an epidemiological study of listeriosis in dairy cattle

3. 4. Discussion:
In this study we tried to determine the farm level risk factors associated
with different outcome variables. Different variables were associated with different
outcomes. This difference may be due to the small number of farms with some of the
outcome or predictor variables: for example nervous signs of disease were only reported
by 26 farms. Alternatively it may reflect true differences in the models. However we
think this may not be the case because 26 of 36 variables examined were associated
with Listeriosis when the total number of cases “overall” was the outcome. The other
associated variables were closely related to these 26 variables. Of the 36 variables 16
were consistently associated with at least 3 of the outcome variables. This consistency
of associations is in agreement with the Evan’s postulates of disease causation
(Thrusfield 1995).
Feeding grass silage and maize silage was associated with clinical Listeriosis.
The relationship between feeding silage and Listeriosis is well documented (Gray
1960a, Gray and Killinger 1966, Gronstol 1979a, Kalac and Woolford 1982, Fenlon
1986b, Fenlon 1988, Wilesmith and Gitter 1986, Sargison 1993) and disease is therefore
called “silage sickness” (Gray and Killinger 1966, Dennis 1993). However the
determinants of this association are not well known. The possible ways in which silage
may play a role have already been explained in the Chapter 1. Where silage has been
implicated its quality has always been described as “inferior” (Gray 1960a, Fenlon
1988, Sargison 1993). There are several factors that influence the quality of silage.
These factors include the whole silage making process; time and stage of harvesting,
type of harvester used, soil contamination, wilting, method of storing etc. A lack of care
at any of these stages will result in poor quality silage that is manifested by improper
fermentation (aerobic fermentation) where the critical pH (4.2) is exceeded and the
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