Outbreak of encephalitic listeriosis in Holstein bulls

Abstract

Five Holstein bulls in a herd of 478 bulls were diagnosed with clinical encephalitic listeriosis. Four bulls survived after intensive therapy with oxytetracycline, dexamethasone, and oral fluid with electrolytes. The bull which did not survive had histological findings of lymphoplasmacytic encephalitis and inflammation of the trigeminal ganglion, which were consistent with the diagnosis of clinical listeriosis. A suspect silage sample was submitted for laboratory evaluation. The silage had a pH of 6.9 and was positive for Listeria monocytogenes. In vitro antibiotic testing of the organism isolated revealed resistance to penicillin, the drug commonly recommended for the treatment of listeriosis. It is concluded that caution should be exercised, however, when evaluating in vitro susceptibility results associated with a listeriosis outbreak because multiple ribotypes may be involved.