The Emergence of West Nile Virus: Online First from theAnnals of Internal Medicine

Tuesday, September 11, 2012 // Uncategorized

Note:  It doesn’t mention the status of the vaccine.

The Resurgence of West Nile Virus FREE ONLINE FIRST

Catherine M. Brown, DVM, MSc, MPH; and Alfred DeMaria, MD

Ann Intern Med. 11 September 2012
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Over the past several decades, public health officials have worked to alert the public to the threat of emerging infections, and the idea has captured the imagination of the American public; consider The Andromeda Strain, Outbreak, and Contagion. However, these theatrical portrayals—in which a scary, mysterious disease emerges but is investigated and controlled by the time the credits roll—fail to reveal the true drama, the ecological complexity of the world where real and deadly diseases occur. Over the years, public health professionals and the public alike have gotten excited about such pathogens as hantavirus, H5N1 avian influenza, the severe acute respiratory syndrome (SARS), monkeypox, and West Nile virus (WNV). Whereas H5N1, SARS, and monkeypox are only vague memories for most Americans because the ecosystem that fostered disease transmission exists overseas and not in our own backyards, WNV and hantavirus are different. While the public and professionals may have become somewhat complacent about both diseases, 2012 is reminding us that perhaps we shouldn’t have.

West Nile virus has become endemic in North America. It is here to stay because we have the right combination of birds and mosquitoes. But this very endemicity has been driving complacency. After all, up to 80% of infected persons have few or no symptoms. However, WNV still causes life-threatening encephalitis and meningoencephalitis in some patients, and we are learning more about long-term sequelae (1), including flaccid paralysis (2). Although no vaccine or effective treatment is available, clinicians must maintain clinical suspicion of infection under the right circumstances of season and mosquito exposure to arrive at the correct diagnosis in a presenting patient. In addition, WNV remains a significant concern for blood collection organizations and transfusion services, requiring screening of donors, either individually or in pooled samples, on the basis of virus activity indicators.

Cases of WNV infection in 2012 in the United States have already exceeded that of any other year—with 1590 cases, 65 deaths, and 303 viremic blood donors as of 28 August 2012 (3)—including the earliest years after the introduction of WNV in 1999 when the U.S. population, both bird and human, was immunologically naive to the virus. The problem this year is so dramatic that cities, such as Dallas (4), have resorted to aerial pesticide application to kill adult mosquitoes (adulticiding) for the first time in 45 years. Texas, South Dakota, Mississippi, Oklahoma, Louisiana, and Michigan have been particularly hard hit. Could this be a new strain of the virus, a reintroduction, or a mutation? Is a long-term temporal cycle of WNV infection emerging? Or, and perhaps most likely, is this year’s experience related to unusual weather patterns? Temperature does have a role in WNV amplification.

This year we have been hearing from local health departments, wildlife rehabilitators, and other observers about an increase in dead bird sightings compared with recent years, which is similar to what had been seen earlier in the WNV experience. This might suggest a new WNV strain, either by natural selection or introduction. A new strain could also account for a change in human epidemiology. Alternatively, the reservoir of infection in birds might be substantial—some species experienced considerable mortality when the virus first arrived, but studies indicate that most populations have at least stabilized, if not recovered (5). But we cannot discount the possibility of increased numbers of the Culex species of mosquitoes that are the prime vectors. C. pipiens, C. restuans, and C. tarsalis are abundant and ubiquitous puddle- and container-breeding mosquitoes broadly distributed geographically in overlapping ranges in urban, suburban, and rural settings. These mosquitoes thrive and reproduce in stagnant, dirty, putrid collections of water in puddles and containers, sewers, storm drains, and catch basins, all of which are part of our residential infrastructure. The drought that has gripped much of the country this year has caused contraction of water sources, creating excellent breeding conditions for these mosquitoes. The record-breaking heat is known to speed up both the reproductive rate of mosquitoes and the rate of virus development within them (6). The interplay of heat, drought, human habitats, increased mosquito populations, and enhanced viral development all act in concert to increase the force of transmission. At least, that’s the theory. The truth of what lies behind the resurgence of WNV activity this year will take time to uncover.

A frequently cited publication states that three quarters of emerging infections are zoonotic, that is, shared in nature by humans and animals (7). West Nile virus is first and foremost a bird virus. But it is spread within the avian reservoir by mosquitoes. And humans live in the same ecosystem as both birds and mosquitoes. So, to truly understand WNV and its manifestations, including the causes of increased incidence, we need to understand the virus and its cycle in nature, including the reservoir and the vector, and how the human-built environment contributes. Similar questions about hantavirus may eventually help explain the concerns about that disease that have also arisen this year (8). This integration of environmental science and veterinary medicine with human medicine is the essence of the One Health initiative (, an international effort toward inclusive collaboration across disciplines. Fully understanding a disease like WNV, with its strong environmental component, will require work on all 3 fronts.

In the meantime, mosquito-prevention messages must be unrelenting, directed at personal protective behaviors (avoidance, repellents, and clothing) and reduction of breeding sites. The public must be constantly prodded, with a balance of sensible precautions and serious awareness of the possibility for severe disease. Reduction of mosquitoes requires an integrated pest-management approach, and we must come to grips with the sometimes controversial issue of pesticide application to kill adult mosquitoes, when benefit outweighs risk, and objectively determine efficacy under various conditions. For the long term, perhaps modifying the way we create our own environment will be an important part of reducing the impact of the disease.


Klee AL, Maidin B, Edwin B, Poshni I, Mostashari F, Fine A, et al.  Long-term prognosis for clinical West Nile virus infection. Emerg Infect Dis. 2004;10:1405-11. [PMID: 15496241]
Jeha LE, Sila CA, Lederman RJ, Prayson RA, Isada CM, Gordon SM.  West Nile virus infection: a new acute paralytic illness. Neurology. 2003;61:55-9. [PMID: 12847156]
Centers for Disease Control and Prevention.  West Nile virus. Accessed at on 4 September 2012.
Kuta S.  West Nile virus: Dallas, Texas attack plan includes aerial insecticide spray assault. Huffington Post. 16 August 2012. Accessed at on 5 September 2012.
Foppa IM, Beard RH, Mendenhall IH.  The impact of West Nile virus on the abundance of selected North American birds. BMC Vet Res. 2011;7:43. [PMID: 21831324]
Brault AC.  Changing patterns of West Nile virus transmission: altered vector competence and host susceptibility. Vet Res. 2009;40:43. [PMID: 19406093]
Taylor LH, Latham SM, Woolhouse ME.  Risk factors for human disease emergence. Philos Trans R Soc Lond B Biol Sci. 2001;356:983-9. [PMID: 11516376]
Centers for Disease Control and Prevention.  Hantavirus: August 2012—Yosemite National Park outbreak notice. Updated 29 August 2012. Accessed at on 5 September 2012.

This article was published at on 11 September 2012.


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