How a few bat clades, not all, shape global spillover risk

A large-scale study reveals that only a few bat lineages harbor viruses with high epidemic potential, reshaping our understanding of zoonotic threats and calling for more targeted, clade-specific disease monitoring rather than a broad fear of bats.

Study: Viral epidemic potential is not uniformly distributed across the bat phylogeny. Image Credit: Faisal_Fatso  / Shutterstock

In a recent study published in the journal Communications Biology, researchers found that the potential for viral epidemics is not uniform across the bat phylogeny.

Zoonotic pathogens differ in transmissibility and virulence, which influence death burden and describe the epidemic potential of a virus. Surveillance and transmission prevention initiatives must prioritize zoonotic pathogens with the highest epidemic potential to reduce the emergence of zoonotic diseases and optimize resource allocation.

Recent studies indicate that bats harbor more viruses with high virulence in humans than other avian or mammalian orders.

Media Perception and the Need for Clarity on Bats

Bats have garnered negative media attention due to their identity as hosts of multiple high-profile viruses, leading to retaliation against bats, which in turn may increase the risk of spillovers. As such, clarifying the distribution of viral virulence across the order Chiroptera (to which bats belong) could provide a better understanding of viral epidemic potential, helping inform outbreak prevention efforts and improve public perception of bats.

Study Aim: Mapping Viral Risk Across Mammals and Bats

In the present study, researchers investigated whether viruses with high epidemic potential in humans are uniformly distributed across mammals or whether specific clades exhibit a higher epidemic potential. First, the Global Virome in One Network database was used to extract 2,637 unique mammal-virus associations.

These associations were detected through a combination of viral isolation, PCR, and serology. The authors caution that such detections do not necessarily imply that each host is a competent reservoir or makes a meaningful contribution to human transmission.

Quantifying Epidemic Potential Using Phylogenetic Models

The researchers quantified typical viral epidemic potential, transmissibility, and death burden in humans attributed to each host. Phylogenetic signal was estimated for the mean and maximum case-fatality rate (CFR), the mean death burden, and the percent of viruses with onward transmission using Pagel’s λ and Blomberg’s K indices. Both K and λ were calculated for all viruses to evaluate whether viral epidemic potential is conserved across species among mammals and within bats.

Identifying High-Risk Clades with Phylogenetic Factorization

Phylogenetic factorization was used to identify clades with unusually low or high viral epidemic potential. It was first performed across all mammals to investigate whether Chiroptera as a whole was a clade with a greater propensity to host viruses with high virulence, death burdens, and transmissibility. Next, it was performed within Chiroptera to identify specific clades with high or low viral epidemic potential.

Patterns of Viral Virulence Across Mammals and Bats

Viral epidemic potential data included 112 unique virus species and 889 mammal species, with 202 bat species. The maximum CFR, mean CFR, and percent of viruses with onward transmission were all between 0 and 1. The mean death burden ranged between 0 and 2.58 million.

Across all mammals and viruses, the mean CFR and death burden showed moderate phylogenetic signal, whereas the percent of viruses with onward transmission showed a high signal.

Results were similar when analyses were restricted to bats. However, the mean CFR and death burden showed a high phylogenetic signal across all viruses in bats, while the percentage of viruses with onward transmission exhibited a weaker signal. Among mammals, 11, 10, four, and nine clades varied in mean CFRs, maximum CFRs, percent of viruses with onward transmission, and mean death burden, respectively.

High-Risk Clades and Geographic Hotspots Identified

Twenty-three out of these 34 clades, including eight clades of bats, were high risk, i.e., those with higher CFRs, death burdens, and propensity for onward transmission. Across viruses, six clades varied in mean CFRs, four of which were high-risk clades.

The first high-risk clade included bat superfamilies Vespertilionoidea and Emballonuroidea, the second included 13 families under the order Carnivora, and the third included residual bat families from Yinpterochiroptera and Yangochiroptera.

The fourth high-risk clade was a subclade of Neotominae, a rodent subfamily. One clade was identified as high risk for the percent of viruses with onward transmission, which included families from Catarrhini, a primate parvorder.

Three clades varied in mean death burden; the first high-risk clade included the bat family Rhinolophidae, and the second high-risk clade included the infraclass Eutheria. The third clade had lower death burdens and included the superorder Cetartiodactyla.

The researchers did not observe that Chiroptera as a whole was a clade harboring high-risk viruses in any measure. Crucially, the researchers also tested whether these findings were simply an artifact of which species are studied most. They found that the high-risk bat clades were not the most heavily studied groups; in fact, some were identified as high-risk despite being undersampled. This suggests the results reflect a real biological pattern, not just scientific bias.

Finally, they mapped the geographic distribution of bats in high-risk clades with anthropogenic footprint data and identified hotspots of high viral epidemic potential in Central America, equatorial Africa, coastal South America, and Southeast Asia across all viruses.

Implications for Targeted Surveillance and Conservation

In summary, the findings suggest that bats do not exhibit uniform viral epidemic potential, as transmissibility, death burden, and virulence are clustered within distinct bat clades, often within cosmopolitan families.

The paper suggests potential reasons for these patterns, noting that many high-risk clades are insectivores whose diet could expose them to vector-borne viruses. Furthermore, the global distribution of some of these families and their ability to roost in human-made structures may increase opportunities for spillover.

Within Chiroptera, high-risk bat clades were identified for both Flaviviridae and Togaviridae in terms of viral virulence (mean case fatality rate, CFR). For Flaviviridae specifically, high-risk bat clades were also identified for onward transmission and death burden.

Across viruses, Rhinolophidae showed unusually high death burden, consistent with previous reports linking them to high-impact viruses. By moving beyond the generalization that all bats are high-risk, these findings allow for more targeted surveillance efforts.

Reframing Human-Bat Interactions and Spillover Risk

Ultimately, the study emphasizes that bats provide critical ecosystem services, such as pollination and pest control, and that culling them out of fear can actually increase the risk of spillover.

The authors hope their work will help shift the conversation, highlighting that human activities, such as habitat destruction, primarily drive the emergence of zoonotic diseases, rather than the inherent nature of bats themselves.