Kerala health authorities are still clueless about the source of the infection which claimed the life of a six-year-old boy in Malappuram last month as the samples of crows and mosquitoes tested negative for the West Nile Virus.

According to a health ministry official, samples of mosquitoes were collected by the Vector Control Department from various parts of Venniyur, where the deceased resided, and remains of four dead crows found in and around the area were sent to the National Institute of Virology in Alappuzha to test for the virus.

The samples have tested negative, the heath ministry official said. "It is a major concern as the source of the virus has still not been ascertained. More samples of mosquitoes are being sent for testing. Also a stern monitoring has been put in place at the Kadalundi Bird Sanctuary to keep a tab on the death of any bird as the virus is also known to spread through migratory birds," the official said.

West Nile Virus is a mosquito-borne disease and related to viruses that cause Japanese encephalitis, Yellow Fever and St Louis Encephalitis. The mosquitoes catch the virus through infected birds. The virus is most often transmitted to humans via mosquitoes and human-to-human transmission of the virus is very rare.

The Kerala government has set up a multi-disciplinary team in Malappuram to investigate various epidemiological aspects of West Nile Fever and also to aid the district administration in preventing and managing the disease. To prevent mosquito bites, the state has been asked to follow the National Vector Borne Disease Control (NVBDCP) programme of personal protection.

Further, all cases of Japanese Encephalitis or Acute Encephalitis syndrome are being inspected as per the guidelines of the disease and also tested for West Nile Virus. The directive comes after the young boy from Malappuram in Kerala succumbed to West Nile Virus on April 1 after being admitted at the Kozhikode Medical College Hospital since early March.