Strangles is a highly contagious disease that affects a horses lymph nodes in its upper respiratory tract. It is caused by a bacterium called streptococcus equi. The name, strangles, was coined due to the strangling breathing sounds made by affected horses, caused by the enlarged sub-mandibular and retro-pharyngeal lymph nodes.
Strangles can affect horses of any age, but most commonly infects those between one and five years of age. The disease is usually acquired after exposure to another horse that is shedding the streptococcus equi bacteria, either during or after its own bout of the illness. This can occur when new horses are introduced to an established herd and can also become established in an area. Although the infectious horse may no longer show signs of strangles, it can still spread the bacteria. Around twenty percent of horses remain contagious for a month after all symptoms are gone.
While direct contact between horses is the most common way that strangles is spread, it can also be spread by contaminated equipment. Improperly cleaned and shared buckets, stalls, and tack can spread the disease between horses. Fortunately, the bacteria die fairly quickly in the environment.
Once a horse is exposed to the bacteria, it will begin to show symptoms in two to six days. If left untreated, it will develop abscessed lymph nodes within one to two weeks after the onset of illness. These lymph nodes will rupture and drain, and the drainage is highly contagious. Most horses will recover, but around ten percent of untreated horses die, usually from a secondary infection which causes pneumonia or due to an inability to move air through the upper respiratory tract.
With onset, the horse appears depressed, dull, and stops eating. Typically, the temperature rises to 103°C. After a few days lymph nodes around the throat swell, forming abscesses. The horse can have difficulty breathing and swallowing. A nasal discharge is at first clear and then becomes purulent (thick with signs of pus), after the abscesses have ruptured in the nasal passages. Sometimes the veterinarian surgically opens the abscesses to help breathing. Abscesses that rupture shed highly infective pus into the environment, which can infect other horses. In some outbreaks and in up to ten percent of cases, these abscesses spread to other parts of the body (a condition known as bastard strangles) which is nearly always fatal.
To control the spread of the strangles bacteria, any new horse with a vague or unknown health history should be isolated for four to six weeks before being added to the general population of the stable or paddock. Nasal swabs can ascertain whether the horse is shedding the streptococcus equi bacteria, but because affected horses shed the bacteria sporadically, one swab test is not enough. Three nasal swabs over a period of seven days are required before it can be assumed that the horse is negative for strangles.
Strangles can also be controlled by vaccinations. Although modern vaccines are more effective than those of the past, providing better protection with fewer side effects, they are not a complete guarantee against the disease. Still, vaccinated horses tend to have a less severe illness if they do contract strangles.
If you suspect that your horse has strangles, notify your veterinarian to confirm the presence of the disease. The sooner a positive diagnosis is reached, the less down time the stable will have to tolerate. Most often strangles is allowed to “run its course,” with the majority of horses recovering with little or no problems. There are more severe strains of strangles that can cause multiple lymph nodes to abscess on the face. These horses maybe started on antibiotics. The treatment of strangles is dependent on the stage of the disease. Also, if a horse begins antibiotic treatment in the early stages of the disease, lymph node abscesses can be prevented. However, once lymph nodes have enlarged and become abscessed, antibiotic treatment will only prolong the horse's illness. It is better to allow the abscess to open, or have the veterinarian lance it, so that it may drain. The best treatment at this point is to flush the drainage site, keep the area as clean as possible, and to maintain strict isolation of the ill horse.
If your horse was stabled near one who had strangles at a show or rodeo, it is reasonable to treat it with antibiotics for at least six days after exposure. This is because horses usually don't show the first signs of the disease for two to six days. However, if your horse is kept in a barn where other horses have strangles, antibiotics will do little to prevent it from getting the disease.