Conservation epidemiology of predators and scavengers to reduce zoonotic risk

The increasing instances of epidemics and pandemics of zoonotic origin—diseases maintained in animal populations but transmitted to humans—show that humanity is fundamentally off track with predator and scavenger conservation. We are off track because most diseases of pandemic potential are zoonotic, with natural reservoirs in predators and scavengers.1, 2 Predators and scavengers, species that consume other animals either live or dead, also play a role in maintaining ecosystem structure and reducing zoonotic risk.3, 4 Leopards, polar bears, insectivorous bats, and carrion-eating vultures are just a few examples of these species that are crucially positioned in food webs, controlling densities of disease hosts and vectors through feeding⁵ or competitive exclusion.⁶ However, most of these species are at increased risk of extinction globally.3, 4 Their decline, coupled with expanding environmental destruction, is known to be a strong driver of zoonotic spillover.

Mammalian predator and scavenger reservoir hosts are found globally, with species richness hotspots in North America, Africa, southeast Asia (eg, order Carnivora; appendix p 1), and central and South America (eg, bats, possums, armadillos, shrews, and anteaters; appendix p 1). Although zoonotic risk is homogeneous among mammalian and avian taxonomic orders,⁷ predator and scavenger species have been linked to some of the most serious epidemics in recent history, serving as reservoir hosts for cross-species transmission.¹ Masked palm civets, raccoon dogs, and horseshoe bats are just three of the species that have been linked to severe acute respiratory syndrome coronavirus 1 and severe acute respiratory syndrome coronavirus 2. Across the world, these and other predator and scavenger species are regularly sold in live animal markets. Indeed, capturing, handling, storing, and processing predators and scavengers for consumption from reservoir hotspots inherently increases zoonotic risk.

Predator and scavenger populations are declining and it is predicted that their numbers will continue to drop in the next 20 years due to overharvesting, habitat loss, and conflict with humans.3, 4 The reduction in the number of predators can increase the risk of zoonotic transmission by maximising the prevalence of infection when transmission depends on the contact rate within the prey population.⁵ Moreover, zoonotic risk in humans is likely to be amplified in areas that are shared with wildlife. For example, leopards in the Sanjay Gandhi National Park in India might reduce stray dog densities (a reservoir host for rabies) in surrounding slums, reducing risk of human deaths from dog bites.⁸ Likewise, foxes help to reduce Lyme disease risk in North America by controlling rodent populations, which are the main reservoir host for infected nymphal tick vectors.⁵ Other predators such as raptors and snakes prey on rodents and bats that are known reservoir hosts of a number of dangerous pathogens. Such potential trophic relationships are found globally, with hotspots in much of far North America, southeastern South America, Africa, and Asia (appendix p 2). Mammalian consumers of reservoir hosts are known to be imperilled in parts of southern and eastern Africa, and mainland Asia, with other areas of concern in far North America and in South America (appendix p 2), highlighting the importance of targeted conservation action in these areas to minimise disease risk.

The decline of predator and scavenger numbers can also increase disease risk in humans through a reduction in competitive exclusion, the act of outcompeting disease hosts for resources.⁴ For example, there has been a 92% decrease in vultures in India between 1990 and 2000, which is associated with a spike in carrion and stray dog populations, and therefore exposure to dog bites and rabies infection.⁶ This increase in carrion from vulture declines could also explain the unusual increase in anthrax-associated deaths.

© 2020 Alexander Braczkowski

While predators and scavengers play a crucial role in maintaining ecosystem structure and reducing disease risk, human activities amplify risk when handling these species and reducing their populations in natural environments. This points to a dire need to not only uphold increasing review and regulation of markets and medicines that contain wildlife, but also to promote predator and scavenger conservation and habitat retention in shared landscapes. Epidemiological research on the avian influenza epidemic revealed the weakness in regulation of domestic animal markets as prominent hubs of disease transmission, and now with increasing regulation, zoonotic risk has been substantially reduced.⁹ This tactic should be met with a conservation epidemiological approach that embraces the integration of ecology and the measurement of the downstream effect of conservation action on disease transmission dynamics at the human-wildlife interface.¹⁰ Conservation epidemiological research will enable discovery on the impact of predator and scavenger conservation action on the epidemiology of zoonotic spillover at the human, wildlife, and livestock interface. Targeted to different stages of the wildlife market chain, conservation epidemiology will inform risk-based surveillance and control of pathogens of zoonotic potential and deliver an evidence base for public health policy that considers the preservation and protection of wildlife known to be reservoirs or regulators of zoonoses to reduce the risk of human exposure.