an epidemiological study of listeriosis in dairy cattle

transmission between animals occurrs. In our study listeriosis in beef cattle and sheep
increased the risk of disease. This may be explained in a number of ways. Dairy cattle
might have contracted the disease from sick beef cattle or sheep by direct contact or
beef cattle or sheep with Listeriosis may have been the source of more pathogenic L.
monocytogenes strains and environmental contamination with these virulent strains
resulted in indirect transmission. Alternatively, farmers who reported cases in beef
cattle and sheep may have had a better knowledge of disease and therefore easily
recognised the signs of disease in their dairy cattle. This association may also be due to
a confounding effect e.g. ring feeders are usually used for beef cattle.
The association between vaccinating animals against Leptospirosis and an
increased risk of Listeriosis is without an obvious biological explanation. It might be
explained by the fact that concurrent infections have been reported to make animals and
human more susceptible to L. monocytogenes (Gray and Killinger, 1966, Rocourt 1996)
or the stress caused by the vaccine may have made animals more susceptible. An
alternative explanation may be that farmers experiencing abortion may have vaccinated
against Leptospirosis with or without advice from a veterinarian because the majority of
cases of abortion are attributed to Leptospirosis in this country without thorough
investigation of actual cause. There may have been even a confusion of name between
the two diseases. Such confusion of name was seen in a study from the USA (Schwartz
1967).
In this study 5 groups of farm level predictor variables were used to determine
their relationship with 5 different outcome variables using univariate analysis
techniques. One of the major drawbacks of univariate analysis is that it does not deal
with confounders. A confounder is a variable that is positively or negatively associated
with both the outcome variable and hypothesised predictor variable that are being
studied. Confounding may result in either overestimation or underestimation of an
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