an epidemiological study of listeriosis in dairy cattle

experimentally been shown that there is a relationship between the excretion of the
organism in faeces and infectious dose. Lhopital and colleagues (1993) reported that
sheep challenged orally with 6x10 6 did not excrete the organism in any samples
examined (blood, buccal and nasal swabs and faeces) but those challenged with 6x10 10
shed the organism in the samples, including faeces. Although this could not be proved
in our study, the inclusion of maize silage in the diet may have increased the quantity of
Listeria organisms in the diet, as the organism was isolated more frequently from maize
silage than grass silage on farms A, B and E. An alternative explanation may be that the
strains of L. monocytogenes obtained from maize silage were responsible for the
epidemics. However, when the strains obtained from maize silage were compared with
those detected in animals at the highest peak of infection it was difficult to support this.
The strains detected in maize silage were not predominant in faecal isolates obtained at
the peak prevalence of infection on farms A and E and the strain detected in maize
silage and animals on farm B were also predominant in the isolates from faeces on farm
C and D.
It is also possible that the strains of L. monocytogenes on these 2 farms (farm C
and D) may have not been capable of propagating in the environment, feed or cows.
This is unlikely. Pattern 1 was predominant in faecal samples on farms B, C and D and
in the environment on farm C.
The absence of a high prevalence of infection on farms C and D may also have
been due to demographic features of these herds. Farm C was the smallest herd and
therefore the number of contacts with susceptible animals might have been too low to
initiate an epidemic. Farm D was a flying herd which by its nature would contain a
small proportion of young animals. There was a negative association between age and
faecal excretion on the farms where ages were available. This was also a feature of the
pilot study. It was reflected also in the much higher incidence of clinical Listeriosis
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