West Nile Virus 2002
*Rate per 100,000 population based on the U.S. Census Bureau’s population data as of July 1, 2002
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 2002, there were 4,156 human cases with 284 deaths. Indiana had 293 reported cases with 11 deaths, representing 7 percent of reported cases and 3.8 percent of deaths nationally.
West Nile virus transmission was enhanced during the summer of 2002 by the abnormally hot and dry weather. During the months June through September, Indiana’s mean temperature ranged between 1 and 6 degrees above normal. Rainfall for June, July, and August ranged between 35 and 90 percent below normal. The high heat and low rainfall created excellent conditions and breeding sites for the vector mosquito, Culex pipiens. Culex pipiens prefers breeding sites of stagnant, organically enriched water such as containers or pools of water containing high levels of septic effluent. The abnormally hot temperatures increased the speed of the mosquito’s life cycle, thus increasing the numbers of mosquitoes available to participate in virus transmission. The hot temperatures also shortened the time needed for the virus to develop within the mosquito. The results were an increased number of mosquitoes with ability to transmit virus more quickly. The onset of the first reported case occurred the week of July 20, 2002 (Figure 1). A sharp increase in the number of reported cases occurred starting the week of August 18, 2002. The outbreak peaked the week of September 14, 2002, with the last case onset the week of November 9, 2002.
In 2002, West Nile virus cases in Indiana ranged in age from 7 to 89 years, with a mean and a median age of 52 years. The 11 deaths ranged in age from 51 to 89 years and had a mean and median age of 72 years. The male:female ratio of cases was 1:1.1.
Age appeared to be the biggest risk factor for clinical WNV infection as case rates increased as age increased (Figure 2). Not only were there more cases in older age groups, but the severity of the case generally increased with age as well. West Nile fever (WNF) was identified in 112 of the reported cases, and 180 reported cases were more severe being identified as neuroinvasive (CNS) (meningitis and encephalitis) (Figure 3). Indiana’s experience and national reports have stressed the risk of severe disease in older age groups, but all age groups are at risk for severe disease and should take steps to prevent exposure.
In 2002, evidence of West Nile virus transmission was found in every Indiana county either in mosquitoes, birds, horses, or humans. Human cases were identified in 45 counties with the majority of cases (45%) occurring in 13 northeast counties. The counties with the highest incidence rates reporting five or more cases were: Adams (23.9), Gibson (21.5), Allen (20.4), and Whitley (16). Allen County reported 69 cases, the most reported cases of any county. Figure 4 shows counties reporting five or more cases of WNV in 2002.
In 2002, West Nile virus was isolated from a human, horse, bird, or mosquito in every county in the state. WNV should be considered endemic in the state, and it should be assumed that there will be WNV activity during the mosquito breeding season in future years. The extent of the activity will depend upon 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: