an epidemiological study of listeriosis in dairy cattle

1988, Husu 1990) and from zero to all animals on different farms (Skovgaar and
Morgen 1988, Husu 1990). The use of different culture techniques and media has been
suggested as a reason for these different frequencies (Husu 1990) and the difference
between the farms has been attributed to different feeding practices (Skovgaar and
Morgen 1988, Husu 1990).
In this study too, there were differences between the farms in the proportion of
animals positive for Listeria spp. and L. monocytogenes. It varied from 48% of cows on
farm D to 89.1% on farm E for Listeria spp. and from 28.2% of cows on farm D to
79.3% on farm B for L. monocytogenes. Similarly differences in the monthly frequency
of excretion were also observed between the farms in this study. The monthly frequency
of excretion reported here showed a seasonal pattern but there were two distinct patterns
of excretion. The first pattern seen on farms A, B and E was characterised by the high
frequency of excretion between October and May. This was similar to the pattern
observed in our pilot study (Chapter 5). This seasonality is also similar to that reported
for clinical listeriosis (Chapter 2). A similar seasonality of excretion of Listeria spp. and
L. monocytogenes in faeces was also reported by Husu (1990). However the frequency
of excretion reported in our study was much higher than those observed by Husu
(1990), where the excretion rates for L. monocytogenes were 0.9 % (3.5% for Listeria
spp.) in September and 16.1% in December (19.4% for Listeria spp.).
In contrast to the first group of the farms, the excretion rates on farms C and D
were lower especially between October and April. The use of different culture
techniques would not explain the differences seen between the farms in this study
because the same culture technique was used throughout the study. There are a number
of possible explanations for these differences between farms. They may be related to the
strains of organism, the systems of husbandry or the proportion of susceptible animals.
The difference in frequency of excretion may also be explained by the absence of some
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