Harvest strategies for the elimination of low prevalence wildlife diseases.

Atle Mysterud,Aniruddha V Belsare,Hildegunn Viljugrein,Christer M Rolandsen

doi:10.1098/rsos.210124

Abstract

The intensive harvesting of hosts is often the only practicable strategy for controlling emerging wildlife diseases. Several harvesting approaches have been explored theoretically with the objective of lowering transmission rates, decreasing the transmission period or specifically targeting spatial disease clusters or high-risk demographic groups. Here, we present a novel model-based approach to evaluate alternative harvest regimes, in terms of demographic composition and rates, intended to increase the probability to remove all infected individuals in the population during the early phase of an outbreak. We tested the utility of the method for the elimination of chronic wasting disease based on empirical data for reindeer (Rangifer tarandus) in Norway, in populations with (Nordfjella) and without (Hardangervidda) knowledge about exact disease prevalence and population abundance. Low and medium harvest intensities were unsuccessful in eliminating the disease, even at low prevalence. High-intensity harvesting had a high likelihood of eliminating the disease, but probability was strongly influenced by the disease prevalence. We suggest that the uncertainty about disease prevalence can be mitigated by using an adaptive management approach: forecast from models after each harvest season with updated data, derive prevalence estimates and forecast further harvesting. We identified the problems arising from disease surveillance with large fluctuations in harvesting pressure and hence sample sizes. The elimination method may be suitable for pathogens that cause long-lasting infections and with slow epidemic growth, but the method should only be attempted if there is a low risk of reinfection, either by a new disease introduction event (e.g. dispersing hosts) or due to environmental reservoirs. Our simulations highlighted the short time window when such a strategy is likely to be successful before approaching near complete eradication of the population.

Highlights

Harvesting is the legal and regulated hunting of game species, and it represents the main cause of mortality among ungulates in Europe and North America [1]
For the Nordfjella population of 1982 individuals with 14 observed chronic wasting disease (CWD) cases, ordinary harvest rates and composition removed less than 50% of the infected deer in most simulations
With the ordinary hunting practices of 2016, only three CWD-positive reindeer were removed, which was very close to the theoretical expectation

Summary

Introduction

Harvesting is the legal and regulated hunting of game species, and it represents the main cause of mortality among ungulates in Europe and North America [1]. Extensive harvesting of hosts by marksmen, often termed culling, comprises a controversial component of the management toolbox in the combat of infectious diseases in wildlife [2,3]. The ability of harvesting to control disease epidemics depends on a number of factors [4], and several mechanisms may be involved. For diseases with density-dependent transmission, harvesting can, in theory, cut transmission rate by lowering population density. Culling of red foxes (Vulpes vulpes) to lower transmission by reducing population density was used to limit rabies before vaccination was common [5]. Lowering the disease prevalence can be achieved for diseases with close to frequency-dependent transmission, but is far more challenging. The hunting and culling of hosts remains a strategy of active mitigation, including the culling of badgers (Meles meles) to limit bovine tuberculosis [8,9], culling of hare (Lepus timidus) to limit louping ill virus [10] and culling of wild boar (Sus scrofa) to limit both tuberculosis [11] and African swine fever [12]

Methods

Results

Discussion

Conclusion