an epidemiological study of listeriosis in dairy cattle

The number of new cases each month and the monthly incidence rate (i.e. the
number of new cases per 100 animal-months at risk) were calculated separately for each
farm (Table 6. 16 and Figure 6. 7).
On Farm A there was a gradual increase in incidence rate until all animals had
become infected. The maximum number of cases in any month occurred in January.
On Farm B there were two peaks; for the incidence rate these occurred in
November and February and for the number of new cases November and January.
December.
On Farm E there was a peak in the incidence rate and number of new cases in
On Farm C the incidence rate showed a small peak in March. The number of
new cases was small with a slight increase in September and highest in March.
On Farm D the incidence rate showed peaks in October and March and the
number of new cases follows a similar pattern.
When compared with management variables the data provide further support for
the association between maize silage feeding and faecal shedding seen using the
prevalence data. The introduction of maize silage in October on Farms B and E is
associated with rapid rise in the number of faecal shedders and incidence rate. On farm
A where maize silage was introduced in January the epidemic curve is less steep
between housing and the introduction of maize silage
Table 6. 16. Incidence rate of L. monocytogenes infection by month.
Visit Farm A (%) Farm B (%) Farm C (%) Farm D (%) Farm E (%)
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