an epidemiological study of listeriosis in dairy cattle

All data were numerically coded, entered onto a database (Microsoft Access 2,
Simpson 1994) and analysed using Epi-info version 6 (Dean and others 1994). A Yates
corrected chi squared test was used to compare the differences between proportions. A
Kruskal-Wallis test was used to compare the differences between median values (Dean and
others 1994). A probability of p< 0.05 was accepted as statistically significant.
2. 3. Results:
2. 3. 1. Response rate:
Of the 1500 dairy cattle farmers, 961 returned questionnaires, giving an overall
response rate of 64.1%. 67 of the 961 questionnaires were returned unanswered because the
farmers no longer kept dairy cattle (36 farms) or were unwilling to take part in the study.
These were removed from the study leaving 61.1% usable questionnaires (894/1464).
2. 3. 2. The prevalence of listeriosis at farm level:
Respondents who did not know whether they had clinical listeriosis in any of the
groups of dairy cattle were removed from analysis. 12.3% (93/759) reported clinical
listeriosis in dairy cattle on their farms between July 1994 and June 1995 . Cases were
diagnosed by a veterinary surgeon or V.I.C. on 83.9% (78/93) of the affected farms. When
the overall proportion of farms affected was estimated using only cases diagnosed by a
veterinarian or V.I.C., the proportion was 10.3%. This was not statistically different from
that of 12.3% (P=0.2).
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