Dr. Philip Johnson
Strangles ("distemper") is caused by a bacteria (Streptococcus equi) that is characterized by inflammation of the upper respiratory tract and most often by abscess in the adjacent lymph nodes. In the worst cases, the glands around the throat get so swollen, the horse looks almost like it's going to be "strangled," but in fact it rarely causes asphyxiation. In some regards, strangles is similar to strep throat, in that it is very contagious and causes painful soreness in the throat of affected horses. The bacterial pathogen that causes strangles rarely affects any other species.
All horses are potentially susceptible to this disease unless, as a result of previous exposure, the individual horse has acquired immunity. Although strangles does sometimes afflict newborn foals and old horses, it is most typically seen in older foals and young adult horses.
The causative agent of strangles, Streptococcus equi equi, is disseminated into the environment by clinically sick horses infected with strangles and by clinically recovered horses, or carriers, that act as an unseen source of infection. It is difficult to identify carrier horses because they do not exhibit any signs of infection. The bacterial pathogen is shed in the nasal secretions of infected horses and in the thick yellow exudate that is expelled in nasal discharges and in fluids drained from abscesses that develop in infected horses.
The disease may be transmitted directly from horse to horse if close contact between infected and non-infected horses occurs. However, unlike viral pathogens, the causative agent for strangles is not spread appreciably in aerosolized fluids released from the airway of infected horses. The most important route of transmission involves indirect contact with Streptococcus equi, through contamination of environmental objects (known as "fomites"), most commonly people. Insects (barn flies) may also play a role in disseminating this bacterial pathogen in the warmer times of the year.
Human contact between infected and non-infected horses (especially at the horse's nostrils) is a common method of transmission. Therefore, veterinarians are very careful as they move among horses, especially between different stables, in order to ensure that they are not inadvertently transmitting this bacterial pathogen on their hands, clothes or equipment. Other common fomites include: food and water troughs, stocks, twitches, dental equipment, bridles, horse trailers, pastures, stable walls and fences.