The disease known as strangles in the horse is caused by the bacteria Streptococcus equi var. equi (S. equi). Horses with strangles usually have a high fever, followed by nasal discharge and enlargement and abscessation of the lymph nodes under the jaw and in the throatlatch region. Affected horses often have a loss of appetite and may stand with their head and neck extended. Occasionally some horses develop abscessation of lymph nodes in the chest and abdomen known as “bastard strangles”.
S. equi is spread through contact with fluid draining from the abscess or nasal
secretions. The incubation period from infection to clinical signs is 3-14 days. Horses infected with Strangles do not shed the bacterium from the nasal secretions for 1-2 days after the onset of fever. If temperatures are regularly taken, new cases can be quarantined before they become infectious, limiting spread through a barn. The organism can be carried on the boots, clothes and equipment of those who have contact with an infectious animal, necessitating strict biosecurity measures to prevent spread. Most horses will shed the organism for 2-3 weeks post infection. A veterinarian can perform nasopharyngeal washes one week apart to ensure a horse is free of infection before returning him to the barn. Some horses may remain persistently infected due to bacteria harbored in the guttural pouches. These horses can shed the organism intermittently and are known as carriers. They may be the source of infection when introduced into a herd of susceptible horses. Carriers can be detected by performing nasopharyngeal washes. S. equi may persist in the environment for a few months.
Treatment of Strangles depends on the situation and severity of the disease. If antibiotics are begun as soon as the horse shows fever, lymph node abscessation may be prevented. Once a horse has lymph node involvement, antibiotics are generally contraindicated. Antibiotics at this stage may prolong the time to abscessation and resolution of the signs. Instead therapy should be directed at encouraging maturation and drainage of the abscess. Some horses develop complications that necessitate additional therapies.
Seventy-five percent of horses that have lymph node involvement and recover have a solid immunity that lasts 5 years or longer. Antibody titers can determine the strength of immunity. Vaccination should be administered on a case-by-case basis by a veterinarian depending on previous exposure and likelihood of future exposure. Horses that have had previous exposure to S. equi may develop immune mediated complications following vaccination.
Most horses that suffer from Strangles will recover without complications on their own. But to prevent the disease, all new horses or horses returning from an event where horses are comingled should be quarantined with temperatures taken daily or twice daily. New horses should be isolated for 3 weeks. Ideally, no new horses should be introduced to a farm without 3 negative nasopharyngeal washes.
Many thanks to Dr. Josie Traub-Dargatz from Colorado State University for allowing us to use her pictures.