an epidemiological study of listeriosis in dairy cattle

Since elimination of L. monocytogenes from the farm environment is not
possible due to its ubiquitous occurrence in nature, the lack of reliable and rapid
methods of detecting the organism when it is present in low numbers and the lack of
understanding of epidemiology of Listeriosis and L. monocytogenes infection, attempts
can only be made to prevent Listeria organisms from multiplying to the level of an
infectious dose, to minimise its presence in the farm environment by improving hygiene
and cleanliness of the farm, and to minimise its intake by animals by preparing
foodstuffs such that L. monocytogenes does not grow. The epidemiology of Listeriosis
in animals is not fully understood (Gray and Killinger 1966, reviewed by Radostits and
others 1994, Donachie and Low 1995) therefore risk factors other than silage feeding
are not known. Where silage is implicated some recommendations can be made. The
proportion of silage in the ration can be reduced, silage feeding can be introduced to the
animals gradually and more attention can be paid to silage making. Spoiled and mouldy
silage should be removed from the feed. When making silage, additives should be used,
soil contamination should be avoided and the silo or clamp should be sealed off as
quickly as possible (Fenlon 1988, reviewed by Radostits and others 1994). It has been
reported that improvement in silage making resulted in a decrease in the incidence of
Listeriosis in Holland (Dijsktra 1986). However, disease has also been reported in some
parts of the world where silage feeding is not practised (Vandegraaff and others 1981,
Aslan and others 1991, Meredith and Schneider 1984). In such circumstances better
farm management practices, such as improvement of nutritional status of animals and
better housing conditions, can also be of some value in preventing disease.
Vaccination: Several attempts have been made to immunise animals against Listeriosis
using killed or live L. monocytogenes. The results obtained using a vaccine prepared
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