an epidemiological study of listeriosis in dairy cattle

Table 6. 11. Isolation of Listeria spp. from the environment on Farm A.
Months
Samples A S O N D J F M A M
Soil 0 0 0 0 Li 0 Li 0 0 0
Grass 0 0 Lm Lm Li 0 Lm 0 Lm Lm
Water 0 0 Lm Li Lm Lm Lm Lm Lm 0
Milk 0 0 0 0 0 Li Li Lm Lm ND
G. Silage NF NF Li 0 Li Li Lm Lm Lm NF
M. Silage NF NF NF NF NF Lm Lm Ls Lm NF
Bedding NH NH NH Lm Lm Li Lm Lm Lm NH
Lm L. monocytogenes, Li L. innocua, Ls L. seeligeri, ND not done, NF not fed, NH not housed.
Farm B:
a) Faecal samples:
A total of 111 animals were examined throughout the study period. 79.3%
(88/111) of the animals excreted L. monocytogenes, 7.2% (8/111) other Listeria spp.
and the rest did not shed Listeria (14.4%, 15/111) during their participation in the study.
Only 54.1% (60/111) of the milking cows were examined at all visits. When only these
animals were taken into account, the proportion of animals shedding L. monocytogenes
was 98.3% (59/60). The remaining animal shed another species of Listeria.
The monthly proportion of animals shedding Listeria spp. varied from 11.4 % in
September to 96.3% in January. A higher proportion of animals were shedding Listeria
organisms in their faeces between October and May with a peak of 96.3% in January
and February. For L. monocytogenes and L. innocua there was a fluctuating rise. The
highest proportion of animals excreting L. monocytogenes and L. innocua was in
February (81.8%) and March (62.5%) respectively. The proportion decreased thereafter
(Figure 6. 3). There was statistically a significant difference between the monthly
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