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.