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Streptococcus equi causes strangles, an infectious upper respiratory tract disease associated with substantial morbidity and occasional mortality in horses. The clinical signs associated with classical strangles include sudden-onset fever, depression, mucopurulent nasal discharge, and inflammation of lymphoid tissue in the head and neck (Sweeney and others 2005). Although the exact prevalence has not been reported, strangles remains one of the most commonly diagnosed and important infectious diseases of horses worldwide (Waller and Jolley 2007). Approximately 10 per cent of horses infected can carry the infection subclinically for months to years after the resolution of clinical signs and continue to shed bacteria periodically (Sweeney and others 2005). Detection of infection becomes difficult after the clinical signs have resolved, making asymptomatic carriers an important source of infection to susceptible animals.
Serology is potentially a quick and easy way to determine recent exposure to S equi. However, a commercially available ELISA (IDEXX Laboratories) for measuring antibody to the S equi-specific protein SeM does not discriminate between vaccinated and infected horses, and interpretation of the results can also be complicated by cross- reaction with antibodies to Streptococcus zooepidemicus (Timoney 1997). An ELISA blood test recently developed at the Animal Health Trust (AHT) uses two antigens (referred to as antigen A and B) that are unique to S equi and targeted by the equine immune system following exposure to this agent (AHT 2011). A positive …