The Epidemic of West Nile Virus in the United States, 2002

Abstract

Since 1999, health officials have documented the spread of West Nile virus across the eastern and southern states and into the central United States. In 2002, a large, multi-state, epidemic of neuroinvasive West Nile illness occurred. Using standardized guidelines, health departments conducted surveillance for West Nile virus illness in humans, and West Nile virus infection and illness in non-human species. Illnesses were reported to the Centers for Disease Control and Prevention (CDC) through the ArboNET system. In 2002, 39 states and the District of Columbia reported 4,156 human West Nile virus illness cases. Of these, 2,942 (71%) were neuroinvasive illnesses (i.e., meningitis, encephalitis, or meningoencephalitis) with onset dates from May 19 through December 14; 1,157 (28%) were uncomplicated West Nile fever cases, and 47 (1%) were clinically unspecified. Over 80% of neuroinvasive illnesses occurred in the central United States. Among meningitis cases, median age was 46 years (range, 3 months to 91 years), and the fatality-to-case ratio was 2%; for encephalitis cases (with or without meningitis), median age was 64 years (range, 1 month to 99 years) and the fatality-to-case ratio was 12%. Neuroinvasive illness incidence and mortality, respectively, were significantly associated with advanced age (p = 0.02; p = 0.01) and being male (p < 0.001; p = 0.002). In 89% of counties reporting neuroinvasive human illnesses, West Nile virus infections were first noted in non-human species, but no human illnesses were reported from 77% of counties in which non-human infections were detected. In 2002, West Nile virus caused the largest recognized epidemic of neuroinvasive arboviral illness in the Western Hemisphere and the largest epidemic of neuroinvasive West Nile virus ever recorded. It is unknown why males appeared to have higher risk of severe illness and death, but possibilities include higher prevalence of co-morbid conditions or behavioral factors leading to increased infection rates. Several observations, including major, multi-state West Nile virus epidemics in 2002 and 2003, suggest that major epidemics may annually reoccur in the United States. Non-human surveillance can warn of early West Nile virus activity and needs continued emphasis, along with control of Culex mosquitoes.

Caspase 3-Dependent Cell Death of Neurons Contributes to the Pathogenesis of West Nile Virus Encephalitis

“…WNV induces neuronal damage and loss in several brain regions, including the hippocampus, brain stem, cerebellum, and anterior horn of the spinal cord (13,21). No therapy or vaccine is currently available for humans, and approximately 10% of individuals with neuroinvasive disease succumb to the infection (36,40). In addition, long-term neurological sequelae with associated motor, sensory, and cognitive complications are frequent (1,19,49).…”

Cost-effectiveness of West Nile Virus Vaccination

“…While the risk is highest in the elderly, severe disease does occur among young adults and children ( 4 , 24 ). The more severe, untreatable manifestations of WNV infection would compel interest in vaccine development and use even if vaccination is expensive, particularly since current prevention strategies such as personal repellent use or environmental reduction of mosquito abundance may not be consistently implemented ( 25 ).…”

“…Based on the limited data from these studies, we assumed that 35% of patients would have lifelong disability after neuroinvasive WNV disease. Of 2,942 patients with neuroinvasive WNV disease reported in the United States in 2002, 276 (9%) died ( 4 ). For our model, we assumed a case-fatality ratio of 9%.…”

“…Although the disability cost of stroke may underestimate the disability cost due to WNV because the median age for WNV neuroinvasive patients is lower (64 years of age [29]) than the median age of persons disabled due to stroke (76 years of age [30]), this will not have significant effect on the results because the cost-effectiveness ratio was not sensitive to changes in the cost of disability. The estimate of the cost of disability due to stroke in 1990 dollars was discounted at a 5% rate.…”

“…From 1999 through 2004, >16,600 WNV illnesses in humans have been reported in the United States; >7,000 of these were neuroinvasive disease, and >600 were fatal. In 2002 alone, 2,942 cases of neuroinvasive WNV disease were reported in the United States, which represents the largest epidemic of neuroinvasive WNV disease ever recorded ( 4 ). Approximately 20% of WNV infections in humans result in symptomatic illness, and ≈1% of infections lead to encephalitis, meningitis, or acute flaccid paralysis ( 1 ).…”