Changes

=='''Environment factors'''==

=='''Environment factors'''==

This category includes a wide variety of factors which are neither directly associated with host or agent characteristics, and encompasses managemental, 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.

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'''==

=='''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, due to the complex interplay between different host, agent,and environmental factors, as well as many diseases being multifactorial in nature.  

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 impossible by philosophers<ref>Rothman, K.J. 'Causation and Causal Inference in Epidemiology', American Journal of Public Health, 2005, Vol 95, No. S1 [http://ajph.aphapublications.org/cgi/reprint/95/S1/S144]</ref>. , due to the complex interplay between different host, agent,and environmental factors, as well as many diseases being multifactorial in nature. In order to assist the investigation of causation, risk factors for disease may be classified as '''necessary''' or '''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

=='''Analytic studies'''==

=='''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 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 the prevalence of lung cancer in the UK is likely to be very different from that in Mongolia, the association between smoking and development of lung cancer in both of these countries is likely to be similar.  

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 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 the prevalence of lung cancer in the UK is likely to be very different from that in Mongolia, the association between smoking and development of lung cancer in both of these countries is likely to be similar.  

[[Category:Veterinary Epidemiology - Introduction|D]]

[[Category:Veterinary Epidemiology - Introduction|D]]