Cerebral listeriosis in sheep and goats: a histopathological and immunohistological study

In a retrospective study of fifty-four cases of sheep (n = 40) and goats (n = 14) diagnosed as cerebral listeriosis, we reevaluated the histological l esions and used a silver stain as well as immunohistochemistry to demonstra te the listeriae in the tissue. Perivascular cuffs consisting of mononuclea r cells, focal purulent lesions (microabscesses) and/or focal accumulation of macrophages (glial nodules) in the caudal brain stem (medulla oblongata and pons) were considered as pathognomonic. In 87% of the cases the organis ms could be visualized within the focal lesions both by silver stain and by immunohistochemistry with comparable sensitivity. They were most numerous in microabscesses and often in very low numbers or absent in glial nodules. Bacterial organisms were absent or very rarely found in the meninges and i n the perivascular cuffs. In only eight out of eighteen cases Listeriae (L. monocytogenes) could be cultered, sometimes only after 4 weeks of incubatio n at 4 degreesC. We conclude that listeriosis in small ruminants can be dia gnosed histologically with high reliability, but negative isolation results by culturing do not exclude listeriosis.