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WEST NILE VIRUS INFECTION IN TREE SQUIRRELS (RODENTIA: SCIURIDAE) IN CALIFORNIA, 2004–2005

West Nile virus (WNV) transmission generally involves a mosquito vector and an avian reservoir host, with mammals as incidental hosts. Although most mammalian WNV infections cause low or no morbidity or mortality, tree squirrels are susceptible to WNV-associated neurologic disease with infection prevalence comparable to that in dead birds. Positive species included fox squirrel (Sciurus niger), western gray squirrel (S. griseus), and eastern gray squirrel (S. carolinensis). Kidney tissue (dissected and swabbed), and oropharyngeal (oral) swab samples from tree squirrels submitted by California vector control and rehabilitation agencies were tested by reverse transcription–polymerase chain reaction; cycle threshold values were similar for all three samples, ranging from 21.9 to 26.5. Kidney tissue was more sensitive than oral swabs for detecting WNV in squirrels. Three of 36 live neurologic tree squirrels had viremia approximately 5 log₁₀ plaque-forming units/mL or greater, similar to WNV-infected birds. Tree squirrels are useful in WNV surveillance and provide localized evidence of WNV transmission to mammals.

Received September 28, 2006. Accepted for publication January 19, 2007.

Acknowledgments: We thank Marzieh Shafii, Ian Holser, Julian Saputo, Jessica Jennings, Karen Ehnert, the staff of the Lindsay Museum, and the staff of the Wildlife Care Association for excellent technical assistance. In addition, we are grateful to the staff of the WNV hotline and all the California vector control districts that submitted squirrels and other mammals for testing: Alameda County Mosquito Abatement District (MAD), Burney Basin MAD, Butte County Mosquito Vector Control District (MVCD), Consolidated MAD, Contra Costa County MVCD, El Dorado County Vector Control (VC), Fresno MVCD, Greater Los Angeles Vector Control District (VCD), Marin-Sonoma MVCD, Napa County MAD, Orange County VCD, Placer County MAD, Sacramento-Yolo MVCD, San Bernardino County VCP, San Mateo County MAD, San Joaquin County MVCD, Santa Clara County VCD, Santa Cruz County MVCD, Shasta County MVCD, Tehama County MVCD, Tuolumne County Environmental Health (EH), and Ventura County EH.

Financial support: This research was supported by the Centers for Disease Control and Prevention, as part of grant U50/CCU923677-02 (Epidemiology and Laboratory Capacity for Infectious Diseases) and grant Number RO1-AI55607 from the National Institute of Allergy and Infectious Diseases, National Institutes of Health to William K. Reisen.

^* Address correspondence to Kerry A. Padgett, Vector-Borne Disease Section, California Department of Health Services, 850 Marina Bay Parkway, Richmond, CA 94804. E-mail: kpadgett{at}dhs.ca.gov

Authors’ addresses: Kerry A. Padgett, Ryan Carney, and Stan Husted, Vector-Borne Disease Section, California Department of Health Services, 850 Marina Bay Parkway, Richmond, CA 94804, Telephone: 510-412-6252, E-mails: kpadgett{at}dhs.ca.gov, rcarney{at}dhs.ca.gov, and shusted{at}dhs.ca.gov. William K. Reisen and Ying Fang, Center for Vectorborne Diseases, Old Davis Road, One Shields Avenue, Davis, CA 95616, Telephone: 530-752-0124, E-mails: arbo123{at}pacbell.net and ylfang{at}ucdavis.edu. Nicole Kahl-Purcell, Sacramento Department of Public Health, 4600 Broadway, Suite 2300, Sacramento, CA 95820, Telephone: 916-874-9231, E-mail: nkahl{at}pacbell.net. Barbara Cahoon-Young, Placer County Public Health Laboratory, 11484 B Avenue, Auburn, CA 95603, Telephone: 530-889-7210, E-mail: bcahoon{at}placer.ca.gov. Nancy Anderson, Lindsay Wildlife Museum, 1931 First Avenue, Walnut Creek, CA, 94597, Telephone: 925-927-2942, E-mail: nanderson{at}wildlife-museum.org. Lynda Zucca, Wildlife Care Association, PO Box 60982, Sacramento, CA 95860, Telephone: 916-722-0400, E-mail: lzdvm{at}mindspring.com. Leslie Woods, Wyoming State Veterinary Laboratory, 1174 Snowy Range Road, University of Wyoming, Laramie, WY, 82070, Telephone: 307-742-6628, E-mail: lwoods1{at}uwyo.edu. Vicki L. Kramer, California Department of Health Services, Vector-Borne Disease Section, 1616 Capitol Avenue, MS7307, PO Box 997413, Sacramento, CA 95899-7413, Telephone: 916-552-9730, E-mail: vkramer{at}dhs.ca.gov.

Reprint requests: Kerry A. Padgett, Vector-Borne Disease Section, California Department of Health Services, 850 Marina Bay Parkway, Richmond, CA 94804, Telephone: 510-412-6252, E-mail: kpadgett{at}dhs.ca.gov.

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