Health experts and physicians have raised concerns about the deadly Nipah virus’ transmission from animals to humans in Kerala’s northern districts.
In the latest case, a 42-year-old woman from Valanchery in Malappuram district is in a coma and continues to be supported by a ventilator after contracting the deadly virus.
However, it is still unclear as to how the virus is getting transmitted in Kerala during the annual outbreaks that have been occurring for the past seven years.
Despite the ongoing dedicated virus surveillance on Pteropus medius bats, which are considered the primary reservoir for the Nipah virus, the gap in understanding the transmission persists.
Research conducted in vulnerable regions has revealed that the Nipah virus present in infected individuals and bats is of the same strain, as confirmed by genome sequencing. However, the exact transmission mode from bats to humans during previous outbreaks remains unclear.
District Medical Officer (DMO) R Renuka confirmed to Down To Earth (DTE) on May 12 that the patient remains stable, with no signs of improvement or deterioration.
The medical board at EMS Memorial Hospital in Perinthalmanna, Malappuram, has administered two monoclonal antibodies to her, indicating progress in treatment. Additionally, medications that are part of the state’s approved treatment protocol for Nipah have also been given, offering hope for the patient’s recovery.
This patient’s positive test for the Nipah virus on May 8 marked the third reported case from Malappuram district in the past two years. The earlier cases identified last year were a 14-year-old boy who tested positive on July 21, and a 24-year-old adult on September 15.
With the latest detection of Nipah in Valanchery, Kerala has recorded two outbreaks in 2018 and 2023 involving human-to-human transmission, four spillovers in 2019 and 2021, and two separate events in 2024.
Meanwhile, Kerala’s Minister for Health Veena George informed in Thiruvananthapuram that 11 more people on the affected woman’s contact list tested negative on May 11, bringing the total number of people who tested negative this time for Nipah to 42.
On May 12, 18 more people were added to the contact list, bringing the total number to 112. Fifty-four were in the high-risk group, and 58 were in the low-risk category.
Among those in the contact list, as many as 81 were from Malappuram district, 25 from Palakkad district, three from Kozhikode district, and one each from Ernakulam, Idukki, and Thiruvananthapuram districts. Ten have been under treatment for various ailments, and two have been admitted to the intensive care unit (ICU), informed the DMO. They all have been given prophylactic treatment.
The latest patient was first tested for the virus on May 6 at the Calicut Medical College, where she was admitted with symptoms of dengue. When the tested samples were found to be positive for Nipah, a sample was sent to the National Institute of Virology (NIV) in Pune on the afternoon of May 7 for confirmation. The NIV confirmed Nipah on the afternoon of May 8.
According to TS Anish, nodal officer at Kerala One Health, Centre for Nipah Research and Resilience, and Professor of Community Medicine at Government Medical College, Kozhikode, samples tested using Truenat came positive for the Nipah virus. Samples tested in the microbiology lab using RT-PCR also turned positive.
It is generally assumed that the Nipah virus can be transmitted to other animals by consuming contaminated fruit exposed to saliva or urine from infected fruit bats or through direct contact with other infected animals. Despite this, research conducted in the state has not pinpointed the exact mode of transmission in all the reported Nipah cases.
Anoop Kumar AS, a critical care specialist involved in diagnosing the Nipah virus during previous outbreaks, mentioned that although fruit samples and various other specimens have been collected and tested, the virus has not been detected in the samples from the state. “All fruit samples have tested negative,” Kumar stated.
He emphasised that determining the precise mode of transmission for the Nipah virus can be challenging, as this uncertainty is also present in many other diseases, including COVID-19. Additionally, he noted that there could be multiple modes of transmission.
Kumar referenced research from Bangladesh, which has experienced regular Nipah outbreaks, where scientists managed to isolate the virus in date palm syrup. This finding helped establish that consuming contaminated date palm syrup is a possible primary transmission pathway from bats to humans in that country, alongside other transmission routes.
According to Renuka, trained health officials visited 2,087 houses in and around Valanchery on May 11 and 12 as part of the ongoing fever surveillance and search for the exact pathways. They visited 3,868 houses, covering 87 per cent of the area’s houses.
The health department also launched a joint outbreak investigation on May 12 with the support of the animal husbandry department. This comprehensive approach includes surveillance of domestic animals in the region, examination of recent changes in animal behaviour, and collection of samples from domestic animals. The investigation will also look into recent deaths among domestic animals and collect samples wherever possible.
“We are limited in including wild animals due to the difficulty of tracking and testing them. Therefore, our focus will be more on cats and dogs, which are more accessible and likely to have close contact with humans,” said Malappuram District Surveillance Officer C Shubin.
Doctors said the latest Nipah patient exhibited acute encephalitis syndrome and had classic symptoms of dengue in the beginning when admitted to the Kozhikode Medical College hospital. She tested positive for dengue, and hence, Nipah was not suspected. It was only when her condition did not improve that samples were sent for Nipah testing two days later.
According to George, the Nipah outbreak is now reported as the seventh instance in Kerala. The first outbreak was reported in 2018, followed by 2019, 2021, 2023, and 2024.
Official sources said there had been cases in Kozhikode in 2018, 2021, and 2023. On two occasions, the cases were from near Perambra and Kuttiyadi and the areas surrounding the Janakikkad forests. These places are on the north-eastern side of the district and are close to Wayanad. The months between May and September were earlier considered crucial because they are the mating season of fruit bats, the principal carriers of the Nipah virus.
However, other studies have found that the mating season could begin in February. In a study by the NIV, the virus was detected in fruit bats in Manathavady in Wayanad. Furthermore, the Nipah-infected child who died in Malappuram in 2024 belonged to an area close to Wayanad.
As part of remedial measures, the health department has proactively identified Kozhikode, Malappuram, Kannur, Wayanad, and Ernakulam as possible hotspots of Nipah because either humans have been found to have been infected there in the past or the presence of the virus has been detected in fruit bats in these districts.
“It is improbable that any index cases had direct contact with the bats; however, they must have somehow come into contact with bat secretions. The respiratory route was probably the means of transmission. We are currently locating evidence of fallen fruits containing virus content to substantiate the hypothesis that the virus was transmitted to humans through fruits. Additionally, there have been some endeavours to develop mathematical models that simulate the probabilities of pathogen transmission,” said Anish.
“With Nipah cases now regularly occurring in Kerala, we cannot overemphasise the need to routinely study fruit bats, which are natural hosts for Nipah. Although the 2018 virus exhibited minor genetic differences from the Bangladesh strain, it is even more crucial to determine whether the virus responsible for spillovers without human transmission has further evolved. More scientists can do such studies only when the genetic sequences are shared in public databases without delay,” said Anoop.