Population reduction is often used as a control strategy when managing infectious diseases in wildlife populations, however it disrupts existing social structures and increases movement of infectives due to the vacuum effect, which may lead to enhanced disease transmission. Using a generic non-spatial model, key characteristics of disease systems are identified for which such effects reduce or even reverse the disease control benefits of population reduction. If population reduction is not sufficiently severe, then enhanced transmission can lead to the perturbation effect, whereby disease levels increase or disease can be stabilised where it would otherwise be unstable. Perturbation effects are enhanced for systems with low levels of disease, e.g. low levels of endemicity or emerging disease.
Mechanisms observed in real systems are examined for their role in the perturbation effect. If population reduction is non-random and fails to target infected individuals, then vertical transmission (an important mechanism in many diseases including tuberculosis and paratuberculosis) can lead to the perturbation effect if horizontal transmission is low. The perturbation effect can also arise when population reduction preferentially targets resistant individuals, or mature individuals with low susceptibility, a factor implicated in wild boar and classical swine fever.
In a stochastic spatial model of demography and disease dynamics with density dependent dispersal (implicated in the spread of rabies in foxes, and tuberculosis in badgers and wild boar due to the vacuum effect), enhanced transmission is found to arise implicitly as an emergent property of the disease-system, even when population reduction is entirely random. Culling strategies are examined, and the spatial heterogeneity of distribution of culling resources and timing of culling intervals are shown to influence the perturbation effect. Whilst the perturbation effect may not always be apparent, the various effects modelled are likely present in many disease systems, mitigating the results of population reduction.
