DISTRIBUTION of THE DISEASE
■ The times of occurrence of cases of a disease constitute its
temporal distribution, whereas places of occurrence comprise its
spatial distribution.
Trends in the temporal distribution of disease
The temporal changes and fluctuations in disease occurrence can be classified into three major trends:
1. short-term;
2. cyclical (including seasonal);
3. long-term (secular).
■ Short-term trends; trends are typical epidemics, which already have been discussed.
■ Cyclical trends; are associated with regular, periodic fluctuations in the level of disease occurrence.
■ Seasonal trends; A seasonal trend is a special case of a cyclical trend, where the periodic fluctuations in disease incidence are related to particular seasons.
■ Long-term (secular) trends; occur over a long period of time and represent a long-term interaction between host and parasite.
– if a balance occurs, then a stable, endemic level of disease is maintained;
– if the interaction is biased to the host, then there is a gradual decrease in disease occurrence;
– if the interaction is biased to the parasite, there is a gradual increase in disease occurrence.
Spatial Distribution of Disease
■ Factors affecting the spatial distribution of diseases include population structure, transmission routes and ecological factors
■ In order to see a disease in a region, there must be a sensitive population of animals.
If the disease is carried by vectors, the vector must be present
■ In order for an epidemic to occur with contact, the number of sensitive animals must be at a certain density
■ The minimum animal density required to form the epidemic is called the threshold level
■ This density is determined by the Kendal threshold theorem.
– There should be at least 12 animals in km2 for the emergence of parvovirus outbreak in dogs.
Reference: Veterinary Epidemiology, 4ed. Michael Thrusfield with Robert Christley, Brown H, Diggle PJ, French N, Howe K, Kelly L, O’Connor A, Sargeant J, Wood H.
