A 67-year-old woman has died of the West Nile virus in Colorado, bringing to five the number of fatalities in the state hardest hit by the bug this year.
Colorado, which had a dozen human cases last year, leads the nation with 166 so far this year, according to state officials.
The latest death was a Boulder woman who died Tuesday, six days after feeling the onset of West Nile symptoms including encephalitis, Boulder County Public Health officials said Sunday.
Lab results returned late Friday confirmed she had the virus, said Heath Harmon, a coordinator with the county health department.
Why Colorado has seen such high numbers of the mosquito-borne disease this year is unclear to experts. Some have blamed the outbreak on a wet June and very hot July, which they say provided the perfect summer for mosquitoes. Others have said the weather doesn't entirely explain the numbers.
"I can't predict what will happen in Colorado, nor can I completely explain why it is happening," said Dr. Julie Gerberding, director of the CDC.
"That certainly makes us very nervous," said Craig Levy of the Arizona Department of Health Services. "If it can increase that dramatically in Colorado, it has the potential to do so in Arizona."
Experts expect the mosquito-borne disease — which killed 284 people nationwide last year — to reach every state in the coming months.
Colorado differs from other states because it reports mild cases of the virus that some do not report, state epidemiologist John Pape has said.
The Centers for Disease Control and Prevention had confirmed only 72 Colorado cases and only one of its deaths as of Friday, according to the organization's Web site. Nationwide, the agency confirmed 182 West Nile cases and five deaths.
The virus is spreading fast nationwide and will likely break last year's record 4,156 cases, Gerberding said.
"The numbers are starting to change very, very quickly," Gerberding said last week. "That is very concerning."
"It indicates we are starting the epidemic with more cases than last year," the CDC director said. She warned of "a great number of infected people."
New cases are likely until the first frost of the year kills the mosquitoes. The good news is that the numbers tend to decrease after a state's second year dealing with the disease.
"Ever since West Nile virus arrived, we've always been forecasting that this year would be the worst year for infections in humans," Heath Harmon, communicable disease control coordinator for Boulder County Public Health. "For the rest of this summer and into next year, we'll try to weather the storm."