After its introduction into the eastern United States in 1999, West Nile virus (WNV) reached California in 2003 (1
). In response, the state enhanced mosquito management programs to reduce vector populations and virus transmission (2
). By late summer 2005, WNV disease was epidemic in Sacramento County, with more cases reported in Sacramento County than in any other county in the nation that year (3
). The Sacramento-Yolo Mosquito and Vector Control District (SYMVCD) responded by conducting emergency aerial spraying over the city of Sacramento and surrounding areas to reduce mosquito populations.
Effective management of infection rates, illness, and death from mosquito-borne pathogens such as WNV requires reduced contact between humans and infected mosquitoes (4
). No effective treatment exists for WNV; prevention of disease relies on management of mosquitoes through various control tactics. Elnaiem et al. (5
) and Carney et al. (6
) examined the efficacy of the 2005 emergency aerial spray in Sacramento County, which used pyrethrins as the active ingredients to control adult mosquitoes. In both studies, an unsprayed area within the county was used as the control. Elnaiem et al. showed a total decrease in WNV-competent vector mosquitoes, Culex pipiens
and Cx. tarsalis
, of 57.5%, compared with the prespray population in the treated area (5
). They also observed a decrease in WNV infection rates in mosquitoes to 3.9/1,000 for trapped females in the treated areas, compared with 6.7/1,000 in the untreated areas (5
). Carney et al. used illness onset dates and residential locations for 152 of the 163 WNV disease cases reported in humans in 2005 to determine the efficacy of the spray event (6
). Their results showed no incident human cases in the treated area after the spray event, compared with 18 cases in the untreated area. Consequently, the emergency aerial spray seemed to effectively reduce both mosquito populations (5
) and human WNV cases (6
WNV infection can be asymptomatic or symptomatic in humans, with a 4:1 ratio (7,8
). The disease can be mild, resulting in influenza-like symptoms (as in West Nile fever [WNF]), or severe, affecting the central nervous system symptom (as in West Nile neuroinvasive disease [WNND]) (7
). Many WNF cases are not reported because they are not recognized as WNF; symptoms can resemble a cold or mild influenza-like illness, for which medical care is not sought, or is underdiagnosed because the additional cost of testing would not provide alternative direction to effective palliative medical care (7,9
Zohrabian et al. (10
) estimated the economic impact of the WNV disease outbreak in 2002 in Louisiana, which resulted in 24 deaths. They included costs of inpatient and outpatient medical care, productivity loss, the state’s public health department, and vector control. Total epidemic costs were ≈$20.14 million for the 329 cases, including $9.2 million for mosquito control and public health agency costs. Zohrabian et al. (11
) used the economic data from their 2004 study to determine the cost-effectiveness of the initiation of a potential WNV vaccination and found that the cost of vaccination would not offset the costs in medical care.
Several studies have demonstrated the efficacy of mosquito management in response to WNV, but only the study by Carney et al. (6
) suggested a reduction in human WNV cases associated with aerial adult-mosquito control. We estimated the economic cost of the 2005 WNV disease outbreak in Sacramento County, California, and evaluated the reduction in WNV disease necessary to offset the cost of emergency vector control. Economic costs for patients’ productivity loss and for treatment of disease symptoms, as well as for emergency vector control conducted in response to the outbreak were also investigated.