West Nile Virus, Texas, USA, 2012

During the 2012 West Nile virus outbreak in Texas, USA, 1,868 cases were reported. Male patients, persons >65 years of age, and minorities were at highest risk for neuroinvasive disease. Fifty-three percent of counties reported a case; 48% of case-patients resided in 4 counties around Dallas/Fort Worth. The economic cost was >$47.6 million.

onset ranged from May 1, 2012, through December 6, 2012 ( Figure 1). The outbreak peaked during week 33 (mid-August) with 225 reported cases, which is historically the same peak for all reported WNV cases in Texas during 2002-2011 (2). The median time from date of symptom onset to date of official report to TxDSHS was 27 days (range 6-274 days).
When examining the demographic characteristics of the reported cases, we found significant differences in sex, age, and race/ethnicity with regard to severity of disease. Overall, a higher percentage of male case-patients were reported (55%), and male case-patients were significantly more likely than female case-patients to have WNND (OR 1.5, 95% CI 1.2-1.8, p<0.001). Median age of all case-patients was 54 years (range 1 month-100 years). As each age category increased, the attack rates also increased (Table). Persons >65 years of age were significantly more likely than younger persons to have WNND (OR 2.1, 95% CI 1.8-2.6, p<0.001). The median age of the 89 case-patients who died was 79 years (range 25-100 years). When examining race/ethnicity of all cases, we observed the highest attack rate (11.1 cases/100,000 population) in white, non-Hispanics. However, minority populations were significantly more likely to have WNND (OR 1.9, 95% CI 1.6-2.4, p<0.001).
Of the 254 counties in Texas, 135 (53%) reported a WNV case ( Figure 2

Conclusions
The 2012 WNV outbreak in Texas was unexpected in terms of the magnitude of virus transmission and number of human cases. We recently observed a 3-year pattern of increases in reported human cases in Texas, as seen in 2003, 2006, and 2009 (2). In 2012, the dramatic epidemic was consistent with this prior observation, with the 1,868 reported cases being more than double the historic high, which occurred in 2003 (735 cases). In addition to the dramatic increase in human cases in 2012, the state also reported an increase in equine cases (121 cases in 2012 compared with 6 cases in 2011). The exact factors that contributed to this epidemic are unknown and most likely complex, considering that successful transmission depends on supportive environmental conditions, vector abundance, avian reservoir and susceptible host abundance, pathogenicity of the virus, and sizeable populations of immunologically naive reservoir species.
WNV more severely affects persons >65 years of age; deaths typically are reported in elderly presons (4,5).  Table, wwwnc. cdc.gov/EID/articlepdfs/19/11/13-0768-Techapp1.pdf). In addition to these acute costs, the outbreak also required an increase in resources for mosquito control and public health efforts to respond to the epidemic. A recent study reported the cost of aerial spraying alone in Dallas County exceeded $1.6 million (7). The long-term economic impact of this outbreak also is expected to be substantial as a consequence of long-term rehabilitation and disability costs (8), possible risk for chronic kidney disease (9), and risk for premature death in severe cases (10).
The unprecedented 2012 outbreak confirms the need for continued vigilance for surveillance to enable timely implementation of control measures to prevent virus transmission. We expect Texas will continue to experience endemic levels of virus transmission with periodic epizootics. Considering the economic and physical costs   to persons severely affected, development of an effective vaccine is urgently needed to prevent disease. Until a vaccine becomes available, public health authorities will need to maintain their focus on surveillance, disease recognition, implementation of control measures, and public education about protective measures.