Strangles, caused by the bacterium Streptococcus subspecies equi (often referred to as S. equi) is a highly contagious disease of horses worldwide. Recent discussion at the AAEP Convention confirms that strangles remains an important clinical problem that can be frustrating to both horse owners and veterinarians.
Despite being first described in the 1200s, there are still many unanswered questions regarding the disease. Classically, strangles presents as an infection of the upper respiratory tract, but any anatomic site can be involved. One of the bacteria's surface proteins, SeM, is important in the bacteria's ability to cause disease. Also, horses that are exposed to or infected with S. equi generally have an immune response to this protein. An assay to measure the concentration of antibody to SeM, the SeM ELISA, is commercially available. Veterinarians sometimes use this test to help diagnose strangles when a sample cannot be obtained for direct identification of the organism, such as in horses with internal abscessation ("bastard" strangles).
It has also been suggested that the test may be used to help determine if vaccination is indicated; horses with low concentrations of antibody may benefit from vaccination while those with high concentrations may already be protected and may actually be at increased risk of developing purpura hemorrhagica, a vasculitis that can be associated with strangles. However, practitioners recognize that an individual immune response can vary widely and more information regarding the SeM ELISA and the relevance of specific titers is needed.
Some aspects regarding the treatment and prevention of strangles remain controversial. Practitioners at the convention generally felt that strategies should be developed on a case by case basis. While antibiotic treatment is often not necessary, data do not support an association between the use of antibiotics and the development of internal abscesses.
Recently, a new product, Streptococcus equi antibody, has been conditionally licensed by the USDA to aid in the treatment of strangles. This plasma product provides the patient with antibodies to S. equi. In one study in which horses were experimentally challenged with S. equi, the administration of the S. equi antibody product was associated with a reduction in the duration and severity of disease, as well as in mortality. Additional studies are in progress. This product has not yet been widely used in the field.
Vaccination for strangles is currently considered a risk-based vaccination by the AAEP, meaning that the decision for vaccination should be made on an individual basis following a risk-benefit analysis. There are two general types of strangles vaccine currently available in the United States, an extract vaccine and an attenuated live vaccine. The development of new strangles vaccines with improved safety and efficacy is the focus of several laboratories around the world. Some new vaccine technologies under investigation include recombinant vaccines and the use of microspheres. It is unknown when such vaccines might become commercially available.
This Table Topic was moderated by M. Keith Chaffin, DVM, MS, Dipl. ACVIM, and Melissa Hines, DVM, PhD, Dipl. ACVIM, who authored this report.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.