an epidemiological study of listeriosis in dairy cattle

housed for 5 days. The proportion of animals shedding Listeria spp. fell in December
and this was mainly due to the decline in cows shedding L. monocytogenes. It rose
again in January associated with an increase in the proportion of animals shedding L.
monocytogenes. The latter reached a peak in February and fell to low level in March
This fall preceded the end of winter housing. Maize silage continued beyond the end of
the study.
The effect of age on excretion was similar to the Farm A (Table 6. 9). The mean
age of cows that were excreting Listeria spp. was lower than non-excretors (Table 6.
10).
b) Environmental samples:
Both L. innocua and L. monocytogenes were isolated from all environmental
samples and milk. L. monocytogenes was isolated from one environmental sample each
month with the exception of October. In February L monocytogenes was detected in all
the environmental samples except grass silage and milk. This was also the time when
the highest proportion of animals were excreting L. monocytogenes (Table 6. 12).
Table 6. 12. Isolation of Listeria spp. from the environment on Farm B.
VISITS
Samples A S O N D J F M A M
Soil 0 0 0 0 Li 0 Lm Li Lm Lm
Grass 0 Lm 0 Li Li 0 Lm Li 0 Lm
Water 0 Lm Li 0 Lm Li Lm Li 0 Li
Milk Lm Li 0 0 0 0 0 0 0 0
G. Silage 0 Lm Li Li Li Li Li Li Li ND
M. Silage NF NF NF Li 0 Lm Lm Lm Lm ND
Bedding NH NH Li Lm Li Lm Lm Li Lm NH
Lm L. monocytogenes, Li L. innocua, Ls L. seeligeri, ND not done, NF not fed, NH not housed.
180