The effects of inoculation of Listeria monocytogenes into the ovine mammary gland.

In each of two experiments, the effects of inoculation of Listeria monocytogenes into the ovine mammary gland were studied. In the first experiment, ewes were challenged with one or other of five different Listeria spp. isolates to study differences in their pathogenicity. In the second, ewes were challenged with L. monocytogenes serotype 1/2a to study the sequential features of the infection. The reaction of the mammary glands was assessed by bacteriological, cytological and histological methods. No distinct variation in the pathogenicity of L. monocytogenes isolates was evident: all produced subclinical mastitis, independently of their origin or serotype; a L. innocua isolate caused only a transient increase of milk somatic cell counts. After challenge, L. monocytogenes was isolated for 88 days from the milk of inoculated glands, whose milk somatic cell counts were greater than 1.0 x 10(6) cells ml-1. The organism was also isolated from the mammary lymph nodes, but not from any internal organ of any inoculated ewe. In early stages of the infection neutrophilic infiltration was the predominant histological feature, but hyperaemia, and degeneration of alveolar epithelial cells were also recorded. Later, chronic inflammatory features predominated, with lymphocytes as the principal cell types, destruction of alveoli and fibrous tissue proliferation. In the final stage of the experiment, fibrosis was the salient finding. It is concluded that L. monocytogenes can cause subclinical mastitis after intramammary inoculation into ewes.