S equi is highly infectious and produces high morbidity and low mortality in susceptible populations. Morbidity may reach 100% in naive horses. Disease is most severe in horses 1–5 years of age; however, it has been reported in foals as young as 3 months and in older adults. Transmissionis via direct contact with infected horses or with any number of contaminated fomites. While horses with overt clinical signs of disease are obvious sources of infection, asymptomatic carriers (up to 40% of naturally infected horses) and horses recovering from infection but no longer demonstrating overt clinical signs play an important role in disease outbreaks and perpetuation of the organism within the population. Survival of the organism in the environment depends on temperature and humidity; wet, cold conditions and protection within mucoid secretions improve survival. S equi is susceptible to desiccation, extreme heat, and exposure to sunlight; most organisms do not survive > 3 days in the environment.
