Clostridium septicum infection in Icelandic horses

This study presents the first known outbreak of subcutaneous clostridial infection in horses in Iceland following subcutaneous antiparasitic treatment. Sixteen out of 32 horses which received an off-label ivermectin product subcutaneously, developed symptoms consistent with a subcutaneous Clostridium septicum (C. septicum) infection, with an additional one case of sudden death in the beginning of the outbreak. Necropsy of one horse revealed severe, acute cellulitis with vasculitis in the neck region, and lymphadenitis in the retropharyngeal and mandibular lymph nodes. Approximately two weeks post-injection, abscesses were observed at the presumed injection site on eight horses. Samples from the necropsied horse and from abscesses of three horses yielded C. septicum on bacteriological culture. Analysis of water samples from the pasture where the herd was kept also revealed C. septicum. The C. septicum isolates from the necropsied horse and abscess from a surviving horse were identical, whereas the strain isolated from the water samples was significantly different from the disease-causing isolates. Subcutaneous injection of a herd of horses whose skin presumably was contaminated by C. septicum spores, with a locally irritating ivermectin product, resulted in a serious outbreak with a mortality of 19%. Similar outbreaks have not been reported in Iceland previously although ivermectin products formulated using the same irritant have been commonly used for at least 30 years. The proportion of animals developing serious infections in this outbreak is alarming and warrants a reconsideration of this practice.