Veterinary Epidemiology and Zoonosis

CASE-CONTROL STUDY

• Case-control study is also referred to as case-comparison study, retrospective study and case history.
• Case-control study is an observational study, that means looking backwards.
• Compares a group of diseased animals with a group of healthy animals with respect to exposure to hypothesized causal factors.
  • Example: Occurrence of Fascioliosis with respect to exposure.
• Comparing individuals having the disease (cases) and those not having the disease (controls) for identifying the suspected cases or factors from the past records.
• It may be conducted with new cases or existing cases (incidence or prevalence).
• There are three important features of this study
  • The exposures to a factor or factors and outcome (disease) have occurred before the start of this study.
  • The study proceeds backwards from effect to cause.
  • It helps to know the incidence rate.

Basic steps

• The four basic steps in the conduct of case-control study are,
  • Selection of cases and control
  • Matching
  • Measurement of exposure
  • Analysis and interpretation
• Selection of cases and control
  • As per the design of the case-control study the cases and controls are properly selected from the population by using hospital data and also from the general population.
• Matching
  • It means the individuals or animals in the case group and the control group should be almost identical regarding the age, sex, breed etc. and the only difference is that the case group contains the individuals suffering from the disease and the control group contains individuals not suffering from the disease.
  • The number of individuals in the control group must be at least or more than one per case group.
• Measurement of exposure
  • Usually this information is obtained by interviews, questionnaire and studying the past records.
• Analysis and interpretation
  • Analysis of data is made to know
  • Exposure rates among the cases and controls to a suspected factor
  • Estimation of disease associated with exposure (odds ratio)

Advantages

• Limited amount of effort.
• Limited time involvement.
• Inexpensive.
• Useful for identifying the suspected cases and for the formation of hypothesis and its testing.
• Useful for studying chronic or rare cases.
• No risk to the subject.

Disadvantages

• Past events may not be available.
• Biased report due to incorrect diagnosis.
• Incomplete information to some variables.
• Incidence rate cannot be calculated due to non-availability of population at risk.
• Exposed and unexposed populations in target populations cannot be estimated.
• Selection of an appropriate control group may be difficult.
• Not suited for the evaluation of therapy or prophylaxis of disease.