Colorado Residents Warned To Take Precautions Against West Nile Virus

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West Nile Virus

Surveillance systems monitoring West Nile virus activity are showing Culex mosquito populations are at or above the counts observed at the same time in 2003, when Colorado experienced a large West Nile virus outbreak.

With three human cases confirmed in the state so far this season, state health officials are warning residents that it is important to immediately begin taking precautions to avoid mosquito bites.

"Based on the surveillance data from around the state, the indicators are suggesting that we're heading into a high-risk year, so it's important for people to take precautions against this preventable disease now," emphasized John Pape, an epidemiologist who specializes in animal-related diseases at the Colorado Department of Public Health and Environment.

Mosquito trapping shows populations of Culex mosquitoes, which transmit West Nile virus, are extremely high for this time of year and rising rapidly. Mosquitoes infected with the West Nile virus have been found in nine counties (Boulder, El Paso, Jefferson, Larimer, Logan, Mesa, Otero, Prowers and Weld). Twenty-six mosquito pools have tested positive so far, with 18 of those detected since July 1. A mosquito pool is a group of up to 50 mosquitoes tested together.

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"Usually we don't pick up the first positive mosquitoes until around this time of year," said Pape, "but then the virus moves quickly. Due to the large numbers of mosquitoes out there now, we expect the infection rates, and therefore the human risk, to rise rapidly over the next several weeks."

The state has confirmed three human cases in individuals from Boulder, Cheyenne and Logan counties. However, four to five weeks generally pass between the time a person is bitten by an infected mosquito and the date the case is reported to the health department. This is due to the one- to two-week incubation period before the onset of symptoms and delays inherent with seeing a physician, getting test results and reporting the case. Thus, people being infected today may not be counted until sometime in early August.

According to case data from the previous four years, it appears approximately 85 percent of people are infected during a period from July 1 through the second week of August. Infected mosquitoes continue to feed through September and early October, depending on the weather, but fewer than 10 percent of human cases are contracted after Labor Day.

"July and August are the months posing the highest risk of infection," Pape explained. "People should be extra diligent during this time and use whatever methods available to avoid mosquito bites, especially during the period from an hour before sunset through about midnight, when Culex mosquitoes are most actively feeding."

"We are entering the peak of the transmission season, even though only a few human cases have been reported," Pape noted.

Pape reminded Colorado residents and visitors to observe the "Four Ds" to protect against West Nile virus:

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