Listeriosis in sheep. Tick-borne fever used as a model to study predisposing factors.

Three groups of 6 months old lambs, each group consisting of 5 animals, were infected experimentally with Ehrlichia phagocytophila (Ep), Listeria monocytogenes (Lm) and Ep/Lm, respectively. All the animals had a period with fever and reduced appetite after infection, and these symptoms were most pronounced in the group with the combined infection (Ep/Lm). One animal in group Lm developed listeric meninigo-encephalitis. Lm was isolated from blood samples from both groups infected with Lm during the first week after infection, and from faecal samples during the first 2 weeks. Lm was also isolated from organs from several animals in these 2 groups at post-mortem examination. Group Ep/Lm developed the highest reciprocal geometrical mean titres and the stromgest delayed hypersensitivity reaction against Lm. After infection, a fall in serum iron and albumin was recorded, and the groups infected with Ep had a substantial fall in neutrophils. The myeloid : erythroid ratio in the bone marrow tended to decrease in Group Ep/Lm after infection. An increase in leucocyte counts and total protein content was found in the cerebrospinal fluid in the 2 groups infected with Lm, The experiment indicates that the blood changes induced by tick-borne fever viz, neutropenia and probably also impaired function of the neutrophils, may predispose for listeric septicaemia, but probably not for listeric meningo-encephalitis.