an epidemiological study of listeriosis in dairy cattle

On this farm, grass silage was introduced in September 12 days after the visit
(9/9/1996). By the October visit the proportion of animals shedding Listeria spp. and L.
monocytogenes had increased slightly but was similar to that seen in August before
silage feeding. The cows were housed on the same day as the October visit and by the
November visit the proportion of animals shedding Listeria spp. had increased
significantly. This was mainly due to an increase in the number of animals shedding L.
innocua. The proportion of animals shedding L. monocytogenes continued to increase
in December and showed a significant increase in January 9 days after the introduction
of maize silage. Maize silage feeding ended at the end of March, 14 days before the
April visit and this was also the time when animals were turned out. The proportion of
animals shedding L. monocytogenes was still high in April but it dramatically
decreased in May 12 days after grass silage feeding ended.
The total proportion of examined animals excreting Listeria spp. before (5.4%,
6/111) and after (16.5%, 15/91) grass silage feeding was significantly lower than the
proportion obtained during grass silage feeding (76.9%, 90/117) (p