Oregon wildlife managers are considering the use of chemical or biological insecticides to kill off mosquitoes that might be carrying the West Nile virus, despite fears that destroying mosquito larvae will cripple an important component of the food chain for waterfowl.

Fears of a West Nile outbreak that could spread to birds, horses and people are driving the considerations.

But nesting hens feast on the mosquito larvae while producing eggs, as do ducklings and goslings. Other invertebrate food sources for ducks and geese consume mosquito larvae as well.

"How does a wildlife area fit into a West Nile world?" asked Bruce Eddy, watershed district manager for the Oregon Department of Fish and Wildlife.

To answer that, the department plans to work more closely with local insect control managers and review its guidelines for spraying, said Eric Rickerson, wildlife habitat program manager in Salem.

Local insect control agencies are keeping an especially close eye on four state wildlife areas: the 6,200-acre Ladd Marsh Wildlife Management Area in Union County, the 1,860-acre Denman Wildlife Management Area near Central Point, the 3,600-acre Klamath Wildlife Management Area just south of Klamath Falls and the 12,000-acre Sauvie Island Wildlife Management Area near Portland.

The state has previously used insecticides at wetlands to control the transmission of the insect-borne Newcastle virus in birds. Meanwhile, "sentinel chickens" are in place at Denman — blood is drawn from the fowls every two weeks to determine if the Newcastle, and now West Nile, virus is present.

But it also appears that the abundance of predators in managed wetlands may scare off mosquito species that spread West Nile, Rickerson said. Rather, they like rain gutters, neglected birdbaths and stagnant water in old tires, he said.

West Nile has been around since at least 1937, when it appeared in Uganda under the name Rift Valley fever. It arrived in the United States in New York in 1999 and began moving west, reaching Oregon in 2004, and now is present in all of the lower 48 states, according to the federal Centers for Disease Control.

People seldom die of the flulike effects of the virus and many don't develop symptoms when infected. Last year, doctors found five cases in Oregon where people caught West Nile — three of the five became sick and all recovered. Still, of the 15,000 human cases reported since 1999, at least 500 people have died in the United States.

In Eastern Oregon, the virus is a serious threat to the large population of horses, which have a 33 percent to 50 percent mortality rate when infected, said Kelly Beehler, insect control manager in Union County. The virus also can be deadly to birds, particularly eagles, hawks, crows, magpies and ravens.

The virus has been found in birds in Jackson, Benton, Malheur and Crook counties, and in horses in Grant, Jackson and Linn counties.