In Southeast Asia, a broad spectrum of coronaviruses (CoVs) are known to circulate in bat populations, with both alpha- and betacoronaviruses detected in a wide range of species in Cambodia, China, Laos, the Philippines, Taiwan and Thailand. These two genera have been shown to infect animals and also to be capable of causing disease in humans.

A recent report, whose authors include CIRAD scientists Julien Cappelle and Véronique Chevalier, has investigated the extent to which targeted coronavirus monitoring in bat populations may offer opportunities for developing prevention measures and early warning systems for public health purposes.

The study was conducted before the global spread of Covid-19, but its findings assume a particular resonance in the light of the pandemic’s massive health and economic repercussions. A core objective of the 17-month surveillance initiative, conducted in Cambodia between 2014 and 2016, was to identify potentially zoonotic viruses in bats and their typical circulation patterns, paying particular attention to the age of bats found to be CoV-positive.

Immunologically naive bats

In some bat species, juveniles are thought to play a significant role in coronavirus circulation, due to the synchronized nature of reproduction. With large numbers of young bats proliferating at the same time, all of them ‘immunologically naive’ – too young to have been exposed to infection and acquire protection – the result can be increased circulation of the coronavirus in question.

By organising monthly live-captures and sampling of 1036 bats belonging to eight species in two provinces of Cambodia, the researchers set out to assess the diversity of CoVs circulating in them and to examine the influence of reproductive timing on levels of infection. A specific aim was to test the link between age and CoV status.

In addition, the authors synthesized existing knowledge on the reproductive cycle of Rhinolophus and other bat species elsewhere in Southeast Asia, with a view to shaping guidelines for future coronavirus surveillance and prevention efforts in the region. Bats within the Rhinolophus genus are believed to be the reservoir of the progenitors of both SARS-CoV-1 and SARS-CoV-2, the latter being the virus responsible for the current Covid-19 pandemic.

Testing for coronavirus in samples of bats from three species captured at the Cambodian sites revealed a pattern, with CoV prevalence peaking during the months following the birth of young bats and significantly higher positivity rates recorded for juveniles and immature individuals.

A review of available literature showed similar results in other parts of Southeast Asia, suggesting that immunologically naive bats may be more susceptible to CoV, which can in turn lead to sharp seasonal upturns in pathogen circulation and persistence of infection.

Targeting surveillance from June to November

Despite some variations, the overall picture points to a strong case for conducting long-term surveillance of CoV in bat species, concentrating monitoring during the periods when juveniles and sexually immature individuals account for a higher proportion of populations, generally from June to November in Southeast Asia, according to the authors.

“This kind of study of the ecology and epidemiology of emerging diseases is key to the understanding of zoonotic risks”, said the report’s lead author Julien Cappelle, a disease ecologist at CIRAD. “Combined with virological, epidemiological and socioeconomic studies implemented within a One Health approach, it supports the design of prevention and early detection strategies aimed at mitigating the impact of emerging zoonotic diseases.”

The study showed that several strains were circulating at the same time in different bat species, underscoring the importance of conducting surveillance, in order to gauge the diversity of coronaviruses present in bat populations in a given area, as well as to identify peak periods of infection, and the consequentially higher risk of spillover to humans in the case of potentially zoonotic forms such as SARS-CoV-2 related viruses.

In terms of cost-effectiveness, particularly in areas where resources are limited, this focused approach appears likely to maximize the probability of detecting CoV surges and taking preventive action.