horse disease caused by Streptococcus equi, a bacterium that invades nasal and throat passages and forms abscesses in lymph nodes and other parts of the body. It is also called distemper of horses. Young horses are most susceptible to it, and outbreaks of the disease usually occur where a number of horses are stabled. Mortality is low. Treatment includes complete rest and antibiotic therapy. Isolation of infected animals and rigid sanitation of quarters help reduce the spread of the disease. Vaccines are available, although they may reduce the severity rather than prevent occurrence of the disease.