An outbreak of dengue fever, a mosquito-borne tropical disease, has killed at least 20 people in India's eastern coastal Orissa state, health authorities said. The latest victim from the coal belt region in the worst-hit Angul district died Tuesday in a hospital in the state capital of Bhubaneswar, the Press Trust of India reported. The report said the disease has spread to 21 districts in the state and more than 690 people have tested positive to the virus. Most of them are receiving treatment at various hospitals, authorities said. "Clean water accumulated on the hard surface of open cast mines and (spare) tires (of coal-transporting trucks) are prefect places for (the) aedes mosquito to breed that spreads dengue," a doctor who works in the coal district told the Times of India newspaper. Authorities said the coal trucks also may be transporting dengue-infected mosquitoes from area to other parts in the state. Scientists have isolated samples of the virus to identify its strain, the report said. Authorities have asked district administrations to launch cleaning, fogging and awareness efforts in the affected areas.