Department of Land Resources & Environmental Sciences
Montana State University
Bozeman, MT 

 

 
©2006 RKD Peterson

West Nile Virus

 

Epidemiology



 Introduction

      WNV infection has become a major public health concern in the United States since 1999, when the first outbreak in the Western Hemisphere occurred in New York City, with 62 cases of human encephalitis and 7 deaths reported (CDC 1999). This particular outbreak in New York City is thought to have affected 2.6% of the population (Hubalek 2001). In 2000, WNV spread to 3 states and 21 human cases of WNV infection and 2 deaths were reported. In 2001, there were 66 human cases and 9 deaths reported in 10 states, before it spread westward, affecting all but 6 states in 2002 and causing the largest arboviral encephalitis epidemic in U.S. history (Huhn et al. 2003). There were 4,156 documented human cases and 284 deaths reported that year (CDC 2003) and numbers continued to grow in 2003, when 9,862 human cases with 264 deaths occurred, reported by 46 states (CDC 2004a). In 2004, there were 2,539 human cases and 100 deaths in 41 states (Hayes et al. 2005). Since its first appearance in the United States, there have been >16, 706 reported cases and 666 human deaths (CDC 2004b, Hayes et al. 2005). Large numbers of human infections may not be detected, and there may be significant underreporting of milder cases of West Nile (WN) fever (Hubalek 2001, Huhn et al. 2003). Given the infection rate observed for the previous years, Peleman (2004) estimated that 1,500,000 persons were infected with the virus in 2003.
      According to a seroepidemiological survey conducted by Mostashari et al. (2001), for every diagnosed case of WN meningoencephalitis, there were approximately 30 additional people with WN fever, and approximately 2.6% of the population in New York were infected during the epidemic of 1999 (Mostashari et al. 2001). Overall, one in five (20%) infected persons develops mild febrile illness (Mostashari et al. 2001) and one person in 150 (0.67%) develops neurologic disease (Fratkin et al. 2004). Among hospitalized patients, about 59.8% develops WN encephalitis, 32.8% develops meningitis, and 14.5% develops acute flaccid paralysis (Asnis et al. 2001, Weiss et al. 2001, Emig and Apple 2003, Sejvar et al. 2003, Brilla et al. 2004, Klee et al. 2004). Extrapolating these numbers to the estimate that 0.67% of infected persons develops neurologic disease, then, among those infected, 0.43% develops encephalitis and 0.24% develops meningitis.

 Epidemiology

 Symptoms

 Risk Factors

 Morbidity

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       Case-fatality rates in the United States ranged from 4% to 18% among hospitalized patients (Nash et al. 2001, Weiss et al. 2001, Emig and Apple 2003, Pepperell et al. 2003, Sejvar et al 2003, Brilla et al. 2004) and from 2.7% to 14% among cases reported to CDC (CDC 2004b). Encephalitis cases have a 19% fatality rate (Nash et al. 2001, Weiss et al. 2001).   After infection, physical and mental capacity returns to normal in about a year, however preexisting comorbid conditions are associated with longer recoveries (Loeb et al. 2008).
     Although WNV produces higher levels of viremia in birds than St. Louis encephalitis virus and infects more mosquitoes, herd immunity in humans doesn't seem to have an effect on the incidence of the disease (Petersen and Hayes 2004). The levels of infection are not enough to decrease the frequency of epidemics or modulate their intensity (Hayes et al. 2005), and the risk of WNV disease is likely to vary considerably over time and place (Petersen and Hayes 2004).


References

Asnis, D.S., R. Conetta, G. Waldman, and A. A. Teixeira. 2001. The West Nile Virus Encephalitis Outbreak in the United States (1999-2000) – From Flushing, New York, to beyond its borders. Ann. N. Y. Acad. Sci. Dec 2001, 951: 161-171.

Brilla, R., M. Block, G. Geremia, and M. Witcher. 2004. Clinical and neuroradiological features of 39 consecutive cases of West Nile meningoencephalitis. J. Neurol. Sci. 220: 37-40.

CDC - Centers for Disease Control and Prevention. 1999. Outbreak of West Nile-like viral encephalitis: New York, 1999. MMWR Morb. Mortal. Wlly Rep 1999. 48: 845-849.

CDC - Centers for Disease Control and Prevention. 2003. 2002 West Nile Virus Activity in the United States. Accessed at http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount02.htm on 08 Feb 2005.

