Monitoring fruit bat colonies could provide early warning for Nipah outbreaks

An outbreak of the bat-borne Nipah virus occurred in the southern Indian state of Kerala in September 2023. As state authorities scramble to control this latest outbreak, fourth in five years, questions on the pathways of transmission have resurfaced.

With each new outbreak, narrowing down on a single pathway of first exposure becomes remote. Nipah virus infections always occur in clusters and have a high rate of fatality. While fruit bats are known reservoirs of the Nipah virus, many risk factors need to overlap in time and space at a local and regional scale, for the virus to spill over into humans.

In our recent work, we identified complicated trade-offs between ecosystem services from fruit bats on the one hand, and the evolving scenario of zoonotic risk on the other.

We described that fruit bats feeding on cashew and areca nuts bring real benefits to people across the Western Ghats, especially in Kerala. Bats pick the ripest fruit, and after chewing the external part, drop the undamaged nuts in clumps, which are then easily collected by farmers, thus reducing labour costs. But as a consequence, seed collectors routinely handle bat-eaten fruits and nuts and could also be potential contact points increasing the risk of viral transmission and spillover of the virus to humans.

Nipah virus can remain alive on fruit surfaces for two hours to over 30 hours, depending on temperature and weather conditions. As plantation workers often prefer to work early in the morning during summer, they potentially expose themselves to bat-eaten fruits from the previous night.

Some of the above indicators seem common in Kerala's September 2023 Nipah spillover case. The first identified case in this cluster of cases (called the index case) owns an areca plantation and some other fruit trees, where bats were commonly seen. One could speculate that the index case could likely have handled an areca fruit in the plantation in the morning. Anyone who has walked in such plantations would know how casual and natural this action is, almost an unforced error.

Letko et al. (2020) have shown that bats had partial immunity in case of the most pathogenic viruses (including Nipah virus), suggesting higher rates of shedding of such viruses than the hundreds of other viruses they harbour. Overall, a lot of factors have to act together -- the probability of bats carrying Nipah virus, the same bats being under physiological stress (due to resource crunch or breeding time) and shedding the virus in the plantation, the index case handling a bat-eaten fruit, changing weather patterns, deforestation, or urbanization at the regional scale.

This was also an El Nino year in India, resulting in a weak monsoon. Links have been drawn between reduced rainfall and potential dampening impacts on resources for fruit bats, a known predictor of stressed bats shedding higher viral loads, leading to Nipah virus outbreaks. Notably, areca nuts are among the few abundant fruits available to these bats in September – because most other bat-preferred fruits have their peak over by summer.

Today, the health of fruit bats is linked to the health of the neighbouring people. This is a One Health perspective, defined as “an integrated, unifying approach that aims to sustainably balance and optimize the health of people, animals, and ecosystems” by the WHO.

Safe coexistence with bats

There is an urgent need to increase the capacity and awareness of local people, especially those associated with fruit plantations, in rightly recognizing the risks while continuing their practices in a safer manner.

The preventive measures could be very simple, even obvious, but often not followed.

These include not directly touching or consuming animal-bitten fruits, using gloves and masks while handling such fruits, not shooting or injuring bats visiting fruit trees or disturbing their colonies, careful handling of inadvertently dead bats or other animals, avoiding consumption of bat meat, not allowing livestock or pets to feed on animal-bitten fruits, dipping collected seeds in diluted bleach solution before use.

On the research side, careful and regular monitoring of the health of fruit bat colonies is especially important, to serve as early warning signals for potential spillovers.

These insights could inform emerging frameworks for interdisciplinary studies on human-bat interactions, as zoonotic diseases increasingly become central to human well-being. We emphasize objective assessments of bat-linked ecosystem services and risks, for effective conservation and public health action. It cannot be forgotten that fruit bats are highly beneficial to plantation economies and forest ecosystem health all along the Western Ghats of India and elsewhere, despite some damage to fruit orchards and the possibility of zoonotic risks that we need to be cognizant of.