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Strangles
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Introduction
Strangles is a disease caused by the bacteria Streptococcus equi. The name comes from the fact that it enlarges the lymphnodes between the jawbone, causing the horse to make strangled breathing sounds. The bacteria only causes disease in equine species (horses, ponies, donkeys and mules). It’s not dangerous for other animals or humans, but strangles is often associated with facilities that experience a high turnover of horses (fairs, horse sales, competitive events, boarding stables). It is one of the most common equine respiratory infections in the world and it can affect horses of all ages and types. Young horses are particularly susceptible. Most animals recover from strangles with no long-term aftereffects.
Some horses are carriers of S.equi, meaning that they are infected with S.equi but do not show any signs of the disease. Carriers of S. equi can be a source of strangles for susceptible individuals.
Horses with strangles
The disease causes major economic losses to the equine industry worldwide due to its prolonged course, extended recovery period and associated serious complications. In Sweden, where strangles and equine flu are notifiable diseases, there are approximately four cases of strangles for every case of flu.
Strangles itself can kill (in simple cases there is a one percent mortality rate), but the major reason for its concern is the speed with which strangles spreads among horses, especially in a stable setting. In large horse populations, established outbreaks may last for months, essentially shutting down stabling premises.
Moreover, “recovered” horses (carriers) can harbour Streptococcus equi with no outward clinical signs. Consequently, new or recurrent outbreaks are likely unless costly diagnostic procedures and aggressive quarantine measures are used.
Transmission of strangles
Strangles is caused by oral exposure of a horse to S. equi bacteria. The bacterium enters the lymph glands via the respiratory tract and may harbour in the guttural pouch. Horse-to-horse contact (pus or nasal discharges from an infected horse is the easiest way) but strangles can also be spread through contaminated tack and equipment, shared drinking bowls and feed, clothing and hands. It is thought that the bacteria can survive in water for at least four weeks, and up to eight weeks on tack or wood.
Flies may also spread the bacteria from horse to horse. The bacteria can survive in the environment for weeks or months. Exposure of a horse to S. equi does not necessarily mean that it will come down with strangles.
Previously exposed horses are often immune to the disease, or do not get as sick as unexposed horses. During the first three to six months of life, foals are often protected by maternal antibodies. Vaccination can also increase resistance to the disease.
Stress (poor nutrition, overcrowding, lengthy transportation or pre-existing diseases) increase the risk of strangles.
Symptoms
• The incubation period (time from exposure to first signs of disease) is 3 to 14 days with abscesses formed up to 2 weeks following infection. The horse has fever (sometimes 41 degrees Celsius or 102 degrees Fahrenheit!), has decreased appetite and is depressed.
• There is a watery nasal discharge that quickly turns thick and yellow.
• The lymph nodes in the upper-respiratory tract become enlarged, the ones between the jawbones being the most noticeable. The inflammation is due to the accumulation of purulent fluid (pus) within the lymph node.
• Affected horses may stand with neck outstretched to relieve pressure in upper neck. Later in disease, lymph nodes often burst and drain pus from openings in overlying skin.
Complications
Complications of disease are seen in 10 to 20 percent of cases (these should be considered emergency situations requiring immediate veterinary care). Difficulty in breathing and signs of distress due to compression of trachea by enlarged lymph nodes in the neck. This can lead to death by asphyxiation (source of the name “strangles”) and may require emergency lymph node drainage or insertion of a breathing tube through the trachea.
Occasionally, abscesses spread by internal relase of pus to other parts of the body: the lungs, the abdominal cavity or even the brain. This is lifethreatening and known as internal or “bastard” strangles. Purpura hemorrhagica (widespread small bleeding along with fluid accumulation (oedema) of the limbs, eye lids and gums) may occur in association with circulating antibody complexes with S. equi M-like protein. The peripheral accumulation of fluid can be so extreme that circulatory failure and death ensue. Strangles is often fatal in young foals (less than six months old) that did not receive sufficient colostral immunity as newborns.
