an epidemiological study of listeriosis in dairy cattle

from 3.1% to 67.2%) but it was high when three correct signs were examined (96.1%). This
may indicate that the farmers were poor at self diagnosis or alternatively they were only
reporting the clinical signs seen in cases on their farm. On the majority of farms only one
type of clinical sign was seen. However, in view of the fact that there was no significant
difference between the overall proportion and those estimates based on veterinarian and
veterinary investigation centre diagnoses we do not consider that the farmers’ misdiagnoses
introduced bias into the results. The failure of farmers to recognise clinical signs other than
nervous disease and to call a veterinarian may have resulted in an underestimation of true
prevalence. As a further validation measure the proportion of cattle with nervous signs
culled or died in this survey was compared with confirmed Listeria cases submitted to CVL
as suspect BSE cases. Those proportions were remarkably similar. The close similarity of
the results adds credence to the results of this survey.
The influence of non-respondents was not measured but an effort was made to
maximise the response rate by sending a number of reminders to the farmers. The 64.1%
response rate was good for the size of samples used in this study (Vailloncourt and others
1991). However, when compared with the two recent surveys conducted by our group in
which response rates were 74.2% and 78.3% (French and others 1992, Cetinkaya and others
1996) the response rate was lower. The difference in the length, format and content of the
questionnaire might have contributed to this lower response rate (del Garso and Wallop
1975). Our questionnaire consisted of 8 pages and 50 questions, where as Cetinkaya and
others (1996) used 39 questions on 5 pages and French and others (1992) used 15 questions
on 2 pages.
At farm level, Listeriosis in milking cows was reported more frequently than in
other groups of animals. However within the affected herds the highest incidence of disease
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