Process & Systems Modelling

Counter-intuitive increases in disease following culling

Although population reduction (often implemented by culling) is a recognised and widely used method of disease control in wildlife populations, counter intuitive increases in disease levels following culling (termed the perturbation effect) indicate that it may be less effective than current theory suggests. Evidence from studies of both badgers (Meles meles) and wild boar (Sus scrofa) suggest that culling disrupts social and demographic structures, leading to enhanced levels of disease transmission. Understanding the cause of this phenomenon will allow us to determine the levels of population reduction which may give rise to a substantial perturbation effect and hence to improve our ability to reduce disease incidence.

We find that epidemiological and demographic characteristics associated with the perturbation effect are common to many wildlife disease systems and reduce the efficiency of population reduction as a disease control strategy even when not leading to an increase in disease levels. Thus, cases where perturbation effects have been observed may be the "tip of the iceberg" and social and demographic mechanisms which enhance transmission as a result of population reduction should be considered routinely when designing control programmes.

simulations of spatial meta-populationsspacer
Simulations of spatial meta-population models with density dependent dispersal illustrate a plausible mechanism for the perturbation effect:

(A) shows that for such a system with initially patchy distribution of infection between groups culling spreads infected individuals to new groups, which then infect additional individuals, especially once control measures are discontinued;

(B) shows that culling causes an increase in the rate of dispersal in the previously stable population, leading to an increase in the proportion of infected groups and in the rate of disease transmission.

Further details from: Glenn Marion

Article date 2013


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