Page 452 - The Veterinary Laboratory and Field Manual 3rd Edition

chapter 9

Epidemiology

9.1 Introduction to epidemiology good coordination between laboratory and field
services if the data gathered is to represent the
Susan C. Cork and Sylvia Checkley
true ‘field’ situation. Field based animal health
Regional and district field programmes play a staff and laboratory professionals should be able
crucial role in gathering routine data for epide- to appreciate the principles of epidemiologi-
miological purposes and can provide important cal studies and to understand the terms used.
information on endemic and emerging diseases. Passive surveillance involves the use of data
Epidemiological data is used to inform decision collected for other means, for example, routine
making at the regional and national level and laboratory data or notifiable disease reporting.
may also be required for the purposes of interna- In order to apply this data more broadly, an
tional reporting (see Chapter 10). Unfortunately, understanding of the type of reporting, and the
field data is often unreliable and/or restricted rationale for selecting the samples submitted, is
due to lack of resources at the extension level necessary.
and a lack of diagnostic services to confirm a Epidemiology is the study of the ‘patterns
diagnosis. However, as a result of concerns sur- of disease’, that is, the frequency, distribution
rounding the global spread of highly pathogenic and determinants of health and disease in a
avian influenza (H5N1) in 2003 a concerted population of animals. The unit of concern in
effort was made to improve veterinary services, epidemiological studies is not the individual
including diagnostic support, at the district level animal but rather the group (pack, herd, flock),
in many countries. This was to ensure early category (age group, sex, breed) or an entire pop-
disease detection, reliable reporting and rapid ulation (cattle in a region, district or country). In
response to safeguard the health of animals and some cases, the disease characteristics identified
the people who rely on them for their liveli- through epidemiology can be used as a diagnos-
hoods. This has resulted in a better network of tic tool to recognize specific diseases due to the
disease reporting across the globe but there are way the disease spreads, the nature and dura-
still many rural and more remote areas where tion of the clinical signs and the outcome. For
the veterinary infrastructure remains limited or example, the now eradicated rinderpest (cattle
non-existent. plague) historically killed a large number of
The setting for most epidemiological work is animals rapidly (that is, an acute disease with a
the ‘field’, that is, where the samples are col- high mortality) and spread very quickly whereas
lected (farm, veterinary clinic and so on) and bovine tuberculosis may spread slowly and
to a lesser extent, the laboratory where the causes illness but not necessarily death until the
samples are analysed. Active surveillance and later stages of infection (that is, a chronic disease
retrospective epidemiological studies rely on with morbidity and mortality late in the disease).

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