Measure of disease_occurence

Describing Describing Disease Disease

OccurrenceOccurrence

Disease Disease occurrence/magnitudeoccurrence/magnitude

Fundamental task of the Fundamental task of the epidemiologist is to express the epidemiologist is to express the amount and temporal and spatial amount and temporal and spatial distribution of disease in animal distribution of disease in animal populationspopulations

Population Population

Group of people with a common characteristic like age, race, sex

Quantify disease occurrence in a populationQuantify disease occurrence in a population

Measures of disease frequency should take into Measures of disease frequency should take into account:account:

Number of individuals affected with the diseaseNumber of individuals affected with the disease

Size of source populationSize of source population

Length of time the population was followed Length of time the population was followed

Disease FrequencyDisease Frequency

Different levels of disease occurrenceDifferent levels of disease occurrence EndemicEndemic EpidemicEpidemic Pandemic Pandemic SporadicSporadic OutbreakOutbreak

Endemic occurrenceEndemic occurrence ‘‘Endemic’ is used in two senses to Endemic’ is used in two senses to

describe the usual frequency of describe the usual frequency of occurrence of a disease in a population. occurrence of a disease in a population. The constant presence of a disease in a The constant presence of a disease in a

population. population. The term ‘endemic’ can be applied not only The term ‘endemic’ can be applied not only

to overt disease but also to disease in the to overt disease but also to disease in the absence of clinical signs and to levels of absence of clinical signs and to levels of circulating antibodies. circulating antibodies.

HyperendemicHyperendemic When a disease is continuously present to a When a disease is continuously present to a

high level, affecting all age-groups equally, it high level, affecting all age-groups equally, it is hyperendemic. is hyperendemic.

Example: Example: ‘Endemic’ is applied not only to ‘Endemic’ is applied not only to infectious diseases but also to non-infectious infectious diseases but also to non-infectious ones: the veterinary meat hygienist is just as ones: the veterinary meat hygienist is just as concerned with the endemic level of carcass concerned with the endemic level of carcass bruising as is the veterinary practitioner with bruising as is the veterinary practitioner with the endemic level of pneumonia in cattle. the endemic level of pneumonia in cattle.

When endemic disease is described, the When endemic disease is described, the affected population and its location affected population and its location should be specified. should be specified.

Epidemic occurrenceEpidemic occurrence

‘‘Epidemic’ describes a sudden, usually Epidemic’ describes a sudden, usually unpredictable, increase in the number of unpredictable, increase in the number of cases of an infectious disease in a cases of an infectious disease in a population. population.

Epidemic is an occurrence of an infectious Epidemic is an occurrence of an infectious or non-infectious disease to a level in or non-infectious disease to a level in excess of the expected (i.e., endemic) level. excess of the expected (i.e., endemic) level.

Epidemic occurrenceEpidemic occurrence Example:Example: Cattle grazed on rough pasture, Cattle grazed on rough pasture,

which could abrade their mouths; there which could abrade their mouths; there might be an increase in the number of might be an increase in the number of cases of actinobacillosis. Although only 2% cases of actinobacillosis. Although only 2% of the animals might become infected, this of the animals might become infected, this would be an unusually high (epidemic) would be an unusually high (epidemic) level compared with the endemic level of level compared with the endemic level of 1% in the herd. 1% in the herd.

Thus, an epidemic need not involve a Thus, an epidemic need not involve a large number of individuals.large number of individuals.

………….continued.continued In epidemic, the population must have been In epidemic, the population must have been

subjected to one or more factors that were subjected to one or more factors that were not present previously. not present previously.

Some epidemics may go undetected for Some epidemics may go undetected for some time after their occurrence. some time after their occurrence.

Some epidemics may be exaggerated. Some epidemics may be exaggerated. Example:Example:

Increased number of deaths in foxes was apparently Increased number of deaths in foxes was apparently an epidemic of a ‘new’ fatal disease. Every laboratory an epidemic of a ‘new’ fatal disease. Every laboratory analyses & identified chlorinated hydrocarbon analyses & identified chlorinated hydrocarbon poisoning as the cause of the increased fox fatality, poisoning as the cause of the increased fox fatality, but only 40% of foxes submitted for postmortem but only 40% of foxes submitted for postmortem examination had died from the poisoning. examination had died from the poisoning.

Other 60% had died of endemic diseases that had not Other 60% had died of endemic diseases that had not previously stimulated general interest. Thus endemic previously stimulated general interest. Thus endemic level of diseases in a population has to be known level of diseases in a population has to be known before an epidemic can be recognized. before an epidemic can be recognized.

Pandemic occurrencePandemic occurrence

A pandemic is a widespread A pandemic is a widespread epidemic that usually affects a large epidemic that usually affects a large population. Many countries may be population. Many countries may be affected. Serious human pandemics affected. Serious human pandemics include; plague (the Black Death), include; plague (the Black Death), cholera, and influenza (spanish flu). cholera, and influenza (spanish flu).

Sporadic occurrenceSporadic occurrence

A sporadic outbreak of disease is one that A sporadic outbreak of disease is one that occurs irregularly and haphazardly. This occurs irregularly and haphazardly. This implies that appropriate, circumstances implies that appropriate, circumstances have occurred locally, producing small have occurred locally, producing small localized outbreaks. localized outbreaks.

Thus ‘sporadic’ can indicated either a single Thus ‘sporadic’ can indicated either a single case or a cluster of cases of a disease or case or a cluster of cases of a disease or infection (without obvious disease) that is infection (without obvious disease) that is not normally present in an area. not normally present in an area.

