an epidemiological study of listeriosis in dairy cattle

1991, Boerlin and Piffaretti 1991, Norrung and Skovgaard 1993, Fenlon and others
1996).
At the primary production stage silage has long been associated with L.
monocytogenes infection and is thought to be the source of the organism (Gill 1933,
Gray 1960a, Gray and Killinger 1966, Fenlon 1985, Gitter 1989). The contamination of
the agricultural ecosystem with L. monocytogenes is well documented and L.
monocytogenes is thought to be a saprophytic organism living in a plant-soil
environment (Welshimer 1968, Welshimer and Donker-Voet 1971, Welshimer 1975,
Weis and Seeliger 1975, Watkins and Sleath 1981, van Renterghem and others 1991,
MacGowan and others 1994). Carrier humans and animals are thought to play an
essential role in the contamination of their environment (vegetation, soil, water etc.) and
therefore foodstuffs. However the primary source of contamination is unknown
although it has been the subject of research for many years.
Research has been carried out to determine the excretion rate in humans
(Kampelmacher and van Noorle-Jansen 1969, Lamont and Postlethwaite 1986,
MacGowan and others 1991, Gray and others 1993, MacGowan and others 1994), sheep
(Gronstol 1979b), wild and domesticated birds, (Fenlon 1985, Skovgaard and Morgen
1988, Idia and others 1991, Casanovas and others 1995) pigs, cats, dogs (Iida and others
1991, van Renterghem and others 1991) and cattle (Kampelmacher and van Noorle-
Jansen 1969, Hofer 1983, Skovgaard and Morgen 1988, Husu 1990, Iiada and others
1991, van Renterghem and others 1991, Ueno and others 1996,). These studies have
reported considerable variation in the proportion of animals excreting L.
monocytogenes in their faeces, from 0% in cats (Iida and others 1991) to 64% in sheep
(Gronstol 1979b). The proportions reported for cattle vary between 1.9% (Iida and
others 1991) and 51% (Skovgaard and Morgen 1988). However little attention has been
paid to rigorous epidemiological sampling strategies in the design of these studies.
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