an epidemiological study of listeriosis in dairy cattle

There were no sample size calculations and the studies involved different study
populations. Some studies involved sampling from slaughterhouses (Hofer 1983, van
Renterghem and others 1991) others from freshly excreted cow pats on the farm
(Skovgaard and Morgen 1988). In only three studies was the source of the organism
investigated (Skovgaard and Morgen 1988, Husu 1990, Ueno and others 1995) and in
only two of these was it done by comparing environmental and animal isolates of L.
monocytogenes on the same farms using serotyping and phagetyping methods. The
same strains have been identified in both environmental and animal isolates (Skovgaard
and Morgen 1988 and Ueno and others 1995). But the usefulness of these techniques
has already been disputed by some researchers (Seeliger and Hohne 1979, Ralovich
1993).
Animal derived foodstuffs such as soft cheese have been implicated in human
Listeriosis in England (McLauchlin 1996) and studies have been carried out to
determine the contamination of these products with L. monocytogenes (Greenwood and
others 1991, MacGowan and others 1994). But there have been no studies to determine
carriage rate of Listeria spp. and L. monocytogenes in dairy cattle in England and little
is known about the contamination of their environment.
In a pilot study carried out between September 1994-August 1995 involving
only one dairy farm, a large proportion of animals were found to be excreting Listeria
spp. and L. monocytogenes in their faeces (Chapter 5). However it is known that
differences occur in excretion rate within and between farms (Kampelmacher and van
Noorle Jansen 1969, Skovgaard and Morgen 1988). A longitudinal study was therefore
designed to include five dairy farms in order to investigate the infection rate of L.
monocytogenes in individual milking cows over a 10 month period. Infection was
assessed using bacteriological and serological methods. Factors which might be
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