Human threats could be exacerbating transmission of bat-related viral infections

Can human-caused factors like habitat loss and climate change trigger outbreaks such as the recent Nipah cases in Kerala? Existing literature does hint at this possibility.

According to a report by the World Health Organization, there is “strong evidence” that the emergence of bat-related viral infections can be attributed to the loss of the animal’s natural habitats. “As the flying fox [fruit bat] habitat is destroyed by human activity, the bats get stressed and hungry, their immune system gets weaker, their virus load goes up and a lot of virus spills out in their urine and saliva,” the report adds.

Environmental stress

“There are studies on Hendra and Nipah viruses that hint at reproductive and nutritional stress [fewer food resources] as potential role players in virus spillover,” wrote Arinjay Banerjee, who works on the immune systems of bats and related viruses in Canada’s University of Saskatchewan and has studied the Nipah outbreak in Bangladesh, in an e-mail to The Hindu.

In the case of the Hendra virus — the Nipah equivalent in Australia — scientists found that when fruit bats are stressed (through habitat fragmentation, habitat reduction and physiological stress), the percentage of bats infected with the virus increases drastically, increasing the likelihood of passing it to humans through horses.

Nutritional stress through the loss of food resources — a direct consequence of habitat loss and climate change — brings bats closer to urban areas. According to a study in Malaysia, rapid urbanization of bat-rich rainforests contributed to the emergence of Nipah virus there: the regions most adversely affected were those that suffered from maximum deforestation. Forest fragmentation and hunting bats for food also bring them closer to humans and is often an important cause of disease transmission, says Rohit Chakravarty who studies bats in India.

Conservationists worry that the recent Nipah outbreak could cause a knee-jerk reaction of calls for bat culling. Culling bat populations may seem like an easy solution — and has been tried in Australia — but studies warn that instead of reducing the outbreak of such zoonotic diseases, it could cause even more damage, chiefly ecological.

That’s because about a quarter of the more than 1,300 bat species seen worldwide feed on fruit and nectar and are crucial pollinators (of fruit trees, including mango, guava and banana), helping maintain genetic diversity in agricultural systems. They are also important seed dispersers; other bat species help bring rodent and insect numbers under control.