On Monday, this year’s first Nipah infection was reported, yet again in Kerala, sending the state government into a scramble to set up isolation wards and initiate surveillance of potential carriers.
More than 58 species of fruit bats (often called flying foxes) are considered natural hosts of the Nipah virus. After the latest Nipah virus infection (NiV), public health and veterinary experts said the virus can resurface anywhere in India.
The virus has so far surfaced only in Malaysia, Singapore, Bangladesh and the states of West Bengal and Kerala in India; however, according to the World Health Organization (WHO), the habitat of these fruit bats extends from the east coast of Africa, across south and South-East Asia, east to the Philippines, Pacific islands and Australia.
“As such, all India is part of the flying foxes territories. It can be conjectured that Nipah virus can emerge as a human pathogen anywhere in these distribution areas. Countries with serological evidence or molecular detection of the virus in these flying foxes territories include Bangladesh, Cambodia, China, India, Indonesia, Madagascar, Papua New Guinea, Taiwan and Thailand," a WHO document said.
“According to the world health organisation, the case fatality rate is estimated at 40-75%. This rate can vary by outbreak depending on local capabilities for epidemiological surveillance and clinical management," said Avinash Srivastava, a Delhi-based veterinary consultant.
“With such high mortality rate with this virus, ideally, government shouldn’t have stopped monitoring. Even the monitoring of people movement from endemic countries including Bangladesh should be regularly watched," said Avinash Srivastava, a Delhi-based veterinary consultant.
Nipah, a paramyxovirus (one of a group of RNA viruses that are predominantly responsible for acute respiratory diseases) whose reservoir host is fruit bats of the genus Pteropus, can infect pigs, dogs and horses, and spreads from animals to humans. “Currently, NiV is limited to Kerala. Presumably, pigs may become infected after consuming partially bat-eaten fruits. There are hoards of bats in various parts of India, especially in abandoned buildings and heritage sites, and it (strains of the virus) is very difficult to find. Besides, almost all piggeries are unregulated in India," said Sunil Gupta, former additional director, microbiology at National Centre for Disease Control (NCDC) who monitored last year’s Nipah outbreak.
“Even if we focus on bats, it is very difficult to know which bat is carrying the virus because bats have not shown any symptom so far in Kerala," he added.
India and neighbouring Bangladesh were both hit by a Nipah outbreak in 2001. Humans were infected with the virus after consuming date palm saps contaminated by infected fruit bats. In Bangladesh, it has resurfaced several times since then, with the last recorded outbreak in 2015. India witnessed a Nipah outbreak in 2007 and then in 2018.
“NiV has been reported only in villages clustered around a strip of central, western and northwestern Bangladesh, which is now known as the Nipah Belt. Most outbreaks happened due to the proximity of certain districts of West Bengal to the Nipah Belt of Bangladesh. There is high possibility of NiV travelling to India due to a spill-over effect in Bangladesh," said Sanjana Singh, an environmental science expert at the United Nations University Institute for the Advanced Study of Sustainability, Japan.
This time, scientists are worried because the outbreak has not occurred near the Nipah belt, but in distant Kerala.
Interestingly, the Consortium for Conservation Medicine in the US had carried out a study which was published in Emerging Infectious Diseases Journal in 2008, and had tested 41 bats from Haryana to find antibodies against Nipah and Hendra viruses. The aim was to determine whether these viruses circulate among fruit bats (Pteropus giganteus) in northern India. The study found at least 20 bats positive for the virus. “The risk for NiV spillover to humans should be considered over a much wider area than previously regarded," it said.
Health authorities have expressed concerns that NiV may surface well beyond Kerala. “The Nipah belt is not in question for spread of NiV this time, because the bats cannot fly such long distances. However, (it is now clear that) the virus can travel any distance, from Bangladesh to Kerala. It may be present elsewhere as well. And the worst part is that we will only come to know when there is an outbreak," said Gupta.
“The people who are exposed to areas inhabited by fruit bats or articles contaminated by secretions, such as unused wells, caves, fruit orchards, etc., are likely to be at higher risk of infection. Also, people in direct contact with sick pigs or their contaminated tissues can also be at danger," he added.
NiV caused havoc in Malaysia and Singapore between September 1998 and May 1999, affecting 276 people and leading to 106 deaths. In Malaysia, NiV was first transmitted to pigs and then to humans. However, in Bangladesh and India, NiV travelled directly from bats to humans.
“With the increase in population, there has been an increase in the rate of urban expansion, leading to fragmentation of forests due to construction of roads and large-scale forest clearing for agricultural purposes. This has been the main cause that has led to the spread of NiV. The only way to curb this would be to control the demographic and economic growth rate," said Singh.