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UC Davis scientists targeting crows in war against West Nile virus

September 8, 2006

September 8, 2006

DAVIS--Where there are communal crow roosts, look for the West Nile virus (WNV).

Corvids, including American crows, Yellow-billed Magpies, Western scrub-jays and other members of the Corvidae family, serve as the primary reservoirs or incubators for the mosquito-borne virus, according to research entomologist William Reisen of the Center for Vectorborne Diseases (also known as CVEC), a unit of the UC Davis School of Veterinary Medicine.

"Corvid surveillance is crucial to stopping the transmission of the virus," he said. September is a crucial month in the war against WNV, which usually peaks in late August and September and ends in October.

"Communal crow roosts help drive the West Nile virus into the Culex (mosquito) populations--that's why it's so important for people to find and report dead birds," said Reisen, a professor with the Department of Pathology, Microbiology and Immunology at the UC Davis School of Veterinary Medicine. "Crows are good hosts for mosquitoes. 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."

Reisen and his research team are targeting crows in work funded by the UC Mosquito Research Program (UCMPR), Sacramento-Yolo Mosquito and Vector Control District (MVCD), and the National Institutes of Health. CVEC works closely with UCMRP, the UC Davis School of Medicine and the UC Davis College of Agricultural and Environmental Sciences.

"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," she said.

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.

West Nile CrowCollaborators Sarah Wheeler and Veronica Armijos collect dead birds in Davis as part of the research, test mosquito pools and set surveillance traps. They agree that crows can die fast. One resident told them that a crow "just fell out of a tree in my backyard--dead."

Wheeler and Armijos, who both have college degrees in ecology, head out on "Bird Patrol" several times a week. 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.

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 remove the kidneys for diagnostic testing.

Next stop: CVEC lab, a biosafety level 3 containment laboratory directed by Reisen. Samples arrive from all over 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 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, CVEC staff chart the dead bird reports on an avian density map. So far this year (as of Aug. 29), 35 birds collected in Davis tested WNV-positive; that number exceeds every other city in California except Fresno, with 42. Yolo County, with 21 confirmed human cases of WNV, including 12 in Davis, ranks as the No. 2 county in the state for human infections. Only Kern County, with 30, has more.

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 in Los Angeles County.

"The sky was so black with crows, it was like the Wizard of Oz," he said. "The crows took five minutes to pass over."

Reisen said Southern California's WNV epidemic appears to be subsiding because most of the crows have died or are immune. He said he expects the transmission to intensify in the Central Valley, driven by American crows and Western scrub-jays. His 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 seem to determine the distribution of human cases in the urban landscapes, and the more elevated the viremia (virus in the bloodstream) in the birds, the more likely the mosquitoes will be infected," he said. Culex mosquitoes, including C. tarsalis, C. quinquefasciatus and C. pipiens, are the major transmitters.

As of Sept. 1, WNV activity has been detected in 49 of California's 58 counties. 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 WNV-positive bird tests are Sacramento and Santa Clara counties, with 56 each, followed by Stanislaus with 49, and Yolo with 48.

Last year, 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.

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."

CONTACT: Kathy Keatley Garvey, (530) 754-6894,

More information on the West Nile virus: 
Sacramento-Yolo MVCD,
UC Davis CVEC,
Centers for Disease Control and Prevention,
The UC Mosquito Research Program,
Latest California statistics are on the DHS Web site at

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