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Measure of disease_occurence


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Measure of disease_occurence

  1. 1. Describing Disease Occurrence
  2. 2. Disease occurrence/magnitude Fundamental task of the epidemiologist is to express the amount and temporal and spatial distribution of disease in animal populations
  3. 3. Population Group of people with a common characteristic like age, race, sex
  4. 4.  Quantify disease occurrence in a population  Measures of disease frequency should take into account:  Number of individuals affected with the disease  Size of source population  Length of time the population was followed Disease Frequency
  5. 5.  Different levels of disease occurrence  Endemic  Epidemic  Pandemic  Sporadic  Outbreak
  6. 6. Endemic 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 population.  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 circulating antibodies.
  7. 7. Hyperendemic  When a disease is continuously present to a high level, affecting all age-groups equally, it is hyperendemic.  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 be specified.
  8. 8. Epidemic occurrence  ‘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.
  9. 9. Epidemic occurrence  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.
  10. 10. …….continued  In epidemic, the population must have been subjected to one or more factors that were not present previously.  Some epidemics may go undetected for some time after their occurrence.  Some epidemics may be exaggerated. Example:  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.
  11. 11. Pandemic occurrence  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 flu).
  12. 12. Sporadic occurrence  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.
  13. 13. Outbreaks  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.
  14. 14. Outbreak  Example 1:  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 premises involved.  Example 2:  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 infections.
  15. 15.  The amount of disease is the morbidity (Latin: morbus  = disease)  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 = writing, description).
  16. 16. Disease quantification  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 described.  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.
  17. 17. …..continued  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 affected.
  18. 18.  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.  Contiguous  Spearated
  19. 19. Contiguous populations  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.
  20. 20. Separated populations  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
  21. 21. 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 replacements.
  22. 22. 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
  23. 23. THANKS