Page 453 - The Veterinary Laboratory and Field Manual 3rd Edition

390 Susan C. Cork

Disease determinants are those factors that causes of the disease outbreak(s) have been
influence health and disease through com- determined it is then important to examine sick
plex interactions, varying with the age of the and dead animals and their environment and to
animal(s) and with different stages of manage- collect appropriate samples for laboratory test-
ment and husbandry. In epidemiology, the term ing to confirm the diagnosis (see also Chapter
can be used to describe any factor which, when 1.5 Clinical examination, sample selection, sub-
altered, produces a change in the frequency or mission and clinical diagnosis). The following
characteristics of a disease. Few diseases have a definitions are provided to assist in building up
single cause and the development and progres- an epidemiological vocabulary for laboratory and
sion of clinical signs can depend on a wide range animal health extension staff.
of intrinsic and extrinsic factors, for example, the
age and physiological state of the herd/flock, the Endemic: The constant presence of a dis-
management of the animals, changes in weather, ease or infectious agent within a given
recent history of movement to a new region, geographic area. It may refer to the usual
nutritional status or the presence of concurrent prevalence of a given disease within an area
disease. Some of the questions most often asked or a relatively new but constant situation.
in an epidemiological study, or during a disease The term enzootic may also be used when
outbreak investigation follow. discussing disease in animals.
Epidemic: The occurrence in a country
1 Which animals (species, breed, age, sex) are or region of cases of an illness (or disease
affected? outbreak) clearly in excess of the expected
2 How many animals are sick (morbidity) and level. The number of cases occurring in an
how many have died (mortality)? epidemic will vary according to the cause,
3 When did the disease start and what were that is, the infectious agent involved and its
the initial and subsequent clinical signs? method of transmission, the size and type
4 Have there been any previous health prob- of population exposed, previous exposure to
lems and what are the routine preventive the disease agent and the time and place of
programmes (for example, vaccination, occurrence. Even a single case of a disease
worming)? which has not been recently recognized in an
5 Have there been any recent changes in diet/ area requires prompt reporting and epide-
management? miological investigation to prevent further
6 Have any new stock been recently purchased? cases. The term ‘epizootic’ can also be used.
7 Are there any signs of the disease on neigh-
bouring farms or in wild animals? Prevalence: This is the number of animals
8 Are any people sick with the same disease? which have a disease (diagnosed on the
9 Is there a seasonal occurrence (that is, are basis of clinical signs or by laboratory tests)
disease outbreaks associated with changes in a stated population at a given time. This
in climatic conditions and the presence of is regardless of when the illness began. The
disease vectors such as ticks)? prevalence rate is defined as the number
10 Did the disease outbreak occur concurrently of positive cases divided by the number of
with wildlife migration? animals in the population tested at a given
point in time.
The subsequent action will depend on the likely Mortality rate: This is the proportion of ani-
cause(s) of the disease outbreak. Once the likely mals in a given population which die during

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