The U.S. Centers for Disease Control and Prevention released the final 2012 national surveillance data for West Nile virus activity on May 14. Last year a total of 5,674 cases of West Nile virus disease in people, including 286 deaths, were reported to CDC from 48 states (excluding Alaska and Hawaii).
Of all West Nile virus disease cases reported, 2,873 (51 percent) were classified as neuroinvasive disease (meningitis, encephalitis, acute flaccid paralysis etc.) The dates of illness onset ranged from March through December 2012.
The numbers of neuroinvasive, non-neuroinvasive, and total West Nile virus disease cases reported in 2012 are the highest since 2003. The number of deaths is the highest since cases of the disease were first detected in the United States in 1999.
In 2012, 62 percent of all reported West Nile virus cases—were concentrated in California, Louisiana, Michigan, Mississippi, Oklahoma, South Dakota, and Texas. Texas reported 33 percent of all reported West Nile virus cases in 2012. According to the report, none of the lower 48 states was unaffected. There were 131 cases in Colorado last year. Most of the cases were clustered along the Front Range but 60 cases occurred in four counties on the Western Slope.
Last summer's outbreak likely resulted from many factors, including higher-than-normal temperatures that influenced mosquito and bird abundance, the replication of the virus in its host mosquitoes, and interactions of birds and mosquitoes in hard-hit areas. Because the factors that lead to West Nile virus disease outbreaks are complex, CDC cannot predict where and when they will occur.
Each spring, the CDC releases the final West Nile virus case surveillance data for the previous year. The data include confirmed and probable human disease cases reported to ArboNET by state and local health departments. ArboNET is the national, electronic surveillance system established by the CDC to assist states in tracking illness caused by West Nile virus and other viruses transmitted by mosquitoes or ticks.
Last year's large outbreak is a reminder that it is important for people to protect themselves from West Nile virus, especially as we head into summer and mosquitoes become more active. The best way to prevent West Nile virus disease is to avoid mosquito bites:
Use insect repellents when you go outdoors. Use repellents containing DEET, picaridin, IR3535, and para-menthane-diol (PMD) because these repellents provide longer-lasting protection than other products. Always follow the instructions on the label.
Wear long sleeves and pants at dawn and dusk when many mosquitoes are most active.
Repair or install screens on windows and doors. Use air conditioning, if you have it.
Help reduce the numbers of mosquitoes around your home. Empty standing water from items such as gutters, flowerpots, buckets, and kiddie pools and change birdbaths weekly.
To access the full report and more tips on how to prevent infection, visit www.cdc.gov/westnile.