Page 454 - The Veterinary Laboratory and Field Manual 3rd Edition

Epidemiology 391

a disease outbreak. Disease specific mortal- may vary within the population. When attend-
ity rate is the proportion of animals whose ing a disease outbreak, it is important that the
deaths can be attributed to a specific disease veterinary team carefully examine both sick and
(that is, confirmed by diagnostic tests). dead animals and to collect appropriate samples
for submission to the laboratory. In cases where
Morbidity rate: This is the proportion of
animals in a given population which develop laboratory testing, to confirm a diagnosis, is
clinical disease. Disease specific morbid- not available the veterinary and animal health
ity rate is the proportion of animals that extension teams will need to rely on clinical
develop clinical signs attributable to the experience and their knowledge of the diseases
disease under investigation and confirmed common to the area.
by diagnostic tests.
Incidence: This is the number of new 9.2 disease surveillance and
cases of a disease occurring in a specified reporting systems, resources and
period. The incidence rate is the number diagnostic laboratory services
of new cases of the disease reported in a
specific time (for example, per year or John Woodford
month) divided by the number of animals
in the population at risk. Incidence does not background
include cases of disease that were already
present in the population at the start of the Disease surveillance has become an increasingly
specified period. important core function of the state veterinary
Population at risk: This is the population services in most developing countries. The need
of animals which are identified as those to establish or strengthen existing active and
likely exposed to a disease or ‘event’. passive disease surveillance and reporting sys-
tems has been driven by several different factors
Probability: Is the likelihood of an event over the recent past. Initially, throughout the
occurring. implementation of the global programme for the

(Case) fatality rate: This is usually expressed eradication of rinderpest, planning groups relied
as a percentage and is the number of animals heavily on analysis of both active and passive dis-
diagnosed as having a specific disease that ease surveillance data at regional, national and
subsequently die as a result of that disease. sub-national levels in order to target the limited
This term is usually applied in an outbreak resources available for prevention and control
of an acute disease in which all cases are programmes more efficiently. In the final stages
monitored for an adequate period of time to of the eradication programme, passive disease
include all attributable deaths. The fatality reporting and active disease surveillance, using
rate gives a measure of the severity of the dis- participatory disease surveillance (PDS) tech-
ease and is not the same as the mortality rate. niques, became key tools used to search for and
detect disease outbreaks, especially in the more
It can be understood from the above definitions remote and inaccessible areas of central and east
that the data submitted to an epidemiology unit Africa, where the last pockets of wild circulating
will only be of value if the correct diagnosis has virus remained. As the disease was progressively
been made. In many cases more than one dis- brought under control, all countries participating
ease may be present and the clinical presentation in the joint Food and Agriculture Organization

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