High subclinical West Nile virus incidence among nonvaccinated horses in northern California associated with low vector abundance and infection.
Abstract: 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.
Publication Date: 2008-01-12 PubMed ID: 18187784
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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This research focused on understanding the transmission of the West Nile virus (WNV) among horses. Carried out over two years at the University of California, Davis, it revealed there was a high rate of WNV infection among non-vaccinated horses despite low mosquito abundance and infection.
Research Overview
- The research was conducted from April to October from 2003 to 2005 at a research facility in the University of California, Davis.
- As part of the study, researchers monitored the mosquito population and infection rates thrice weekly.
- They observed a group of 32 non-vaccinated horses that were part of a vaccine study, testing their serum monthly for signs of WNV infection.
Findings
- In 2004, one horse developed a confirmed WNV disease which was associated with a single infected Culex pipiens mosquito pool.
- The ratio of apparent to non-apparent WNV infections among non-vaccinated horses was 1:4, indicating that for every horse showing symptoms, there were four infected but not showing symptoms.
- In 2005, four infected mosquito pools were associated with a much higher equine infection incidence of 62%, along with a higher apparent to inapparent ratio of 1:17.
- Research revealed a significant number of blood-engorged Cx. pipiens mosquitos had fed on birds, whereas the Cx. tarsalis mosquitos primarily fed on horses.
Interpretation and Implication
- These results showed non-vaccinated horses were susceptible to WNV infection and served as a sensitive indicator of WNV activity in an area.
- The presence of Cx. pipiens and Cx. tarsalis mosquitos was associated with an increased risk of WNV infection in horses.
- These two mosquito species served as both enzootic and bridge vectors, amplifying the WNV among birds and transmitting it to horses.
- The high infection rate among non-vaccinated horses, despite low mosquito abundance, speaks to the efficiency of transmission and the importance of vaccination and vector control to prevent dissemination of the disease.
Cite This Article
APA
Nielsen CF, Reisen WK, Armijos MV, Maclachlan NJ, Scott TW.
(2008).
High subclinical West Nile virus incidence among nonvaccinated horses in northern California associated with low vector abundance and infection.
Am J Trop Med Hyg, 78(1), 45-52.
Publication
Researcher Affiliations
- Graduate Group in Epidemiology, Center for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, and Department of Entomology, University of California, Davis, CA 95616, USA. cfnielsen@ucdavis.edu
MeSH Terms
- Animals
- Antibodies, Viral / blood
- Birds / physiology
- California / epidemiology
- Culex / physiology
- Culicidae / physiology
- Disease Transmission, Infectious / veterinary
- Enzyme-Linked Immunosorbent Assay / veterinary
- Female
- Horse Diseases / blood
- Horse Diseases / epidemiology
- Horse Diseases / etiology
- Horse Diseases / mortality
- Horse Diseases / prevention & control
- Horse Diseases / transmission
- Horses
- Incidence
- Insect Vectors / physiology
- Seasons
- Survival Analysis
- Temperature
- Viral Vaccines / therapeutic use
- West Nile Fever / epidemiology
- West Nile Fever / transmission
- West Nile Fever / veterinary
- West Nile virus / genetics
- West Nile virus / immunology
- West Nile virus / isolation & purification
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