CDC - Centers for Disease Control and Prevention. 2004a. 2003 West Nile Virus Activity in the United States. Accessed at  http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount03_detailed.htm on 08 Feb 2005.

CDC - Centers for Disease Control and Prevention. 2004b. West Nile virus. Accessed at  http://www.cdc.gov/ncidod/dvbid/westnile/index.htm on 08 Feb 2005.

Emig, M. and D.J. Apple. 2003. Sever West Nile Disease in Healthy Adults. Clin. Infec. Dis.38(2): 289-292.

Fratkin, J.D., A.A. Leis, D.S. Stokic, S.A. Slavinski, and R.W. Geiss. 2004. Spinal cord neuropathology in human West Nile virus infection. Arch. Pathol. Lab. Med. 128: 533-537.

Hayes, E.B., N. Komar, R.S. Nasci, S.P. Montgomery, D.R. O’Leary, and G.L. Campbell. 2005. Epidemiology and transmission dynamics of west Nile Virus disease. Emerg. Infect. Dis. 11(8): 1167-1173.

Hubalek, Z. 2001. Comparative symptomatology of West Nile fever. Lancet Infect. Dis. 358: 254-255.

Huhn, G.D., J.J. Sejvar, S.P. Montgomery, and M.S. Dworkin. 2003. West Nile Virus in the United States: an update on an emerging infectious disease. Am. Fam. Physician 68: 653-660.

Klee, A.L., B. Maldin, B. Edwin, I. Poshni, F. Mostashari, A. Fine, M. Layton, and D. Nash. 2004. Long-term prognosis for clinical West Nile virus infection. Emerg. Infect. Dis. 10(8): 1404-1411.

Loeb, M., S. Hanna, L. Nicolle, J. Eyles, S. Elliott, M. Rathbone, M. Drebot, B. Neupane, M. Fearon, and J. Mahony. 2008. Prognosis after west nile virus infection. Annals of Internal Medicine 149: 232-241.

Mostashari, F., M.L. Bunning, D.A. Singer, D. Nash, M.J. Cooper, N. Katz, K.A. Liljebjelke, B.J. Biggerstaff, A. D. Fine, M.C. Layton, S.M.Mullin, A.J. Johnson, D.A. Martin, E.B. Hayes, and G. L. Campbell. 2001. Epidemic West Nile encephalitis, New York, 1999: results of a household-based seroepidemiological survey. Lancet Infect. Dis. 358: 261-264.

Nash, D., F. Mostashari, A. Fine, J. Miller, D. O’Leary, K. Murray, A. Huang, A. Rosenberg, A. Greenberg, M. Sherman, S. Wong, and M. Layton. 2001. The outbreak of West Nile virus infection in the New York City area in 1999. N. Engl. J. Med. 344(24): 1807-1814.

Peleman, R.A.A.M. 2004. New and re-emerging infectious diseases: epidemics in waiting. Curr. Opin. Anaesthesiol. 17: 265-270.

Pepperell, C., N. Rau, S. Krajden, R. Kern, A. Humar, B. Mederski, A. Simor, D.E. Low, A. McGeer, T. Mazzulli, J. Burton, C. Jaigobin, M. Fearon, H. Artsob, M.A. Drebot, W. Halliday, and J. Brunton. 2003. West Nile virus infection in 2002: morbidity and mortality among patient admitted to hospital in southcentral Ontario. CMAJ. 168(11): 1399-1405.

Petersen, L.R. and E.B. Hayes. 2004. Westward Ho? – The Spread of West Nile Virus. N. Engl. J. Med. 351: 2257-2259.

Sejvar, J.J., M.B. Haddad, B.C. Tierney, G.L. Campbell, A.A. Marfin, J.A. Van Gerpen, A. Fleischauer, A.A. Leis, D.S. Stokic, and L.R. Petersen. 2003. Neurologic manifestations and outcome of West Nile virus infection. JAMA 290(4): 511-515.

Weiss, D., D. Carr, J. Kellachan, C. Tan, M. Philips, E. Bresnitz, and M. Layton. 2001. Clinical findings of West Nile virus infection in hospitalized patients, New York and New Jersey, 2000. Emerg. Infec. Dis. 7(4): 654-658.

 

 

 

 

 

 

       



        Although we provide general information on West Nile Virus (WNV), this web site is designed primarily to provide information on human-health and ecological risk assessments of WNV and tactics used for mosquito management.