an epidemiological study of listeriosis in dairy cattle

There was no evidence from this farm that grass silage feeding alone was
associated with a high frequency of faecal shedding of Listeria. Grass silage was fed all
year round and in October L. monocytogenes was isolated from it at a time when the
proportion of animals shedding this organism was low.
There was a dramatic increase in the proportion of animals shedding L.
monocytogenes in November but it was difficult to attribute this to any one
management factor as both the introduction of maize silage feeding and housing had
occurred. L. monocytogenes was only isolated from bedding at the November visit.
The peak prevalence of L. monocytogenes excretion in February corresponded
with the highest frequency of recovery from food and environmental samples This was
also the case for L. innocua in March.
The proportion of animals shedding L. monocytogenes declined in March at a
time when maize silage from which L. monocytogenes had been isolated was still being
fed. This continued to be the case in April.
this farm.
Farm C:
L. monocytogenes was isolated from the liver of an aborted foetus in January on
a) Faecal samples:
67 milking cows were examined. 37.3% (25/67) of them shed L.
monocytogenes, 25.4% (17/67) L. innocua and 6.0% (4/67) L. seeligeri. Listeria spp.
were not isolated from the remainder (31.3%, 21/67) during their stay in the herd. When
the animals that left or entered the herd during the study were excluded only 41 (61.2%)
animals were consistently tested. Of these 46.3% (19/41) shed L. monocytogenes,
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