an epidemiological study of listeriosis in dairy cattle

1989, Sargison 1993). It is also possible that the bags are subjected to physical damage
by birds, mice or other rodents when stored outside and that this results in rapid
deterioration (Fenlon 1985, Sargison 1993). Storing big bales outside uncovered was
associated with an increased risk of reporting Listeriosis in milking cows. Similarly the
association between storing hay outside covered and an increased risk of disease in
milking cows may be due to the fact that outside storage exposed hay to factors (wet,
cold, air and physical damage) that may have led to the propagation of L.
monocytogenes to the level of an infectious dose. But only small number of farms
reported storing their hay outside covered.
In this study feeding maize and grass silage, hay and straw in ring feeders or ad
libitum (indoor or outdoor), increased the risk of disease whereas feeding on the floor
decreased the risk. This may be an effect of the clinical forms of the disease reported in
this study. The predominant clinical sign was silage eye (iritis). Although the exact
relationship between L. monocytogenes and silage eye awaits further studies, the
association between silage feeding and silage eye cases has been acknowledged
(Morgan1977, Watson 1989, Mee and Rea 1989, Sargison 1993, Bee 1993, Welchman
and others 1997). It may well be that animals eating silage in ring feeders were at
greater risk of physically damaging their conjunctival membranes which may have led
to infection by L. monocytogenes (Sargison 1993, Welchman and others 1997). It is
also possible that encephalitis may follow eye infection (Asahi 1957, Dennis 1993).
Another explanation may be that in ring feeding systems animals are in very close
contact with a possibility of animal to animal spread. Alternatively unlike other
methods, ring feeders are not cleaned regularly and silage remaining in the feeder may
have created an ideal environment for L. monocytogenes to multiply. This suggestion is
supported with the finding that feeding forages (hay and grass silage) on the floor was
101