoroughly disinfected after use, including forks and wheelbarrows used to muck out. The premises must be isolated so that no in-contact animals leave the yard and risks spreading the disease further. No infected or in-contact animal should be released from isolation until three negative nasopharyngeal swabs have been taken over a two week period. Horses should not enter the premises unless they can be kept in strict isolation away from any possible source of infection.
All new horses entering the yard should be monitored closely and any horse that develops a nasal discharge must be isolated and swabbed immediately. If facilities are available, new horses should ideally be kept in strict isolation for two weeks. The people involved with caring for the horses in quarantine must not move from the isolated horses to others on the establishment. We do not recommend any strangles vaccines currently available. Our experience is that the possible benefit is outweighed by the incidence of reactions to the vaccine.
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