Strangles, also known as Streptococcus equi subsp equi (S. equi) infection, is the most commonly diagnosed, infectious upper respiratory tract diseases of horses younger than 5 years of age. It occurs in horses worldwide, and is characterized by acute onset of respiratory disease and swollen lymph nodes. Strangles is associated with several secondary complications as a result of the infection.

Early clinical signs of strangles include fever, depression, and reduction in appetite. Lymph nodes may be sensitive and firm to touch. Nasal discharge is usually initially serous but changes to become more mucopurulent as the disease progresses. Approximately 7 days into the course of the infection, lymph node enlargement and abscess maturation occurs. Abscessed lymph nodes generally rupture 7 to 10 days following initial onset of fever. Once they rupture, if there are no complications, horses often can recover within 1 to 2 weeks.

Complications occur in up to 20% of horses and up to 40% of those horses may die or be euthanized. One of the most common complications of strangles is the development of guttural pouch empyema, caused by the rupture of the abscessed lymph nodes into the guttural pouch or extension of infection from the pharynx from the salpingo-pharyngeal opening. Less than 1% of horses with strangles develop "bastard strangles", which are metastatic or internal abscesses that occur if S. equi gets into the horse's bloodstream.

Horses that receive the strangles vaccine and horses that recover from the disease are at risk of developing purpura hemorrhagica, an acute, non-contagious syndrome caused by immune-mediated, generalized vasculitis. The severity of clinical signs vary and don't typically develop until 2 to 4 weeks following initial signs of strangles or exposure to S. equi antigens through the vaccination.

Transmission
S. equi is most commonly transmitted through oral and nasal routes, however, can also occur through direct contact with an infected or carrier horse. S. equi is also capable of spreading through contaminated fomites such as tack, halters, blankets, cleaning tools, grooming equipment, etc. Some horses may be infected with S. equi but show no external clinical signs of being infected. These horses can still infect other horses and can maintain S. equi in their bodies for months after recovering from clinical illness.

Incubation Period
The incubation period from time of infection to manifestation of clinical signs varies from a few days to a few weeks, as it depends on the dose the horse received, immunity, and pathogen virulence. Strangles has an average incubation period of 3 to 8 days.