Strangles Exposure and Antibiotics
I often get asked "My horse has been exposed to a horse with strangles. Can I treat him with antibiotics to prevent infection?" It's a reasonable question, but unfortunately there is no clear answer.
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Strangles is an infection caused by the bacterium Streptococcus equi subsp equi, which is often simply called Strep equi or S. equi for short. This pathogen is always circulating somewhere in the equine population, and it can cause sporadic cases of strangles in individual horses, or large outbreaks in groups. It is transmitted mainly by nasal secretions and pus from infected horses when they come in contact with other horses. The classical and most prominent feature of strangles is the formation of large abscesses in the lymph nodes between the jaw bones.
Treatment of strangles with antibiotics is typically frowned upon because antibiotics do not penetrate abscesses well. Draining the abscesses is the key to treatment in most situations. However, antibiotics can kill the S. equi if they are used before abscesses develop. Therefore, if a horse has been exposed to strangles and is in the very early stages of developing an infection, but it does not have any abscessed lymph nodes, then treatment with antibiotics could be useful.
The downside of this kind of treatment is, while it stops the immediate infection from developing, the horse does not develop immunity against S. equi like it would if its body had to fight the infection. This is not a problem if good infection control measures are in place to prevent further S. equi transmission. However, if transmission is not controlled, then the horse is at risk of being re-exposed, and could be infected again after the antibiotics are stopped. This can lead to a vicious cycle of exposure-treatment-susceptiblity-exposure-treatment... I've seen farms where this goes on for a long time with horses receiving multiple courses of antibiotics, and they often eventually getting the disease anyway.
Antibiotics can be useful in horses with early disease (i.e. fever, no lymph node enlargement) IF they are used as part of an overall infection control program that has a heavy emphasis on implementation of sound infection control measures. If potentially infected horses can be rapidly detected and isolated, thus minimizing the risk of exposure to other horses, then early antibiotic treatment of horses that are developing strangles can be useful. Antibiotic treatment alone, without concurrent use of good infection control measures, is bound to fail. While uncommon, antibiotics can be associated with adverse effects in horses, so we want to make sure that we are only using them when they are needed.
More information about strangles can be found on the equIDblog Resources page.
