Demographic Processes Drive Increases in Wildlife Disease following Population Reduction

Abstract

Population reduction is often used as a control strategy when managing infectious diseases in wildlife populations in order to reduce host density below a critical threshold. However, population reduction can disrupt existing social and demographic structures leading to changes in observed host behaviour that may result in enhanced disease transmission. Such effects have been observed in several disease systems, notably badgers and bovine tuberculosis. Here we characterise the fundamental properties of disease systems for which such effects undermine the disease control benefits of population reduction.By quantifying the size of response to population reduction in terms of enhanced transmission within a generic non-spatial model, the properties of disease systems in which such effects reduce or even reverse the disease control benefits of population reduction are identified. If population reduction is not sufficiently severe, then enhanced transmission can lead to the counter intuitive perturbation effect, whereby disease levels increase or persist where they would otherwise die out. Perturbation effects are largest for systems with low levels of disease, e.g. low levels of endemicity or emerging disease.Analysis of a stochastic spatial meta-population model of demography and disease dynamics leads to qualitatively similar conclusions. Moreover, enhanced transmission itself is found to arise as an emergent property of density dependent dispersal in such systems. This spatial analysis also shows that, below some threshold, population reduction can rapidly increase the area affected by disease, potentially expanding risks to sympatric species.Our results suggest that the impact of population reduction on social and demographic structures is likely to undermine disease control in many systems, and in severe cases leads to the perturbation effect. Social and demographic mechanisms that enhance transmission following population reduction should therefore be routinely considered when designing control programmes.

Hog badger (Arctonyx collaris) latrine use in relation to food abundance: evidence of the scarce factor paradox

“…Hog badger removal operations to manage rabies (Hu et al 2012) would seem likely to disturb patterns of latrine use and social contacts, potentially causing perturbation effects similar to those reported in other species (Prentice et al 2014), and resonating with lessons from bovine tuberculosis management in European badgers (e.g., Tuyttens et al 2000, Riordan et al 2011. Habitat management, such forestry operations, influence hog badger latrine use, with latrines being used most in logged and selectively-logged forest and least in farmland; this too has implications for disease epidemiology (White et al 1993, Delahay et al 2007).…”

Using contact networks to explore mechanisms of parasite transmission in wildlife

“…In a radio‐tracking study of an undisturbed badger population, contacts between badgers of the same sett accounted for almost 90% of contacts relative to contacts between individuals of different setts (Böhm et al , ). Culling disrupts setts such that surviving badgers range into new territories, potentially increasing inter‐sett contacts and the risk of bTB transmission to new locales (Carter et al , ; McDonald et al , ; Prentice et al , ). In network terminology, the undisturbed badger populations had a higher modularity than post‐culling populations because of their stronger intra‐sett associations.…”

Dynamic, spatial models of parasite transmission in wildlife: Their structure, applications and remaining challenges

“…In the first, the number of contacts scales with the density of the population, and in the latter, the transmission rate scales with the number of infected individuals (Begon et al., ). Although the concept of a density‐dependent threshold is central to the underpinnings of why interventions like culling and vaccination should be effective in wildlife (McCallum, Barlow, & Hone, ; Prentice, Marion, White, Davidson, & Hutchings, ), proof of existence for such thresholds is scant (Lloyd‐Smith, Cross, et al., ). In fact, in wildlife, transmission is often a combination of frequency‐ and density‐dependent transmissions that may depend on host density, the social structure of the population and the scale of investigation (Borremans et al., ; Cross et al., ; Davis et al., ; Manlove, Cassirer, Cross, Plowright, & Hudson, ).…”