S equi equi is highly contagious and produces high morbidity and low mortality in susceptible populations. Transmission occurs via fomites and direct contact with infectious exudates. Carrier animals are important for maintenance of the bacteria between epizootics and initiation of outbreaks on premises previously free of disease. Survival of the organism in the environment depends on temperature and humidity; it is susceptible to desiccation, extreme heat, and exposure to sunlight and must be protected within mucoid secretions to survive. Under ideal environmental circumstances, the organism can survive ~4 wk outside the host. Under field conditions, most organisms do not survive 96 hr.