Drought may have been a blessing, West Nile fever research indicates

Both 2002 and 2003 were dry years in much of the state, helping hold down the number of Culex mosquitoes. Although it will be a couple of months before the West Nile Fever threat ends in Nebraska for 2005, through this past week no birds and only one horse had tested positive and no human cases were reported.

The good news has emerged even though there’s a large increase in the number of Culex this year in western Nebraska, apparently because of the wet and warm weather the area has experienced. Through July 29, a reporting station in Dawes County had measured 23.07 inches this year, about three times the total for 2002.

“We’ve been catching a much higher number of Culex mosquitoes this year than we did either in 2003 or 2004,” said Dr. Randy Lawson, a Chadron State biology professor who for the third straight year is working with the Nebraska Department of Health and Human Services to monitor the West Nile Fever situation in western Nebraska. “If we would have had this many Culex two years ago when the West Nile wave went through the state, we undoubtedly would have had two or three times more cases and probably two or three times more deaths.”

Lawson initially thought the Culex population was about the same each year regardless of the precipitation, but this year’s much higher numbers have changed his mind.

West Nile fever was first detected in the United States in New York in 1999, 42 years after it broke out in Uganda. Since ’99 it has swept across the nation from east to west. It is now hitting California and Arizona the hardest. While the disease usually lingers in what is called an endemic pattern, its impact has greatly declined in the U.S. after the first wave took its toll.

Last year, just 53 cases of WNF and no deaths were reported in Nebraska. By then the wave had moved into states such as Colorado, where 291 cases were reported; Arizona, which had 391; and California, which had 779.

All the California cases last year were in the southern part of the state. This year it has spread to the central and northern portions and is taking a heavy toll again.

The Chadron State contingent, made up of Lawson and four students, set mosquito traps in Dawes, Sheridan and Sioux counties. They also receive mosquitoes that have been collected in Scotts Bluff, Lincoln, Cherry and Garden counties, then sort the Culex from the other species, the Aedes, which is not a carrier.

A microscope is needed to determine the difference in the pests. The Culex has a rounded body; the Aedes is more pointed.

Pools of up to 50 Culex are then sent to the University of Nebraska Medical Center in Omaha for testing. While a Culex pool from Dawes County was the first in the state to test positive this year, it is the only one from the dozens collected and tested from the county that has been found positive.

Adam Wells, a senior from St. Paul, Neb., who also helped Lawson with the WNF project last summer, said about three times more Aedes than Culex mosquitoes were found last year. This year, he said it’s about 50-50 with the Culex probably in the lead.

A recent shipment of about 10,000 mosquitoes from Scotts Bluff County contained approximately 7,000 Culex. Some 1,400 Culex were caught in Dawes County last week.

“We didn’t find large numbers of Culex last year until it was much later in the summer,” said Wells. Lawson said that also was true in 2003.

“Culex apparently thrive during a wet year,” Lawson explained. “They lay their eggs on water while the Aedes lay theirs in dirt. Snow melt will trigger the Aedes hatch, but it takes water and warm weather to hatch the Culex. This year, we’ve had a lot more water standing around and, with the high temperatures in July, the hatch really took off. For the Culex to survive after they hatch, organic material such as leaves, grasses or a dead animal is needed. With so much more water this year, they’ve had no problem surviving.”

The Chadron State researchers also have collected and sent 10 birds to UNMC for testing, but so far none of the 350 birds from across the state that have been examined have tested positive, indicating that the major WNF threat has indeed passed through Nebraska.

In 2002, 61 percent (490 of 796) of the birds tested were positive. In 2003, the figure was 49 percent (396 of 800) while last year it was 25 percent (90 of 355).

Three species of birds are the primary carriers. They are crows, blue jays and magpies. Anyone finding one of these birds that recently died is asked to call Lawson’s office at 432-6298.

Lawson said that the spraying for mosquitoes that takes place in most towns undoubtedly helps hold down the number of Culex, but his research shows that it is not eradicating them. He said he and his staff will continue to collect and sort mosquitoes until the first hard freeze occurs or about mid-October.

He added that just because WNF does not appear to be a problem in Nebraska this year, does not mean people should not use a repellant if they were in an area where mosquitoes are prevalent.