As many as 25 children died of Chandipura virus in Gujarat in about five weeks, but health specialists have found no trace of the virus in any of the known virus carriers screened so far, the most common being sandflies found in the crevices of mud houses and livestock.Results from a massive screening exercise taken up after the outbreak from June 30, covering thousands of sandflies, and later on mosquitoes and ticks suspected to be carriers of the virus, returned negative.So where exactly is the virus breeding? The question continues to haunt researchers.Notably, similar screenings conducted during the 2024 outbreak, when 28 children died out of the 78 suspected Chandipura virus cases, have also failed to find the virus carriers.In the current outbreak, the Gujarat government has confirmed 39 Chandipura virus cases, all of them children.The Chandipura Vesiculovirus (CHPV) predominantly affects children below 15 years old.It presents with sudden high fever that rapidly escalates to acute encephalitis syndrome (AES) with brain swelling and seizures. The disease was first identified and catalogued in a village of Chandipura in Nagpur, Maharashtra, in 1965.Story continues below this adSandflies were established as vectors of CHPV in a 2018 outbreak. In the paper ‘Molecular Evidence of Chandipura Virus from Sergentomyia species of Sandflies in Gujarat’, researchers from ICMR-NIV stated, “A highly zoophilic species of the sandfly Sergentomyia was found to be a potential vector of CHPV in Gujarat. This is probably the first report from India of male sandflies testing positive for CHPV in RT-PCR analysis…”Sandflies are common in eastern Gujarat, but patients infected with the CHPV have been found in Saurashtra as well, which is in the west. This has given rise to the preliminary conclusion that there could be more than one vector (insect) transmitting the disease to humans.Single clusters, vector yet to be tracedResearchers from Gujarat and five national institutes in the state as part of the National Joint Outbreak Response Team (NJORT) have not been able to find any trace of the virus in any of the over thousand samples of sandflies and other vectors, collected during both the 2024 and current 2026 outbreaks in Gujarat and Rajasthan.The confirmed CHPV cases in the current outbreak are distributed over 12 districts of Gujarat and three districts of Rajasthan, with the Patan district having the highest number of cases at five, followed by four from Sabarkantha district.Story continues below this adAn animal husbandry official told The Indian Express, “There are no clusters of human cases, just single cases from every area. We are not even finding multiple cases from the same locality. So the number of carriers (vectors) will also be less, is what we assume. Usually, the more the number of victims in any vector-borne disease, the greater the chance of finding the vector.”A senior scientist from a national institute studying vector-borne diseases told the Indian Express, “We have collected several hundred samples of sandflies and are currently carrying out Reverse Transcription – Polymerase Chain Reaction (RT-PCR) to amplify and check if they were infected with Chandipura Vesiculovirus. Unfortunately, we haven’t found any positive results so far. This is similar to the outbreak in 2024 when we did not find any infected sandflies. However, we are also looking at Aedis aegypti and Culex genus of mosquitoes to see if they have acted as vectors for CHPV, but there are no positive results so far.”Testing of samples from cattle in the vicinity of the human cases have also not yielded any clues.A scientist, who has worked on these entomological samples, said: “After entomologists collect samples at the outbreak sites, they are sent to the laboratory where they are frozen at minus 20 degrees Celsius. They are then crushed, the nucleic acid is extracted, and it is then run on RT-PCR to check whether it contains CHPV.”Expanding screening to ticks and mitesStory continues below this adResponding to a questionnaire sent by The Indian Express to the Union Ministry of Health and Family Welfare (MoH&FW), the PIB said in a statement, “A key focus of the investigation is to unravel the virus transmission cycle. While sandflies are the established vectors of Chandipura virus, scientists are also examining whether other arthropods – including mosquitoes, ticks and mites — have a role in transmission. A large number of vector samples collected from affected areas are currently undergoing laboratory analysis. Officials have emphasised that it is premature to identify the vector responsible for the current outbreak until scientific investigations are completed.”The statement added that even in the investigations in the 2024 outbreak, “no specific vector could be conclusively implicated”.The government statement added that the animal surveillance had also been expanded to determine whether the virus was circulating among domestic animals. “Blood samples from cattle, buffaloes and goats, along with milk samples, have been collected for testing. Around 70 animal blood samples have been collected so far and are being analysed for Chandipura virus and other relevant pathogens. Authorities have clarified that no conclusions can be drawn until laboratory investigations are completed,” the statement said.Expert flags surveillance shortfallsDr Sujatha Sunil, Group Leader of Vector-borne Diseases at the International Centre for Genetic Engineering and Biotechnology (ICGEB), an inter-governmental organisation in Delhi, said that researchers could be missing the “timing of the surveillance and the depth of the surveillance during non-transmitting time”. Though not involved in the current investigation, Dr Sujatha has been working for years on Chandipura virus with other researchers in Maharashtra where the disease was first identified.“During the 2005 Chikungunya outbreak, the virus was able to jump from its classical Aedes Aegypti (mosquito) vector to the non-classical Aedes Albopictus mosquito species on the basis of a single mutation in the virus, which enabled it to change its vector between two species of mosquitoes”, Dr Sujatha told The Indian Express.Story continues below this adOn the need for constant and stringent entomological surveillance, especially in hotspot areas, Dr Sujatha said, “We need to be looking for all possible vectors, not just sandflies as well as animal reservoirs that maintain the virus between human outbreaks. And since it is an India-specific problem, we need proper vector surveillance especially in these hotspots. The collection of (vector) samples too has to be denser. We cannot use dengue or malaria as examples to say we will pick up sandflies and mosquitoes in hundreds and check for the virus. You have to sample several times more. The depth of surveillance is required in this case because of the difference in population numbers between mosquitoes and sandflies, and low CHPV infection rates. Additionally, surveillance should not be limited to outbreaks but in the time of the year starting from June to August regardless of whether there is an outbreak or not. That will be conducive to finding the vector.”