OutbreaksOutbreaks The Office International des Epizooties (OIE) The Office International des Epizooties (OIE)

defines an outbreak as ‘an occurrence of defines an outbreak as ‘an occurrence of disease in an agricultural establishment, disease in an agricultural establishment, breeding establishment or premises, including breeding establishment or premises, including all buildings as well as adjoining premises, all buildings as well as adjoining premises, where animals are present’, the term where animals are present’, the term generally kept as separated populations and generally kept as separated populations and so ‘outbreak’ can be applied unambiguously to so ‘outbreak’ can be applied unambiguously to an occurrence of disease on an individual an occurrence of disease on an individual farm.farm.

Outbreak Outbreak Example 1:Example 1: In UK during 2001, reported 2030 infected premises In UK during 2001, reported 2030 infected premises

(i.e., outbreaks), which all originated from a single (i.e., outbreaks), which all originated from a single infected pig farm. In contrast, the term sometimes infected pig farm. In contrast, the term sometimes also is used in the context of a single source, also is used in the context of a single source, irrespective of the number or premises involved.irrespective of the number or premises involved.

Example 2:Example 2: In the US in 2002-2003, exotic Newcastle disease In the US in 2002-2003, exotic Newcastle disease

occurred in 21 commercial flocks in California, and occurred in 21 commercial flocks in California, and over 1000 ‘backyard flocks’. This was documented over 1000 ‘backyard flocks’. This was documented as only one outbreak because it was considered to as only one outbreak because it was considered to have arisen from a single introduction of the disease. have arisen from a single introduction of the disease.

Definition of ‘outbreak’ may include criteria other Definition of ‘outbreak’ may include criteria other than the presence of clinical cases, and may be than the presence of clinical cases, and may be tailored to specific infections.tailored to specific infections.

The amount of disease is the The amount of disease is the morbidity morbidity (Latin: (Latin: morbusmorbus

= disease)= disease) the number of deaths is the the number of deaths is the mortalitymortality.. The times of occurrence of cases of a disease The times of occurrence of cases of a disease

constitute its constitute its temporal distributiontemporal distribution Places of occurrence comprise its Places of occurrence comprise its spatial spatial

distributiondistribution.. The measurement and description of the size of The measurement and description of the size of

populations and their characteristics constitute populations and their characteristics constitute demography demography (Greek: demo= people; -graphia = (Greek: demo= people; -graphia = writing, description).writing, description).

Disease quantificationDisease quantification

A necessary part of the investigation of A necessary part of the investigation of disease in a population is the counting of disease in a population is the counting of affected animals so that the affected animals so that the amount amount of of disease can be described. disease can be described.

Furthermore, it is usually desirable to Furthermore, it is usually desirable to describe when and where disease occurs, describe when and where disease occurs, and to relate the number of diseased and to relate the number of diseased animals to the size of the animals to the size of the population at risk population at risk of developing disease so that a disease’s of developing disease so that a disease’s importance can be assessed.importance can be assessed.

……..continued..continued Example:Example: A report of 10cases of infectious A report of 10cases of infectious

enteritis in a cattery, for example, does not enteritis in a cattery, for example, does not indicate the true extent of the problem indicate the true extent of the problem unless the report is considered in terms of unless the report is considered in terms of the number of cats in the cattery: there the number of cats in the cattery: there may be only 10 cats present, in which case may be only 10 cats present, in which case all of the cats are affected, or there may be all of the cats are affected, or there may be 100 cats, in which case only a small 100 cats, in which case only a small proportion of the cats is affected. proportion of the cats is affected.

The structure of animal populationsThe structure of animal populations The structure of population influences The structure of population influences

the extent to which the size of the the extent to which the size of the population at risk can be assessed, as population at risk can be assessed, as well as affecting the ways in which well as affecting the ways in which disease occurs and persists in animals. disease occurs and persists in animals.

Contiguous Contiguous SpearatedSpearated

Contiguous populationsContiguous populations In which there is much contact between In which there is much contact between

individual in the population and members of individual in the population and members of other populations. Contiguous population other populations. Contiguous population therefore predispose to transfer and therefore predispose to transfer and persistence of infectious diseases over large persistence of infectious diseases over large areas because of the inherent mixing and areas because of the inherent mixing and movement of animals. Populations of small movement of animals. Populations of small domestic animals also are usually contiguous. domestic animals also are usually contiguous. Dogs and cats that are not confined to houses Dogs and cats that are not confined to houses moves freely within cities, coming into contact moves freely within cities, coming into contact with other urban, suburban and rural animals with other urban, suburban and rural animals of their own and different species. of their own and different species.

Separated populationsSeparated populations

Separated populations occur as discrete Separated populations occur as discrete units such as herds and flocks. They are units such as herds and flocks. They are particularly common in countries that particularly common in countries that practice intensive animal production, practice intensive animal production, with many animals on one farm.with many animals on one farm.

Close separated populationClose separated population Open separated populationOpen separated population

Closed separated populationClosed separated population A separated population can be A separated population can be

closed, with no movement of animals closed, with no movement of animals into or out of the unit (except to into or out of the unit (except to slaughter). An example is a diary slaughter). An example is a diary herd raises its own replacements. herd raises its own replacements.

Open separated populationOpen separated population

A separated population can also be open, with A separated population can also be open, with limited movement of individuals in and out. limited movement of individuals in and out. Examples include beef herds where animals are Examples include beef herds where animals are brought in from other farms and markers for brought in from other farms and markers for fattening, and dairy herds where animals are fattening, and dairy herds where animals are brought in from other farms and markets for brought in from other farms and markets for fattening, and diary herds that receive fattening, and diary herds that receive replacements from other farms. replacements from other farms.

Separated populations, especially of the closed Separated populations, especially of the closed type, are less likely to be infected with agents from type, are less likely to be infected with agents from other areas than contiguous populationsother areas than contiguous populations

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