The death toll due to Nipah is at least 17 in Kerala, and the Health Department is working to ensure that the outbreak is contained and that there are no more casualties. But in order to do that, they need to find the source of the virus in the state. And while they have a strong suspicion that fruit bats carried the virus that struck the first victims in Kerala, their search has hit a wall, as the test sample of these bats has returned negative.
But that doesn’t mean that fruit bats were not the carriers, officials point out. It only means that the samples were not collected soon enough, they say.
While fruit-eating bats are the natural carriers of Nipah virus, the samples of fruit-eating bats collected from Sooppikkada village in Kozhikode's Perambra, that were sent for testing at the National Institute of High Security Animal Diseases Laboratory in Bhopal, have returned negative. The samples were collected from a roost near the house of Moosa in Perambra, where the virus first struck.
Kerala Additional Health Secretary Rajeev Sadanandan told TNM that although the samples sent for testing in the Bhopal lab has turned negative, the possibility of fruit-eating bats being the source of the virus cannot be ruled out.
"It is true that as time passes by, the possibility of the samples testing positive diminishes. This is because Nipah remains at a low level in fruit-eating bats for most of the time and due to some hormonal changes, there is a spike in the virus levels. This is when it becomes infectious. This also means that our current surveillance system should be improved. We need to conduct epidemiological studies," Rajeev Sadanandan said.
In an article titled "Nipah virus: Anatomy of an outbreak" published in The Hindu recently, Priyanka Pulla quoted from a 2011 study published in the American Journal of Tropical Hygiene and Medicine, which said that it was difficult to locate the virus in the fruit-eating bats weeks after the virus spread.
“Unless your timing is good, and you are collecting samples close to when the first case was exposed, your chances diminish,” the report quoted from US-based epidemiologist Jonathan Epstein's study.
The first person to have died of Nipah virus is a son of Moosa, who passed away on May 5. Subsequently, three more people including Moosa died of the same symptoms, which was confirmed to be that of Nipah.
Rajeev Sadanandan agrees that considering the first death happened in the first week of May, the chances of finding the virus in the fruit-eating bats in Perambra was bleak.
However, Kozhikode district animal husbandry officer AC Mohandas said that the reason for the samples turning negative was because the sampling was done randomly.
"It is not necessary that all the samples we collect turn positive for Nipah. We know that fruit bats are the natural habitats of the virus, based on the epidemiological history. We collected the samples from the fruit bats in order to establish this. But we collected three samples from a roost near the home of Moosa. We also sent urine samples of the roost, along with samples of a rabbit from the family's house. As expected, the samples of the rabbit have returned negative," Mohandas said.
He, however, said that since fruit bats are the reservoirs of the virus, detection may not be a problem, even if the samples are collected weeks after the first infection was reported.
"They shed the virus when the virus load becomes high in their bodies. This happens when they go through stress, be it pregnancy stress or environmental stress or man-induced stress. While starvation can be a reason, it is unlikely in this case, since there is no dearth of fruits in Perambra region," he said.
For the state health department, arriving at the source of the virus is like a "wild hunt," Mohandas pointed out.
"We are not ruling out anything, we are conducting all possible tests to confirm the source. If it had been a crime, then there will be leads that will take us forward. But in this case, we are on a blind chase, there are no leads here," Mohandas said.