THE EFFECTS OF INOCULATION OF LISTERIA-MONOCYTOGENES INTO THE OVINE MAMMARY-GLAND

In each of two experiments, the effects of inoculation of Listeria mon ocytogenes into the ovine mammary gland were studied. In the first exp eriment, ewes were challenged with one or other of five different List eria 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 ma mmary glands was assessed by bacteriological, cytological and histolog ical methods. No distinct variation in the pathogenicity of L. monocyt ogenes isolates was evident: all produced subclinical mastitis, indepe ndently 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 gla nds, whose milk somatic cell counts were greater than 1.0 x 10(6) cell s ml(-1), The organism was also isolated from the mammary lymph nodes, but not from any internal organ of any inoculated ewe. In early stage s of the infection neutrophilic infiltration was the predominant histo logical feature, but hyperaemia, and degeneration of alveolar epitheli al cells were also recorded. Later, chronic inflammatory features pred ominated, with lymphocytes as the principal cell types, destruction of alveoli and fibrous tissue proliferation. In the final stage of the e xperiment, fibrosis was the salient finding. It is concluded that L. m onocytogenes can cause subclinical mastitis after intramammary inocula tion into ewes. (C) 1998 Elsevier Science B.V.