Naturally occurring subclinical ovine mastitis associated with Listeria monocytogenes

Mammary infection with and faecal excretion of Listeria monocytogenes and somatic cell counts in milk were monitored in 98 ewes in a population of 335 in two flocks. From these 98 ewes, three samples were obtained throughout lactation: first at 15-30 days after lambing, second 6-7 weeks later and third 6-7 weeks after the second sampling. No clinical mastitis was recorded in any ewe, but the prevalence of subclinical mammary infection with L. monocytogenes was 3.1% on the first and the second sampling and 6.2% on the third; in 83% of these ewes, bilateral infection was recorded. Somatic cell counts in the milk of ewes with mammary infection ranged from 1.75 x 10(6) to 2.96 x 10(6) cells ml(-1). Furthermore, L. monocytogenes was isolated from faecal samples of 19.4% of the 98 ewes. Four ewes with L. monocytogenes subclinical mammary infection were euthanatized. L. monocytogenes was isolated from mammary tissue samples of the four ewes and from the liver of two. Chronic inflammatory features predominated in histological sections of mammary tissue samples. It is concluded that L. monocytogenes causes subclinical mastitis in ewes; infection of the mammary gland appeared to be haematogenous. (C) 1998 Elsevier Science B.V. All rights reserved.