an epidemiological study of listeriosis in dairy cattle

human Listeriosis and animal Listeriosis prompted us to investigate this disease and its
cause, L. monocytogenes , in more details in order to determine:
a) the frequency of Listeriosis in dairy cattle in England. The figures on prevalence and
incidence of clinical Listeriosis in dairy cattle were not known at the beginning of this
study. Listeriosis is not a notifiable disease and the only figures available are those
estimated from cases submitted to the Central Veterinary Laboratories (CVL) in most
cases for the diagnoses of other disease such as BSE and Brucellosis. Cases of
Listeriosis other than encephalitis are not usually reported to these centres. Therefore a
cross sectional study using a postal questionnaire was carried out to determine the
prevalence, incidence and some characteristics of clinical Listeriosis in dairy cattle in
England (Chapter 2).
b) risk factors associated with disease at the farm level. The relationship between silage
feeding and L. monocytogenes is established but the exact manner in which silage
plays a role is not known (Wilesmith and Gitter 1986, Gitter 1989). There is an
evidence that farm management may also be important (Meredith and Schneider 1984).
In the cross sectional study we also attempted to identify and test some hypothesises for
farm level risk factor(s) associated with the occurrence of clinical Listeriosis (Chapters
3 and 4).
c) infection rate in individual animals. Several studies have been carried out to
determine carriage status of this organism (Skovgaard and Morgen 1988, Husu 1990).
These studies lacked epidemiological and statistical design and differences were
detected in excretion rate between them. There have been no studies in the United
Kingdom aimed at determining the infection rate in dairy cattle. We therefore conducted
43