an epidemiological study of listeriosis in dairy cattle

CHAPTER 3
The relationship between farm management practices and
clinical listeriosis in dairy cattle in England: univariate
3. 1. Introduction:
analysis
Infectious disease is the result of a complex set of interactions between the
infectious agent, host and environment. The infectious agent is an essential component
of disease but it may not be sufficient to trigger disease on its own. Other factors are
also needed for disease to develop. Listeriosis is a good example of this disease process.
Listeria organisms (mainly L. monocytogenes) are necessary causative agents of
disease but the isolation of L. monocytogenes from the brain (Gronstol 1980b) and
faeces (Husu 1990) of healthy individuals suggests that other factors are also involved.
Some other risk factors have been identified both in epidemic and sporadic cases of
human listeriosis; various physiological states of host such as extremes of age and
pregnancy (Ciesielski and others, 1988), underlying conditions such as cancer (Niemen
and Lorber 1980), immunosuppression, such as HIV infection (Jurado and others 1993),
organ transplantation (Lorber 1990) and concurrent infections (Rocourt 1996). However
less is known about the risk factors associated with listeriosis in animals. The
association between silage feeding and the occurrence of Listeriosis is well documented
(Gray 1960a, Wilesmith and Gitter 1986, Fenlon 1988, Sargison 1993) but the exact
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