Demonstration of Listeria monocytogenes by Immunohistochemistry in Formalin‐Fixed Brain Tissues from Natural Cases of Ovine and Bovine Encephalitis

In the present work, evidence of Listeria monocytogenes antigens based on the avidin–biotin complex (ABC) immunoperoxidase technique was performed on formalin‐fixed central nervous system tissues (CNS) from a total of 23 natural cases of encephalitis (four ovine and 19 bovine). Listeria monocytogenes serotype 4 was isolated from 10 of 17 cultured specimens. Meningoencephalitis characterized by focal necrosis, microabscesses, perivascular cuffing, and gliosis with presence of macrophages and/or neutrophils was observed at histological examination. Positive L. monocytogenes antigens were successfully identified by immunohistochemistry (IHC) in the CNS of all 23 cases. Paraffin‐embedded tissues assayed were stored up for 17 years. Morbidity of the outbreaks was between 0.3–3% and 0.1–1% for ovine and bovine cases, respectively. In all the ovine cases, flocks involved were under extensive grazing conditions. In nine of the 19 bovine cases (47.3%), supplementation with corn silage was used. The ABC test can help as a practical tool for the diagnosis of natural cases of L. monocytogenes encephalitis on formalin‐fixed specimens from ovine and bovine.