As winter approaches, European authorities are carefully examining the signals of a resumption of the epidemic linked to Sars-CoV-2. With, in the shadows, the specter of a return of the mask on public transport or other measures which revive an era that we thought was over. Several thousand kilometers away, in India, the threat has become reality again: nurses wearing masks and overalls, a Ministry of Health talking about high-risk contacts, closed schools, confined areas… For several weeks, the most populous country in the world lives to the rhythm of a virus largely unknown to the general public. Her name: Nipah. For now, the figures are not chilling. Six cases identified, including two fatalities, in Kerala, a state in southern India. But this rare virus that can be transmitted from animals to humans is monitored like milk on fire by the World Health Organization (WHO). Nearly 300 scientists even recently classified it among the pathogens deserving priority research due to their high epidemic potential…
“What makes it particularly dangerous is its case fatality rate, which can be around 70%, the fact that there is currently no treatment or vaccine available, and a variety of symptoms ranging from high fever to life-threatening inflammation of the brain”, specifies virologist Hervé Fleury, professor emeritus at the CNRS and the University of Bordeaux and author of Emerging and re-emerging viruses (Elsevier Masson). Another factor of concern: this virus of zoonotic origin has the potential to be transmitted from human to human. All these characteristics make it a real health threat. However, should we be worried about a future pandemic caused by the Nipah virus?
To answer this question, let’s start by rewinding the movie. Officially, the first outbreak of Nipah virus infections in humans was recorded in 1998 after it spread among pig farmers in Malaysia. Scientists quickly discovered that the pathogen is generally transmitted to humans through contaminated animals or food, but that it can also be transmitted directly between humans.
The origin of Nipah has been traced to a population of fruit bats of the genus Pteropus (the flying fox, in French), which live mainly in Asia and Madagascar and naturally harbor this virus belonging to the family Henipavirus – which also includes those of Hendra (discovered in Australia) and Langya (identified in China). The first Nipah outbreak killed more than 100 people in Malaysia (out of 300 cases) and led to the culling of 1 million pigs in an effort to contain it. It then spread to Singapore, with 11 cases including one fatality among slaughterhouse workers who came into contact with pigs imported from Malaysia.
Since then, the disease has been reported mainly in Bangladesh and India, with both countries recording their first outbreaks in 2001. Bangladesh has been hardest hit in recent years, with more than 100 people dying from Nipah since 2001. But the he Indian state of Kerala is currently experiencing its fourth epidemic wave in five years, the virus having killed 17 people during its first appearance in 2018. An acceleration which is of great concern to world health authorities.
In these regions, a traditional practice is at the origin of contamination: the harvesting of palm sap. The farmers drill holes in the trunks of the palm trees, and place large terracotta pots underneath in which the bats come to drink the juice during the night. Driven from their natural habitat due to deforestation, these animals are coming closer and closer to humans to feed. In the morning, a person can come and drink the palm juice without knowing that it has been contaminated by bats which may have, for example, urinated in the pot. And here is the first infected patient. Once sick, he will contaminate his loved ones and the healthcare teams. A chain of transmission is born.
Fear of a future pandemic?
However, there is good news: Nipah is still transmitted laboriously from man to man. “Where Covid can be transmitted asymptomatically and via close contact, the transmission of Nipah is known to require close contact,” assures Julien Cappelle, health ecologist at CIRAD. The chain of transmission is therefore quite short and the number of patients very limited. However, the more transmissions there are to humans, the greater the risk of seeing a virus better adapted to our species emerge. “One of the fears is that it adapts and mutates, in particular by becoming transmissible by aerosols, like Sars-CoV-2. This would take years, but it is not impossible,” continues the researcher.
But, if a variant of the Nipah virus evolves and becomes more contagious, will it still be as lethal? “Let’s look at Ebola, which shares certain epidemiological characteristics with Nipah (high lethality, small chains of transmission, etc.). During the great epidemic in West Africa of 2013-2014, the disease changed its face with 28,000 cases including 11 000 deaths. It was unheard of. A variant made this virus more transmissible, but also less deadly at the same time. This could happen with Nipah, but it is very difficult to imagine what a variant of this would look like virus that we still know little about”, judges Julien Cappelle.
For virologist Hervé Fleury, the risk of a pandemic caused by Nipah is currently almost zero without an extraordinary event, and therefore, by definition, difficult to predict. “The viruses that are conquering the globe are mainly respiratory, this is not the case here. On the other hand, there is a risk of more and more localized epidemics in the future, with many deaths,” judges -he. And the virologist shares the fears of his ecologist colleague: “If it mutated to become more transmissible, it could be catastrophic.”
Faced with the potential danger of the Nipah virus, how can we react locally? While it would be tempting to remove the colonies of fruit bats that establish themselves near human populations, Julien Cappelle warns against any hasty decision: “These animals play an important role as pollinators, particularly for crops. And, in many regions, bats coexist with people without problem. In Cambodia, for example, where we have not seen cases of Nipah emerge among humans, the method of collecting palm juice is different. We do not collect palm sap but nectar: the flowers are pressed into small bamboo or plastic containers, and it is much more difficult for bats to have access to them. A difference which could explain the lack of emergence.” For there to be human contamination, there must therefore be a route of transmission of the virus from bats to humans, which is favored by a certain number of practices that should be adapted.
Increasing transmissions from animals to humans?
If zoonoses have multiplied over the last twenty years, it is not because of bats which have carried within them, since the dawn of time, an incalculable number of viruses. By occupying larger and larger areas of the planet, humans contribute to the disruption of ecosystems and increase the likelihood of random viral mutations transmissible to humans. Industrial agriculture increases the risk of pathogens spreading between animals while deforestation increases contact between wildlife, domestic animals and humans.
By mixing more and more, species transmit their viruses more, which encourages the emergence of new diseases. Climate change will push many animals to flee their ecosystems for more habitable lands, warns the scientific journal Nature in a study published in 2022. According to estimates published in the journal Science in 2018, there were 1.7 million unknown viruses in mammals and birds, of which 540,000 to 850,000 had the capacity to infect humans.