an epidemiological study of listeriosis in dairy cattle

groups were excluded from analysis. In calculating the within herd incidence rates for the
different clinical signs those cases where clinical signs were not reported were excluded
from the analysis.
Three alternatives were given for the diagnosis of the disease; veterinary surgeon,
veterinary investigation centre (V.I.C) or self diagnosis. In order to validate diagnosis by
farmers, they were asked to select the clinical signs of listeriosis from a list of eight clinical
signs, two commonly associated with listeriosis (nervous sign and silage eye-
keratoconjunctivitis), four occasionally associated with the disease (abortion, sudden death,
diarrhoea and mastitis) and the remaining two not linked to listeriosis (lameness,
pneumonia). The criterion validity (Abramson 1988) of “self diagnoses” was made by
comparing the reporting of clinical signs by farmers with the combination of veterinary
surgeon or V.I.C. diagnoses. The questionnaire was also externally validated using data on
the frequency of listeriosis collected by Central Veterinary Laboratory (CVL) during
statutory Bovine Spongiform Encephalopathy (BSE) reporting between July 1994 and June
1995.
Information about the month of illness, treatment and its outcome was also
collected. Cases reported to occur over a period of two or more months were divided
equally between the months in order to estimate their seasonal and monthly distribution.
Those cases where no month of diagnosis was given were excluded from analysis. Cases
where clinical signs and treatment were not indicated were also excluded from analysis.
2. 2. 5. Data processing and analysis:
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