West Nile Virus 2003
*Rate per 100,000 population based on the U.S. Census Bureau’s population data as of July 1, 2003
The West Nile virus (WNV) was first identified in Indiana in 2001. In that year, WNV was identified in 47 birds and 1 horse from 7 counties. In 2002, Indiana was one of 39 states, including Washington, D.C., to report human WNV cases. Nationally in 2003, there were 9,862 human cases with 264 deaths. Indiana had 48 reported cases with 4 deaths, representing less than 1 percent of reported cases and 1.5 percent of deaths nationally.
In 2003, the 48 reported cases of WNV in Indiana represented a rate of less than 1 case per 100,000 population (Table 1). This is over an 83 percent decrease from 2002 (293). There was also a decrease in deaths from 11 reported deaths in 2002 to 4 reported in 2003. This decrease was due to a more normal weather pattern in 2003 than in 2002. In 2003, the initial cases had onset of illness dates starting in mid-June, which was a full month earlier than in 2002, followed by a three-week period in July without any reported cases (Figure 1). Reported cases reoccurred with onset dates in early August and a decrease in reported cases in mid-to-late September, unlike 2002 where case onset dates continued until early November. This biphasic pattern has been attributed to two periods of heavy rainfall across the state, one on the Fourth of July weekend and the other on Labor Day weekend. The heavy rainfall flushed breeding sites of Culex mosquitoes disrupting mosquito development and the virus amplification cycles necessary for virus transmission to humans. Seventy-nine percent of reported cases were in individuals aged 40 years and older (Figure 2).
Twenty-four Indiana counties reported human West Nile virus cases in 2003. However, only two counties had five or more reported cases (population adjusted): Allen (2.05) and Lake (1.43).
West Nile virus is endemic in Indiana and virus activity will continue to occur during the mosquito breeding season in future years. The extent of activity will depend on the weather, presence of mosquito and bird populations for virus amplification, equine vaccination rates, and human activities to prevent transmission.
You can learn more about West Nile virus by visiting the following web sites: