an epidemiological study of listeriosis in dairy cattle

with others (Kanuk and Berenson 1975, Vaillancourt and others 1991). Farmer
questionnaires are particularly useful when the disease is easily visible or has distinct
clinical signs. In previous studies we have taken advantages of this to estimate the
prevalence of blowfly strike (French and others 1992) and Johne`s disease (Cetinkaya and
others 1996). Listeriosis differs from these diseases by manifesting itself by three different,
rarely overlapping, syndromes; encephalitis, septicaemia and abortion. In spite of this we
considered that three aspects of listeriosis would make it suitable for a farmer based
questionnaire; its sporadic occurrence, the likelihood that a veterinarian would be involved
in diagnosis and its distinctive name (reviewed by Radostits and others 1994).
A key component in the use of questionnaires as measuring instruments is the
repeatability and validity of these measures.
In this study we have no measure of repeatability but we attempted to validate the
questionnaire in three ways; by asking for the method of diagnosis and stratifying the
results to include only those cases diagnosed by a veterinarian or V.I.C.; by asking farmers
to identify the clinical signs of listeriosis from a list which included signs which were not
typical of listeriosis and by comparing the proportion of culled nervous cases of listeriosis
in our study with the proportion of nervous listeriosis diagnosed at necropsy during
statutory BSE reporting where cows showing any nervous disorder had to be culled during
the same period as our study covered. The majority of the cases were diagnosed by a
veterinarian or V.I.C.. On 83.9% of farms reporting cases between July 1994 and June 1995
diagnoses were made by veterinarian or V.I.C.. When these data were used to calculate the
farm prevalence, the results were within the confidence limits of the overall estimates and
did not statistically differ from them. When we asked farmers to identify clinical signs of
listeriosis the sensitivity of farmers reporting a single correct clinical sign was low ( range
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