The current scientific understanding of the Nipah virus outbreak. Since its emergence in late August, the bat-borne Nipah virus has infected six humans in the southern Indian state of Kerala, two of whom have died. Over the past week, over 700 individuals, including healthcare workers, have been tested for infection. The government has shuttered several schools, offices, and public transportation networks.
This is the fourth Nipah outbreak to strike Kerala in the past five years; the most recent occurred in 2021. Although such spells typically affect a relatively limited geographical area, they can be fatal, and some scientists fear that the virus could become more contagious if it spreads among more people. According to Rajib Ausraful Islam, a veterinary physician specializing in bat-borne pathogens at the International Centre for Diarrheal Disease Research, Bangladesh in Dhaka, the fatality rate for the Nipah virus ranges between 40% and 75%, depending on the strain. “Each outbreak is cause for concern,” he explains. Every outbreak provides the pathogen with an opportunity to evolve.
The virus can cause fever, vomiting, respiratory difficulties, and cerebral inflammation. It is carried primarily by fruit bats but can also infect domestic animals such as pigs and humans. It is transmitted by contact with the bodily secretions of infected animals or humans. No authorized vaccines or treatments exist, but scientists are researching potential candidates.
The Nipah virus was first identified in 1998 during an outbreak among Malaysian swine farmers. Within months, infected pigs had transmitted the disease to Singapore. The explosion caused nearly 300 cases and over 100 fatalities.
Since then, no additional Nipah virus outbreaks have been reported in Malaysia. In contrast, the virus emerged in Bangladesh and India in 2001, where periodic epidemics have continued. Almost every year, there are epidemics in Bangladesh, and studies have linked them to drinking date palm sap contaminated with bat urine1. Scientists are investigating precisely when and how the virus jumped from bats to humans in the current outbreak in Kerala.
Stephen Luby, an epidemiologist at Stanford University in California, explains that the strain prevalent in India and Bangladesh differs from the one that emerged in Malaysia. While the Malaysia strain transmitted from animals to humans, transmission between humans was minimal. However, the version responsible for the recent outbreak in Kerala can be transmitted from person to person and is significantly more lethal. “It serves as a reminder that this is a dangerous virus,” says Luby.
Danielle Anderson, a virologist at the Royal Melbourne Hospital in Australia, says that despite the Nipah virus’s lethal potential, it does not spread as readily between humans as other animal-borne infections, making it less likely to cross national borders. Approximately one-third of Nipah virus infections in Bangladesh over the past 14 years were caused by person-to-person transmission, according to a study of nearly 250 cases conducted in 2019. Anderson states, “I would not anticipate it would spread globally.” Different from what we’ve observed with COVID-19.
According to Christopher Broder, an expert on emerging infectious diseases at the Uniformed Services University Medical School in Bethesda, Maryland, the high fatality rate of the virus reduces the likelihood of its rapid dissemination through populations. It is not in the best interest of the virus to harm everyone it infects. He adds that the strain circulating in Kerala has mostly stayed the same since its emergence in Bangladesh over two decades ago. However, future epidemics could be more significant if the strain mutates into a milder but more contagious form. Broder believes that there are likely already-circulating variants that have not been detected.
Andrew Breed, a veterinary epidemiologist at the University of Queensland in Brisbane, Australia, asserts that improving wildlife management near human settlements is crucial for preventing epidemics of Nipah and other bat-borne viruses. Breed notes that studies on Hendra virus, another bat-borne pathogen closely related to Nipah, indicate that stressed bats release more virus particles, increasing the likelihood of the disease spreading to humans. Breed suggests that restoring forest areas to provide more habitat for bats, which would keep them at a secure distance from humans, could go a long way toward preventing future outbreaks.
According to Islam, another method to reduce the risk of bat-borne diseases spreading to humans is to plant more trees with fruit that is palatable to bats but not to humans. This may prevent infected bats from contaminating food supplies. “We need to learn how to live safely with bats,” he says.