Veterinary Epidemiology and Zoonosis

Definition and Formulating a causal hypothesis

FORMULATING A CAUSAL HYPOTHESIS

• A proposition that can be tested formally and then the hypothesis may be supported or rejected.

Formulating a causal hypothesis

• The first step in any epidemiological investigation of cause is descriptive. Initially description of time, place and population is useful.

Time

• Association with year, season, month, day or even hour may provide information on climatic influence, incubation period and source of infection.
• For example, an outbreak of salmonellosis in a group of cattle may be associated with the introduction of infected cattle feed.

Place

• The geographical distribution of a disease may indicate an association with local geological, management or ecologi­cal factors, for example nutritionally deficient soil or arthropod transmitters of infection.

Population

• The type of animal that is affected is often of considerable importance. Hereford cattle are more susceptible to squamous cell carcinoma of the eye than other suggesting that the cause may be partly genetic.

• When the major facts have been established, alternative causal hypotheses can be formulated.