Co-extinction of carnivore hosts and parasites pose threats to human health

Background/Question/Methods

Zoonotic parasites capable of infecting humans comprise nearly 60% of all human parasites and are arguably, the most prevalent of human-natural coupled systems. Thus, the species composition, diversity, and spatial distribution of zoonotic parasites are sensitive to host dynamics. Though largely ignored to date, the loss of host species can result in the subsequent loss of affiliate species (i.e., co-extinctions) and pose threats to human health.  Mammalian carnivores (Order Carnivora) are a useful model group for investigation because they include species threatened by parasites (e.g., rabies, canine distemper) and other species that serve as reservoirs of zoonoses with over half of the 125 emerging zoonotic diseases associated with carnivores. Here, we used presence-absence matrices of >1300 host-parasite associations for 29 North American carnivores and simulated changes in parasite composition and species richness with the random extinction of hosts.  We also explored the geography of zoonotic parasite richness under three carnivore host composition scenarios and examined variation in corresponding levels of human exposure. 

Results/Conclusions

Of the 394 parasite species found in North American carnivores roughly a third were zoonotic (n=114) including half of all bacteria; thirty-two percent of zoonotic paraistes were nematodes. Some of the most frequently reported (largest number of host species) zoonotic parasites included: Yersinia pestis (causative agent for Plague found in n=15 carnivore hosts), Trichnella nativa (n=16); Toxoplasma gondii (n=15), and Leptospiro interrogans (leading cause of foodborne illness in US found in n=10 hosts).  Conversely, 35% of zoonotic species were found only in a single carnivore species (i.e., carnivore specialist parasites), though some of these are present in other non-carnivore animal species. We found that the parasite assemblage changed independently across parasite groups with the proportion of zoonotic viruses markedly increasing with carnivore extinctions.  Co-extinction of carnivore parasites is unlikely, given that few specialist parasites exploit hosts of conservation concern.  However, local extirpations of widespread carnivore hosts can reduce overall zoonotic richness and shift distributions of parasite-rich areas.  Whether host diversity and the corresponding zoonotic diversity amplifies or dilutes health risks is debatable.  What remains conspicuous in our results is that hosts vary in their contribution to human health risks and anticipating changes in host composition in future environments can help inform both parasite conservation and disease mitigation efforts.