DENVER (Reuters) - Three elderly women have died from West Nile virus in Colorado, which leads the nation in confirmed cases of the mosquito borne disease, state health officials said on Thursday.

"All the women were elderly. They cannot fight this off and tend to succumb," said Cindy Parmenter, spokeswoman for the Colorado Department of Health and Environment.

She attributed the large number of cases in Colorado to a wet, cool spring and a hot July, which caused the mosquito population to soar.

She also said the second year of an outbreak is always harsher than the first year and Colorado is home to a particular mosquito that easily transmits the disease.

There have been 111 confirmed cases in Colorado so far this year and four deaths, including the most recent three, who died over the past several days.

Of the 111 cases, 84 are West Nile fever, 12 have advanced to encephalitis and 15 others have progressed to meningitis.