Strangles is sometimes considered to be a disease of foals and young adult horses. Older horses are often immune to strangles due to prior exposure during their youth, or they may display only lethargy and nasal discharge without obvious lymph node swelling (however, these animals can still spread the disease).
Atypical strangles presents as ‘flu-like symptoms, which is a complication that becomes persistent on a yard.
Diagnosis
Although strangles can be diagnosed based on symptoms alone, it can be confirmed by identification of the Sterptococcus equi in fluids from the throat, nasal passages or abscesses.
Treatment
Treatment of strangles depends on the stage of the disease. Although S. equi is usually sensitive to antibiotics such as Penicillin, antibiotic treatment is generally discouraged if a horse is already exhibiting signs of disease such as fever and nasal discharge. Once the lymph nodes become enlarged and abscessed, treatment with antibiotics will only prolong the disease. Antibiotic treatment at this time may increase the risk of complications such as internalized abscesses.
Antibiotic treatment of strangles-exposed horses before they display signs of disease may reduce the rate of morbidity (percentage of sick animals). This is still a controversial subject and the final decision should be left to your veterinarian. In contrast, complicated cases of strangles may require intensive antibiotic and supportive veterinary therapy to save the horse’s life.
• Observe sick animals for signs of shock, high fever (above 103 degrees F), and difficulty in breathing or abnormal swelling of the legs or ventral abdomen.
• Swollen lymph nodes may be softened by the application of hot compresses. This may hasten abscess drainage. Large abscesses that interfere with breathing may be lanced under veterinary supervision.
• If the horse is cooperative, draining abscesses may be cleaned with dilute antiseptic washes to promote skin healing and avoid secondary infections.
Outbreak control
Once the first case of strangles on a yard has been diagnosed, all other horses will be at risk.
The affected horse needs to be isolated from the other horses. The yard should be closed to horses from outside.
All healthy animals on the yard should be monitored closely to pick up new cases of strangles as early as possible. People who are in regular contact with horses from outside the yard should stay away from the infected yard as much as possible.
Effectively, this could mean that a yard will be closed down. Unfortunately, this situation can last for months.
Prevention of strangles
Several measures can be taken to minimise the risk of strangles on a yard:
• Try and avoid contact with horses of unknown origin.
• Make sure a yard doesn’t become overcrowded.
• If horses are introduced, keep them in quarantine for a few weeks to observe the onset of any clinical signs. Care must then be taken with personnel attending the isolated horses – they must not move from the isolated horses to the other horses on the establishment.
• Reduce sharing of tack or equipment from horses of unknown health status.
• Restrict movement of people onto the premises who have arrived from an affected yard.
• Minimise the risk of horses on the yard coming into physical contact with horses on neighbouring yards whose health status is not known.
Avoid contact with other horses (left). Vaccination into the upper lip of a horse (right)
Vaccination
Vaccinate if the horse is at risk of contracting the disease, and ensure that new animals arriving onto the yard have an up-to-date vaccination program. Vaccination as part of good stable management forms a critical element in preventing strangles outbreaks on yards.
A vaccine is now available to reduce clinical signs and the incidence of lymph node abscesses. The vaccine can be used in horses from just four months of age and is administered by administering a very small volume of vaccine into the upper lip of the horse. After vaccination, a small pimple will develop on the inside of the lip. This is perfectly normal and will disappear within a few days.
Where vaccination is required and to minimise the risk of strangles taking hold, all horses in a yard should be vaccinated.
The basic vaccination schedule is two submucosal vaccinations four weeks apart.
Horses in high-risk situations should be revaccinated with a single dose every three months.
For horses in medium-risk situations revaccination every six months rather than every three months can be considered. If horses are revaccinated every six months, a prompt booster with a single dose should be given if an outbreak of strangles occurs more than three months after the last booster (see table).
There is generally no need to vaccinate horses in low-risk situations.
