an epidemiological study of listeriosis in dairy cattle

associated with a decreased risk of disease as the floors (feeding passage) are cleaned
after feeding.
The association of grass silage with disease may also be explained by the length
of exposure to L. monocytogenes as an increased duration of grass silage feeding was
associated with an increased risk of disease.
Another finding was that purchased grass silage increased the risk of disease
whilst home made grass silage decreased the risk of reporting silage eye. This may be
explained by the fact that purchased silage would be mainly in the form of big bale
silage or if bought in as clamp silage might be of poor quality because it had been
exposed to air. Air in silage has been associated with an increase in Listeria growth
(Fenlon 1986, Woolford 1990, Fenlon and others 1995b). Alternatively farmers may
sell inferior quality or old silage which is surplus of their own needs.
Feeding straw during housing had a protective effect. Straw feeding may have
played a role by reducing silage intake therefore decreasing the exposure to L.
monocytogenes or the provision feed straw may have reduced the intake of dirty
bedding straw.
Wilesmith and Gitter (1986) suggested that housing animals did not have any
association with the occurrence of ovine Listeriosis. This was also the case in our study
although only a small number of farmers did not house their animals. When different
types of housing were evaluated it was found that housing cattle in buildings (mainly
cow sheds) other than cubicles and loose yards had a protective effect. It is difficult to
provide a biological explanation for this association, it may be due to a confounding
effect.
The use of straw as bedding material and straw bedding in cubicles was
associated with an increased risk of disease. The use of straw as bedding and straw
bedding in cubicles may result in a build up of L. monocytogenes in the environment.
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