an epidemiological study of listeriosis in dairy cattle

eported in heifers in the questionnaire survey (Chapter 2). Although the influence of
age on clinical Listeriosis is controversial younger animals have been reported to be
more susceptible (Barlow and McGorum 1985, Nash and others 1995). In this study
younger animals were more likely to excrete the organism in their faeces than the older
animals. This finding contradicts with the finding of Hofer (1983) where the organism
was more frequently isolated from beef cattle older than 5 years. However, in the study
of Hofer the sampling was not statistically based and samples were collected from an
abattoir. It is possible that stress induced shedding contributed to this finding or that the
older animals had taken longer to reach slaughter weight because of intercurrent
disease.
This negative association suggests that animals may became immune to
infection following repeated exposure to Listeria organisms. In our study measurement
of serum antibody indicated that almost all animals had been exposed to Listeria
infection prior to the study. It has experimentally been demonstrated that anti-LLO
antibodies develop after oral challenge (Low and Donachie 1991, Low and others
1992b, Miettinen and Husu 1991) and that antibodies developed against L.
monocytogenes could sustain at higher levels for as long as 4 (Lhopital and others
1993) or 6 (Miettinen and Husu 1991) or 7.5 months after initial exposure (Baetz and
Wesley 1995). This long lasting nature of anti-LLO antibodies may explain the
seropositivity of almost all animals at the beginning of this study. Whilst the
measurement of serum antibody may not correlate with protection against infection
because Listeria is an intracellular organism, it may however be considered as a
measure of exposure to infection or re-infection.
On only one of the farms, farm A, was there an increase in serum antibody
between August and December and December and April. This increase showed a broad
agreement with the proportion of new faecal excretors observed during these two
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