NEW DELHI - Doctors in New Delhi expressed concern Tuesday after a second deadly mosquito-borne virus was discovered in the city which is already struggling to deal with an outbreak of dengue fever.

Seven cases of chikungunya, a rare mosquito-borne fever, have been diagnosed at a top hospital in the capital in the past two or three days, N.K. Ganguly, the director general of the Indian Council of Medical Research, told reporters.

The council is affiliated with the All India Institute of Medical Sciences, known as AIIMS, which has been struggling to deal with an outbreak of dengue fever that has killed 55 people across the country, including 22 in New Delhi, in the past month.

"Twenty-eight samples were done and seven (chikungunya) cases are confirmed," Ganguly said.

Chikungunya has been blamed for 81 deaths in southern India over the last month, though health experts have said chikungunya only weakens the immune system, allowing people to succumb to other diseases.

Both fevers are spread by female aedes mosquitoes and symptoms include high fever, joint pain, headache and vomiting. There is no known cure for either disease, although dengue is more deadly.

Doctors are concerned that misdiagnoses might occur because the diseases have appeared simultaneously in the same place could lead to misdiagnoses.

"There may be some amount of misdiagnoses," Arvind Rai, a director of the National institute of Communicable Diseases told the Hindustan Times newspaper. "Monitoring too is going to be difficult but dengue is definitely the greater cause for concern," he said.

Dengue is fatal in rare cases.

The mosquito-borne diseases usually spread as the annual monsoon season tapers off and leaves puddles of stagnant water for the insects to breed in and normally fade with the end of the mosquito breeding period in November.

On Monday, municipal workers in New Delhi went door-to-door to check for possible places where mosquitoes could breed, and sprayed trouble spots with insecticide.

Workers with fogging machines mounted on bicycles also went through congested neighborhoods spraying clouds of pesticide to kill mosquito larvae.

Authorities also stopped all water fountains in the Indian capital.