Strangles is caused by the bacterium Streptococcus equi. Strangles is very contagious and often all horses in a stable become affected, unless they are immune, i.e. still carry maternal antibodies or have gone through the disease during the last year. The disease lasts for about a month and affected horses must be kept isolated for a long time. Horses can be carriers for a very long time, even after the symptoms are gone.

Globally, the number of cases of strangles increases and in Sweden there are some 100's outbreaks reported annually. For each outbreak, one or more horses can be affected. In Sweden and some other European countries strangles is a notifiable disease, which means that a riding facility undergoes restrictions such as quarantine and inhibition of transportations.

Besides horse influenza caused by a virus, strangles is the most costly infectious disease in the horse.