HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Nielsen, C. F. Articles by Scott, T. W. Search for Related Content PubMed PubMed Citation Articles by Nielsen, C. F. Articles by Scott, T. W. Related Collections West Nile Medical Entomology

High Subclinical West Nile Virus Incidence among Nonvaccinated Horses in Northern California Associated with Low Vector Abundance and Infection

Although horse cases frequently are reported during West Nile virus (WNV) outbreaks, few investigations have focused on the epidemiology of this transmission. From April to October 2003 to 2005, mosquito abundance and infection were monitored 3 days per week at an equine research facility at the University of California, Davis. Thirty-two nonvaccinated horses enrolled as controls in a vaccine study were bled monthly, and their serum was tested for evidence of WNV infection by plaque reduction neutralization test (PRNT). In 2004, one positive Culex pipiens pool was associated with a single horse that presented with confirmed WNV disease in late September. The annual incidence of clinical and subclinical WNV infection in the nonvaccinated horses was 16%, with an apparent to inapparent ratio of 1:4 among infected horses. In 2005, two Culex tarsalis and two Cx. pipiens WNV-positive pools were associated with an equine infection incidence of 62%, with an apparent to inapparent ratio of 1:17. The majority (79%) of 70 blood-engorged Cx. pipiens fed on birds and the remaining on equines (21%). Conversely, Cx. tarsalis fed primarily on equines (n = 23, 74%), followed by birds (n = 7, 23%) and 1 (3%) fed on a lagomorph. These data indicated that nonvaccinated horses were a sensitive indicator of WNV activity and that their risk of infection was associated with the presence of infection in Cx. pipiens and Cx. tarsalis, which served as both enzootic and bridge vectors amplifying WNV among birds and transmitting WNV to horses.

Received April 30, 2007. Accepted for publication October 23, 2007.

Acknowledgments: The authors thank Barbara Cahoon-Young and the staff at the Center for Vectorborne Diseases laboratory for testing the mosquito pools and Ying Fang for providing protocols for the PRNTs. Dave Brown and the SYMVCD provided the mosquito-trapping equipment and allowed us to use their laboratory facilities. Kara Kelley, SYMVCD, assisted with the blood-meal identifications. Tim Carpenter reviewed the manuscript. Pam Hullinger, Gregory Ferraro, and the staff at the Center for Equine Health are thanked for their excellent organizational assistance and maintenance of the horses, and they also provided the sera for the PRNTs. The authors thank 2 anonymous reviewers for their knowledgeable comments.

Financial Support: Funding was provided, in part, by the Sacramen-to–Yolo Mosquito and Vector Control District (SYMVCD). Research grants from the Center for Equine Health, University of California, Davis, and the Maurice Stans Foundation provided funding for the equine research.

^* Address correspondence to Carrie Nielsen, Department of Medicine and Epidemiology, Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Davis, CA 95616. E-mail: cfnielsen{at}ucdavis.edu

Authors’ addresses: Carrie F. Nielsen, William K. Reisen, and M. Veronica Armijos, Center for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, University of California, Old Davis Road, Davis, CA 95616, Telephone: +1 (530) 752-0124, Fax: +1 (530) 754-6360, E-mail: cfnielsen{at}ucdavis.edu, arbo123{at}pacbell.net, mvarmijos{at}ucdavis.edu. N. James MacLachlan, Department of Pathology, Microbiology, and Immunology, School of Veterinary Medicine, University of California, Davis, CA 95616, Telephone: +1 (530) 752-1163, Fax: +1 (530) 752-3349, E-mail: njmaclachlan{at}ucdavis.edu. Thomas W. Scott, Department of Entomology, University of California, Davis, CA, Telephone: +1 (530) 754-4196, Fax: +1 (530) 752-1537, E-mail: twscott{at}ucdavis.edu.

This article has been cited by other articles:

				C. F. Nielsen, M. V. Armijos, S. Wheeler, T. E. Carpenter, W. M. Boyce, K. Kelley, D. Brown, T. W. Scott, and W. K. Reisen Risk Factors Associated with Human Infection during the 2006 West Nile Virus Outbreak in Davis, a Residential Community in Northern California Am J Trop Med Hyg, January 1, 2008; 78(1): 53 - 62. [Abstract] [Full Text] [PDF]