Fundamental task of the epidemiologist is
to express the amount and temporal and
spatial distribution of disease in animal
Group of people with a common
characteristic like age, race, sex
Quantify disease occurrence in a population
Measures of disease frequency should take
Number of individuals affected with the
Size of source population
Length of time the population was followed
Different levels of disease occurrence
‘Endemic’ is used in two senses to
describe the usual frequency of
occurrence of a disease in a population.
The constant presence of a disease in a
The term ‘endemic’ can be applied not only to
overt disease but also to disease in the
absence of clinical signs and to levels of
When a disease is continuously present to a high
level, affecting all age-groups equally, it is
Example: ‘Endemic’ is applied not only to
infectious diseases but also to non-infectious ones:
the veterinary meat hygienist is just as concerned
with the endemic level of carcass bruising as is the
veterinary practitioner with the endemic level of
pneumonia in cattle.
When endemic disease is described, the
affected population and its location should
‘Epidemic’ describes a sudden, usually
unpredictable, increase in the number of cases
of an infectious disease in a population.
Epidemic is an occurrence of an infectious or
non-infectious disease to a level in excess of
the expected (i.e., endemic) level.
Example: Cattle grazed on rough pasture,
which could abrade their mouths; there might
be an increase in the number of cases of
actinobacillosis. Although only 2% of the
animals might become infected, this would be
an unusually high (epidemic) level compared
with the endemic level of 1% in the herd.
Thus, an epidemic need not involve a large
number of individuals.
In epidemic, the population must have been
subjected to one or more factors that were not
Some epidemics may go undetected for some
time after their occurrence.
Some epidemics may be exaggerated.
Increased number of deaths in foxes was apparently an
epidemic of a ‘new’ fatal disease. Every laboratory
analyses & identified chlorinated hydrocarbon poisoning
as the cause of the increased fox fatality, but only 40%
of foxes submitted for postmortem examination had died
from the poisoning.
Other 60% had died of endemic diseases that had not
previously stimulated general interest. Thus endemic
level of diseases in a population has to be known before
an epidemic can be recognized.
A pandemic is a widespread epidemic that
usually affects a large population. Many
countries may be affected. Serious human
pandemics include; plague (the Black
Death), cholera, and influenza (spanish
A sporadic outbreak of disease is one that
occurs irregularly and haphazardly. This implies
that appropriate, circumstances have occurred
locally, producing small localized outbreaks.
Thus ‘sporadic’ can indicated either a single case
or a cluster of cases of a disease or infection
(without obvious disease) that is not normally
present in an area.
The Office International des Epizooties (OIE)
defines an outbreak as ‘an occurrence of disease
in an agricultural establishment, breeding
establishment or premises, including all buildings
as well as adjoining premises, where animals are
present’, the term generally kept as separated
populations and so ‘outbreak’ can be applied
unambiguously to an occurrence of disease on
an individual farm.
In UK during 2001, reported 2030 infected premises (i.e.,
outbreaks), which all originated from a single infected pig
farm. In contrast, the term sometimes also is used in the
context of a single source, irrespective of the number or
In the US in 2002-2003, exotic Newcastle disease occurred
in 21 commercial flocks in California, and over 1000
‘backyard flocks’. This was documented as only one
outbreak because it was considered to have arisen from a
single introduction of the disease.
Definition of ‘outbreak’ may include criteria other than the
presence of clinical cases, and may be tailored to specific
The amount of disease is the morbidity (Latin: morbus
the number of deaths is the mortality.
The times of occurrence of cases of a disease constitute
its temporal distribution
Places of occurrence comprise its spatial distribution.
The measurement and description of the size of
populations and their characteristics constitute
demography (Greek: demo= people; -graphia =
A necessary part of the investigation of disease
in a population is the counting of affected
animals so that the amount of disease can be
Furthermore, it is usually desirable to describe
when and where disease occurs, and to relate
the number of diseased animals to the size of
the population at risk of developing disease so
that a disease’s importance can be assessed.
Example: A report of 10cases of infectious
enteritis in a cattery, for example, does not
indicate the true extent of the problem unless
the report is considered in terms of the number
of cats in the cattery: there may be only 10 cats
present, in which case all of the cats are
affected, or there may be 100 cats, in which
case only a small proportion of the cats is
The structure of animal populations
The structure of population influences the
extent to which the size of the population
at risk can be assessed, as well as
affecting the ways in which disease occurs
and persists in animals.
In which there is much contact between
individual in the population and members
of other populations. Contiguous
population therefore predispose to
transfer and persistence of infectious
diseases over large areas because of the
inherent mixing and movement of animals.
Populations of small domestic animals also
are usually contiguous. Dogs and cats that
are not confined to houses moves freely
within cities, coming into contact with
other urban, suburban and rural animals of
their own and different species.
Separated populations occur as discrete
units such as herds and flocks. They are
particularly common in countries that
practice intensive animal production, with
many animals on one farm.
Close separated population
Open separated population
Closed separated population
A separated population can be closed,
with no movement of animals into or out
of the unit (except to slaughter). An
example is a diary herd raises its own
Open separated population
A separated population can also be open, with limited
movement of individuals in and out. Examples include
beef herds where animals are brought in from other
farms and markers for fattening, and dairy herds where
animals are brought in from other farms and markets for
fattening, and diary herds that receive replacements
from other farms.
Separated populations, especially of the closed type, are
less likely to be infected with agents from other areas
than contiguous populations