Discussion
Continent-wide improvements to African housing have precipitated a myriad of positive human health outcomes, including reductions in soil-transmitted helminths, diarrhoeal disease, leishmaniasis, malaria, and respiratory disease, and improved mental wellbeing22,26. In this way, house design is integral to the United Nations Sustainable Development Goal 3: ‘ensure healthy lives and promote well-being for all at all ages ’. We report an inadvertent and so-far undocumented outcome of housing improvement in sub-Saharan and rural Africa; that changes to building practices are simultaneously creating habitat for bats, and potentially increasing human exposure risk to bat-associated pathogens. Of the 1,109 buildings surveyed in our study, almost one-in-ten showed evidence of bat occupation (9.2%), and one-in-13 were active roosts (7.6%). We identified modern-build style and triangular roofing as building-level predictors of bat occupation, and the proportion of modern buildings as a landscape-level predictor of bat occupancy. Given the international focus on building improvement in Africa, and the increasing access to improved housing already reported in literature 22, we suggest that this is a rapidly accelerating exposure interface that needs urgent attention and investment.
Synanthropic free-tailed bat species are highly aggregative and use social cues to locate existing roosts 27. Modern-build structures typically have more spaces for bats to roost and can support higher densities of individuals. The reported occupation of modern-build houses possibly reflects a preference in habitat by these bats; roosting with conspecifics in large roosts, as opposed to roosting separately in smaller roosts, has several advantages including reduced risk of predation 28, and access to mates during the breeding season 29. It is possible that the removal of natural habitat, particularly large hollow-bearing trees, could be contributing to the use of anthropogenic structures, similar to other bat systems30,31. However, there is insufficient data on historical and current vegetation in this area to investigate. Practically, large roosts likely pose a greater risk of exposure to humans, and a greater burden to residents. Ceiling collapse due to the weight of accumulated faeces was frequently reported by owners of modern buildings with ceilings containing large numbers of bats, for instance.
Not all modern structures that were available were occupied. This may be partly associated with building-level nuances not captured in this dataset, such as successful blockage of roost access points by owners, or recent bat eviction efforts. Alternatively, this may indicate that availability of ideal refugia is not a limiting factor in bat occupation, and instead, that landscape patterns in building use are driven by the presence of at least a few ideal habitats in which animals can aggregate. Speculatively, this could indicate that synanthropic free-tailed bat occupation is unlikely to saturate available buildings beyond a certain threshold of availability. Baseline information and continued monitoring of bat roosting would be needed to investigate this, though the positive association between landscape-level occupancy and the proportion of modern-style buildings suggests that this threshold (if it exists) has not been reached.
We caution that bat occupation of housing may be an accelerating exposure interface. The African continent’s population is the fastest growing in the world, with an estimated increase of 1.3 billion people expected between 2015 and 2050 32. This growth will necessitate hundreds of millions of new homes. In addition, shifting economic and demographic profiles in the continent will continue to promote access to improved housing. The proportion of houses built with finished materials increased from 32% in 2000 to 51% in 2015, with hundreds of millions of Africans accessing improved housing22. These changes present a powerful opportunity to improve human wellbeing, but only with proper consideration of local ecological context. Investment in bat- and human-friendly housing infrastructure is needed to ensure that vulnerable populations are not left exposed to bats and bat-associated pathogens. Investment in key areas will also contribute to the global effort in preventing emerging infectious diseases. African countries are hotspots for bat species diversity and disease spillover, yet rural and remote regions of Africa often have limited resources to detect or combat the first stages of disease emergence after exposure and spillover 7,8. Intervention at exposure interfaces will help pre-empt spillover of potentially new emerging infectious diseases, and prevent large-scale emergence in-country or globally.
Housing improvement initiatives should include proper consideration of local bat ecology to reduce human-bat exposure risk. Individual risk can be immediately reduced by pre-emptively and thoroughly sealing access points to bats. This a highly successful mitigation strategy but can be prohibitively expensive in rural areas 33. Depending on availability and affordability of materials, sealing access points can also leave design features unfunctional (e.g., sealing ventilation points and chimneys with wire mesh vs with cement or clay). Improvements to the availability and affordability of suitable and bat-safe materials should be made a priority. In addition, the restoration of natural roosting habitats should be considered as a long-term mechanism for reversing building use 34. This could be achieved by providing support to existing environmental stewardship programs involving tree planting (e.g., local groups like the Taita Taveta WCK Action Group, and broader ACK initiatives) 35.
The removal of bats using pesticides or chemicals was frequently reported by building owners, and/or by blocking entrance points with thorny plants that get caught in bat wings and cause death. These approaches are both non-ethical and not effective for long-term exclusion of bats, as killing bats does not prevent roost repopulation36,37. Extermination attempts have also been linked with increased pathogen shedding by bats, and transmission to humans38,39. Pre-emptive exclusion is the safest and most ethical approach for limiting bat-human exposure in the immediate term.
This is the first study to provide empirical estimation on building roost density of synanthropic free-tailed bats, and describes the conditions facilitating human-bat exposure in remote parts of Africa. These results set the foundation for additional research and management actions to alleviate the risks posed by identified bat-human exposure interfaces. Surveillance for priority zoonotic pathogens at interfaces where transmission risks are identified, and the development of culturally appropriate and locally feasible interventions that can be used to reduce the risk of contact and transmission at these high-risk interfaces, should be prioritized for future research and management action. Analysis of temporal patterns in occupation and/or within-roost density could also be used to highlight variation in bat exposure risk within and between years, and prioritise educational efforts on disease risk and mitigation. Empirical information provided by this study, on the building-roost density of anthropogenic free-tailed bats, will be crucial for understanding baseline ecological states for these species, and could be used for future management efforts for these species. This information could also be utilised to develop host-virus models to examine spillover risk now, and under conditions of anthropogenic change.