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Crows Targeted In War Against West Nile Virus

September 3, 2006
University Of California, Davis
California scientists fighting a war on the West Nile Virus are collecting dead birds in the crow family, Corvidae, which include American crows, Yellow-billed Magpies and Western Scrub-Jays. They also pick up an occasional House finch, House sparrow or egret, but mainly they target corvids.

The scientists inside the aging Ford Bronco that's chugging through residential Davis in the stifling Sacramento Valley heat are like soldiers on a combat mission.

They've seen the aerial assaults; they've studied the weapons of mass destruction; they know the method of operation. Now, equipped with "body bags," they're on a "hunt-and-gather" mission to pick up the victims of the serial killer. The feathered victims.

They are soldiers in a war against the West Nile virus. The battleground is Davis, zip code 95616, county of Yolo, one of the nation's hot spots.

Veronica Armijos studies the city map. "Two blocks down and take a left," she tells driver Sarah Wheeler.

"Got it," Wheeler says.

The scientists, who work at the Center for Vectorborne Diseases (CVEC) at the University of California, Davis, are collecting dead birds in the crow family, Corvidae, which include American crows, Yellow-billed Magpies and Western Scrub-Jays. They also pick up an occasional House finch, House sparrow or egret, but mainly they target corvids.

"Corvids are more susceptible to the virus than other species of birds," said CVEC research entomologist and principal investigator William Reisen, a professor with the Department of Pathology, Microbiology and Immunology at the UC Davis School of Veterinary Medicine.

"Corvids serve as the primary reservoirs or incubators for the virus," said Reisen. "Corvid surveillance is crucial to stopping the transmission of the virus."

Crows are good hosts for mosquitoes, Reisen said. "There's an amazing amount of virus in the bloodstream of infected crows, sometimes as much as 10 billion virus particles in one millimeter of blood. They're like a big sack of virus."

The research is funded by the UC Mosquito Research Program, Sacramento-Yolo Mosquito and Vector Control District (MVCD), and the National Institutes of Health. "We're investigating how the distribution of host-seeking mosquitoes and infected corvids intersect in time and space to effectively amplify the West Nile virus in an urban California landscape," said Reisen. "We're studying how landscape features, mosquito abundance patterns and corvid roosts affect the distribution and abundance of West Nile virus in Davis."

Collaborator Carrie Nielsen, a UC Davis doctoral candidate in epidemiology, said the research includes work on the vector (Culex mosquitoes), the host (birds, but especially corvids) and incidental hosts (humans and horses). We're studying the increased risk of infected mosquitoes around sites of positive dead birds or roosts and the increased risk of human infection."

Infected crows usually die very quickly, said Reisen, who's seen crows in Southern California literally "fall out of the sky" during a WNV epidemic.

Wheeler and Armijos, too, receive reports of quick crow deaths. Davis resident Lis Fleming, who phoned in a dead bird, reported that "it just fell out of a tree in my backyard."

Entomologist Lynn Kimsey, director of the Bohart Museum of Entomology at UC Davis, said she spotted a crow in West Davis "lying on its side, pumping its legs and tossing its head back and forth. It was making these loud squawking sounds. It died within 5 to 10 minutes."

As part of the Davis-centered research, Wheeler and Armijos respond to dead bird reports from the 95616 zip code. It works like this: Davis-generated reports sent to the California Department of Health Services (DHS) hotline, 1-877-WNV Bird, or the West Nile Web site,, are forwarded to the Reisen research team.

September is a crucial month, the researchers noted. WNV usually peaks in late August and September and ends in October.

Wheeler and Armijos, who both have bachelor of science degrees in ecology, head out on "Bird Patrol" several times a week when they're not testing mosquito pools, setting surveillance traps or working in the lab. Collecting dead birds, they agree, is not the most glamorous of jobs, especially when the birds aren't fresh. "We cannot accept them if they're maggoty or anty (full of maggots or ants) or mummified," Wheeler said.

Armijos said most people would be amazed at "how fast" ants and maggots can target dead birds.
Wearing protective gloves, Wheeler and Armijos examine the birds, zip them into plastic bags, slide the bags into a cooler labeled "Dead Birds," and deliver them to the California Animal Health and Food Safety Laboratory System (CAHFS) on the UC Davis campus. There technicians necropsy them, removing the kidneys for diagnostic testing.

