What is it?

As the name suggests, fruit bats, or Pteropodidae, are a bat family that eats fruit. Since the Nipah virus broke out in Kozhikode, Kerala, fruit bats have attracted attention as the wildlife reservoir for the virus. This means the virus survives in the bat’s body without causing disease, allowing it to jump to susceptible mammals like humans or pigs, when bats come in contact with them. Such contact is becoming increasingly frequent as agriculture and urbanisation destroy bat habitats, forcing them into human dwellings. In the world’s first Nipah outbreak, which occurred in 1998 in Malaysia, virologists isolated the virus from the urine of the Island Flying Fox, a fruit bat species. In Bangladeshi outbreaks, researchers found antibodies to Nipah in the Indian flying fox.

This is why, when a bat colony was spotted in a well at the home of Kozhikode’s first Nipah victim, virologists zeroed in on these mammals as a possible source of infection. However, things have not been as straightforward as expected. When animal husbandry officials collected bats from the well, they only found the insect-eating kind, which belong to a different family. There is some evidence that insectivorous bats can host Nipah, but they have not been connected with human infections so far, says Jonathan Epstein, an epidemiologist studying the virus at the U.S.-based EcoHealth Alliance. So, Kerala’s animal husbandry officials are scanning the region for other fruit bats in the hope of finding the wildlife reservoir.

How did it come about?

All bats can carry viruses, some of them deadly. The Marburg virus, a relative of Ebola, was isolated in 2009 from the Egyptian Rousette, a fruit bat, in Uganda’s Kitaka Cave. After the 2003 outbreak of Severe Acute Respiratory Syndrome (SARS) in China, researchers found antibodies to the SARS Coronavirus in cave-dwelling insectivorous bats. Similarly, Ebola antibodies were found in species like the Hammer-headed fruit bat. In the case of SARS and Ebola though, the virus was never isolated from the mammals. This means other animals may also play a critical role in the outbreaks. Why are so many emerging diseases linked to bats? For one thing, with around 1,200 species, bats comprise 20% of the earth’s mammalian diversity. So, it ought not to be surprising that they host many viruses. Not all of these viruses are threats to humans. The bigger question is how bats stay healthy despite carrying these pathogens. The Indian Flying Fox, for example, hosts over 50 viruses. So far, researchers have only hypotheses to explain this viral diversity in bats. One explanation — the “flight as fever” hypothesis — suggests that long periods of flying raises the temperatures of bats, boosting their immune responses. This helps them survive the microbes’ pathogenic effects.

Why does it matter?

Identifying the source of the Nipah infection will help prevent future spread. In the Kozhikode epidemic, the virus seems to have moved from bats to humans in one “spillover” event. After this, it moved from one human to another. Nipah spreads differently in different countries. In the 1998 Malaysian outbreak, the virus moved to pigs first — perhaps after a domestic pig consumed fruit contaminated with bat saliva. Once it spread widely on pig farms, the virus began jumping to humans who came in contact with the animals. Around 260 people fell ill after such contact, but no person-to-person transmission seems to have occurred in Malaysia, unlike in Kozhikode.

What next?

Officials are trying to identify the bat species behind the outbreak. Even if the outbreak is eventually linked to these mammals, the transfer of bat viruses to humans is a rare event. Given how critical bats are to ecosystems, the Kerala government has taken a stand against culling bats in response to the outbreak.