an epidemiological study of listeriosis in dairy cattle

dairy calves in July 1994 and June 1995, the remaining parts collected information on farm
level variables which might influence the occurrence of listeriosis. This included questions
on feeding, (type, preparation, storage, feeding regime), housing, (season of housing, type
of housing, type of bedding, storage of bedding, cleaning of house) and general information
(cases of listeriosis in other species of animals, vaccine use etc.).
Pilot questionnaires were tested by sending them to 9 dairy farmers on the 5th of
August, 1995. The questionnaire, covering letter, and a stamped addressed envelope were
sent to farmers on the 25th of August, 1995. Two reminder post cards and second copy of
the questionnaire were sent to non-respondents and a final fourth reminder letter was sent to
the remaining non-respondents on the 23rd of December, 1995 (Appendix 1).
Questionnaires returned after the 12th of January, 1996 were not included in the study.
2. 2. 4. Data Analysis
Two estimates of the frequency of listeriosis were calculated in this study; the farm
level prevalence and the crude within herd incidence rate. They were estimated from
clinical cases reported between July 1994 and June 1995 for three groups of dairy cattle;
milking cows, replacement heifers and dairy calves. For the same groups, the incidence
rates were recalculated for two clinical signs; silage eye and nervous signs.
To calculate the within herd incidence rate the mean numbers of milking cows,
replacement heifers and dairy calves reported in July 1994 and June 1995 were used to
estimate the number of animals at risk in each group. The within herd incidence rate of
listeriosis was calculated separately for herds reporting clinical disease and for all herds.
Respondents who did not know if they had had any cases of listeriosis in any of these
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