Next stop: the CVEC lab, a biosafety level 3 containment laboratory directed by Reisen. Samples arrive throughout the state as either tissue, frozen in lysis buffer, or as oral swabs, tucked in special viral transport media. Staff research associates thaw the samples, prepare them for RNA extraction, and use reverse transcriptase-polymerase chain reaction (RT PCR) to test for WNV-RNA. The entire procedure for a round of samples, from preparation to testing to data entry to reporting, takes about 24 hours.

Using geographic information systems much like police do with crime cases, CVEC staff chart the dead bird reports on an avian density map. So far this year (Aug. 29), 35 birds collected in Davis tested WNV-positive; that number exceeds every other city in California except Fresno, with 42. And so far this year, 12 Davis residents have tested positive for WNV, according to the Yolo County Health Department's Sept. 1 report. With 21 confirmed human cases of WNV, Yolo County ranks as the No. 2 county in the state for human infections. Only Kern County, with 30, has more.

Reisen, Nielsen, Wheeler and Armijos collaborate on the research project with the UC Davis Wildlife Health Center, directed by veterinarian Walter Boyce, and the Sacramento-Yolo MVCD, managed by Dave Brown.

CVEC, a unit of the UC Davis School of Veterinary Medicine, works closely with the UC Mosquito Research Program, the UC Davis School of Medicine and the UC Davis College of Agricultural and Environmental Sciences.

"Carrie Nielsen and Veronica Armijos are doing most of the mosquito collections," Reisen said. Sarah Wheeler and Stan Wright (Sacramento-Yolo MVCD) are collecting wild birds to detect WNV antibodies; Kara Kelly (MVCD) is doing most of the testing of mosquito pools and wild bird sera; and Ying Fang (CVEC lab manager) is providing conformational studies and other quality control aspects."

Reisen, who began studying the role of corvids in the epidemiology of WNV shortly after the virus spread to California in 2003, recalls seeing communal roosts with some 30,000 to 50,000 crows in the Whittier Narrows Wildlife area, Los Angeles. "The sky was so black with crows, it was like the Wizard of Oz," he said. "The crows took five minutes to pass over."

"Communal crow roosts help drive the virus into the Culex populations," Reisen said. "That's why it's so important for people to find and report dead birds."

Reisen's research indicates that crows are a common host for the virus in urban areas and that scrub jays are a common host in rural and desert areas. "The distribution patterns of corvids," he said, "seem to determine the distribution of human cases in the urban landscapes, and the more elevated the viremia in the birds, the more likely the mosquitoes will be infected." Culex mosquitoes, including C. tarsalis, C. quinquefasciatus and C. pipiens, are the major transmitters.

Reisen said Southern California's WNV epidemic appears to be subsiding because most of the crows "have either died or the other birds are now immune." He said he expects the transmission to intensify in the Central Valley, driven by American crows and Western scrub-jays.

So far this year, WNV activity has been detected in 49 of California's 58 counties, Brown said. Statewide, callers reported 34,194 dead birds from Jan. 1 through Aug. 29. Of the 4,000 birds tested for the virus, 688 tested positive. The counties with the highest number of positive birds? As of Aug. 29, Sacramento and Santa Clara counties each had 56, followed by Stanislaus with 49, and Yolo with 48.

As of Aug. 29, Sacramento County dead bird reports totaled 4,035, more reports than any other county in the state. The 56 positives came from a total of 315 birds tested. Yolo County dead bird reports totaled 1,422, with 48 positives from the 179 tested.

Last year, Brown said, Sacramento and Yolo counties proved to be the nation's hot spot for WNV activity, with 189 human cases, one human fatality, and 19,431 dead bird reports. The 2005 statistics show 16,640 dead bird reports in Sacramento County and 2,791 in Yolo County. Those 2005 figures also indicate that of the 127 birds tested in Sacramento County, 70 tested WNV-positive. Of the 50 tested in Yolo, 17 tested positive.

Infected female mosquitoes transmit WNV when they seek a blood meal to develop their eggs. Discovered in the West Nile district of Uganda in 1937, the virus spread to New York in 1999 and reached California in 2003. Entomologists agree that the virus "is here to stay."

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Materials provided by University Of California, Davis. Note: Content may be edited for style and length.

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University Of California, Davis. "Crows Targeted In War Against West Nile Virus." ScienceDaily. ScienceDaily, 3 September 2006. <>.
University Of California, Davis. (2006, September 3). Crows Targeted In War Against West Nile Virus. ScienceDaily. Retrieved May 28, 2017 from
University Of California, Davis. "Crows Targeted In War Against West Nile Virus." ScienceDaily. (accessed May 28, 2017).