Analytic epidemiological studies

Analytic epidemiological studies aim to investigate and identify factors associated with the presence of disease within populations, through the investigation of factors which may vary between individual members of these populations. These factors can be classified as one of the components of the 'epidemiological triad' of Host, Agent and Environment:

Host factors

These include all the characteristics of the individual animal which affect the occurrence of disease, and include 'innate' characteristics such as sex, breed, genetics or species, as well as 'acquired' characteristics such as age, nutritional status, previous exposure to pathogens and stage of pregnancy. Age is a common example of a host factor which can impact upon disease susceptibility - for example, older animals

Agent factors

Of course, these factors are only applicable in the case of diseases with causative or associated agents (which may include prions, viruses, bacteria, protozoa and transmissible cancers). Features of the agent which affect the levels of disease within an infected population include pathogenicity (the ability of the agent to produce disease) and virulence (the severity of the resultant disease), along with characteristics of the life cycle of the pathogen such as incubation period (the time between infection and disease), latent period (the time between infection and infectiousness) and the infectious period (the duration over which the pathogen can be transmitted to others).

Environment factors

This category includes a wide variety of factors, encompassing managemental, climatic and ecological features.

Analytic studies

Analytic studies may compare exposure to these factors in animals with disease to those without, or may compare the occurrence of disease amongst 'exposed' animals to 'unexposed' animals. Although there are considerable difficulties in establishing causation of disease, the results of these studies can provide useful evidence regarding possible risk factors.