There has been much speculation over the source of the Nipah virus that triggered an outbreak in Kozhikode, Kerala. Last week, the state animal husbandry department captured and tested insectivorous bats (Megaderma spasma) from a well in the compound belonging to a family whose members were the virus’s first victims – and found no virus in them.

News reports have pointed out that the virus’s reservoir is different: fruit bats of the species Pteropid medius (formerly Pteropid giganteus). A fresh effort is underway to capture and test fruit bats – but the delay could mean that the virus may not be found in them at all. Should this happen, it still wouldn’t mean that bats were not the source of the virus.

This is because it is difficult to find a bat that is infected.

Bats “may only be infectious for a week or two, and then they clear the virus and they’re no longer infectious,” said Jonathan Epstein, a veterinarian and epidemiologist at EcoHealth Alliance, New York, who has, for over a decade, studied Nipah outbreaks and the bats that cause them, in Malaysia, India and Bangladesh. “That’s why these outbreaks are relatively rare events, given the fact that these bats are so abundant and so common but very few of them are ever actually shedding virus at a given time.”

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