Experts in vector-borne diseases from New Delhi visited Budge Budge and Bishnupur on Thursday to assess the outbreak of dengue, chikungunya and an unknown fever in the area. They were accompanied by officials of the state health department.

State health minister Surya Kanta Mishra is scheduled to tour South 24-Parganas on Friday.

The spread of the diseases left the experts stumped even as the number of people affected by the unknown fever crossed 3,000 in seven blocks, including Mograhat, Falta and Joynagar.

On Thursday, health officials confirmed that of the 47 blood samples sent for tests from Falta, three have tested positive for chikungunya and dengue. In Budge Budge II, three more cases of chikungunya have been confirmed, taking the total number of affected people to seven.

“All block medical officers (health) have been asked to send samples to the two referral centres, ICMR and the School of Tropical Medicine. Once a case is confirmed, the patient can be isolated immediately to prevent spread of the virus through mosquitoes,” said Sacchidananda Sarkar, the chief medical officer (health) of South 24-Parganas.

The death of a 13-year-old boy, who had been suffering from fever, in Howrah’s Bigachi spread panic among residents. Many turned up at medical camps with high fever, headache and joint pain.

Health officials said they were finding it difficult to sensitise people in some pockets about the danger of storing clear water.

“Residents refuse to believe that mosquitoes causing dengue and chikungunya breed in clear water,” said an official in Sathgachia.