Colorado mosquitoes biting – but minus virus |

Colorado mosquitoes biting – but minus virus

Jennifer Brown
The Denver Post

Attacked by mosquitoes at a weekend barbecue?

Don’t panic about West Nile virus; the mosquito-population explosion in the past week is probably centered on the species that doesn’t spread the disease.

So-called “floodwater mosquitoes” – the type that lay eggs near water and then experience a mass hatching when rains come – are the likely culprits currently irritating Coloradans, experts said Monday.

“They are more of an annoyance than they are a real public-health risk,” said Dr. Richard Vogt, executive director of the Tri-County Health Department for Adams, Arapahoe and Douglas counties.

Rain, hail and wind gusts aren’t conducive breeding conditions for the kind of mosquitoes – Culex – that spread West Nile. Their fragile eggs, stuck together in batches of a couple hundred each, break apart when pounded by rain or tossed in the wind, said Dr. Adrienne LeBailly, director of Larimer County Department of Health and Environment.

Also, Culex eggs thrive in heat, not this month’s unusually cool weather. Eggs that take two or three weeks to hatch in cool spring weather can burst within four or five days later in the summer, when temperatures hit 90 degrees.

Epidemiologists say it is too early to predict whether this year will end up a particularly bad one for West Nile.

There is a worrisome sign, however.

Larimer County on June 5 trapped a mosquito infected with the virus – five weeks earlier than average and three weeks sooner than the earliest previous positive test. Utah has trapped a handful of West Nile-infected mosquitoes this spring, and a few other states also have confirmed cases.

“There is a chance this could be a bad year, but it’s really too early to answer that question,” LeBailly said.

It is possible Larimer County mosquito-trappers got lucky earlier than usual, perhaps catching one of the pests infected last year that survived the winter in hibernation.

“To find that one mosquito this early in the year is kind of a needle in a haystack,” said Marshall Lipps, an environmental health specialist with Boulder County Public Health.

Based on previous years, late June is the start of “peak transmission time” of West Nile to humans, although cases aren’t usually confirmed for weeks. Typically, the worst period is July and August.

Colorado’s deadliest year for West Nile was 2003, when 63 people died.

Jennifer Brown: 303-954-1593 or

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