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. Details on study designs appropriate for these investigations are given elsewhere. Conceptually, this involves investigating the disease experience amongst different 'groups' of animals within an overall population, distinguished according to the factor(s) of interest. These factors can be classified as one of the components of the 'epidemiological triad' of Host, Agent and Environment, many of which are closely interrelated with each other:

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.

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 infectivity (the ability of the agent to establish infection), 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). It is important to note that many of these characteristics are as much characteristics of the host as of the agent, and indeed this relationship is often dynamic - with host characteristics impacting upon agent characteristics and vice versa.

Environment factors

This category includes a wide variety of factors which are neither directly associated with host or agent characteristics, and encompasses animal husbandry, climatic and geographical features, amongst others. Again, the relationship with other factors is dynamic, and environmental characteristics such as the population density of susceptible animals the availability of water sources, and the ambient temperature can all have a large impact on host and agent characteristics.

Causation

The ultimate aim of most analytic studies is to demonstrate evidence of a causative association between a factor of interest and a disease. Definitively establishing a causal connection between an exposure and disease is very difficult - indeed, it is considered to be conceptually impossible by philosophers. , Additionally, the complex interplay between different host, agent,and environmental factors, as well as many diseases being multifactorial in nature makes the identification and interpretation of possible 'causative' factors difficult. In order to assist the investigation of causation, risk factors for disease (component causes) may be classified as necessary and sufficient causes. A necessary cause is one which is required for disease to occur - i.e. it is present in every case of disease (this is commonly the case with transmissible diseases, where a pathogenic agent is often identified as the necessary cause). A sufficient cause is a combination of component causes which would result in disease, even if the individual components alone will not, and may or may not include necessary causes.

Analytic studies

Analytic studies include cohort, case-control and cross sectional studies, and may investigate possible associations between risk factors and disease by either comparing the risk factor exposure status in animals with disease to those without, or by comparing 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 for disease. Although it is important to carefully consider the source population when undertaking these studies, the results may be able to be extrapolated out to a wider population than for descriptive studies. One example is that of the association between smoking and lung cancer - although a descriptive study of the prevalence of swine influenza amongst pigs in the UK is unlikely to be of much use in estimating the prevalence amongst pigs in the USA, an analytic study investigating risk factors for infection may (or may not) be similar. When considering whether or not results can be extrapolated out to a larger 'target' population, host, agent and environmental factors (such as breeds of pigs, strains of virus, farming systems and wildlife exposure) should be considered.