an epidemiological study of listeriosis in dairy cattle

In the second part of the study a longitudinal survey of five dairy farms was
carried out to determine the infection rate with Listeria monocytogenes and its
behaviour in the environment, individual cows, pastures, soil, water sources, forage and
bulk milk tanks on the five farms monitored bacteriologically and serologically between
August 96 and May 97. The infection rate varied between farms and months. Two
patterns of infection rate were observed. On 3 farms the highest prevalence of infection
rates were obtained between November and April, around 90% of animals excreted L.
monocytogenes in their faeces during this period whereas on the other 2 farms the
infection rate was lower, around 30% of animals excreted L. monocytogenes (that was
only in March). An ELISA assay employing Listeriolysin O (LLO) was used to
determine seroconversion before, during and after silage feeding and winter housing.
Almost all animals examined on each farm had anti-LLO antibodies to L.
monocytogenes before silage feeding and housing. The antibody level remained
unchanged throughout the study with only a small number of animals exhibiting
changes on only one farm.
Three species of Listeria were isolated from the environmental samples (soil,
grass, silage, water, bedding). The commonest isolate was L. monocytogenes. L.
innocua was less common and the rarest was L. seeligeri. L. monocytogenes was
isolated from bulk milk tank on three farms. Random amplified polymorphic DNA
(RAPD) assays were used to identify the source of infection in dairy cows and to
determine the variation between the strains of different origin. In total 113 isolates (40
from the environment and 73 from the animals) were examined and 12 different RAPD
patterns were obtained. The results indicated that different “strains” exist between and
within farms. There also were similar patterns of RAPD between environmental and
animal strains.
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