Emerging Diseases: West Nile Virus

Introduction

Not many years ago, a prevailing notion held that the majority of infectious diseases would soon be eradicated. This optimism was fueled in part by rapid advances in science and technology that introduced disinfected water, antibiotics, vaccinations and myriad life-saving drugs to society, increasing the quality and duration of human life.

However, the rise of new diseases-AIDS, Lyme disease, E. coli 0157:H7, among others-and the resurgence and expansion of old diseases-including Malaria, Tuberculosis and West Nile Virus-give us a very different perspective. In his book, Man and Microbes: Disease and Plagues in History and Modern Times, Arno Karlen reminds us that infectious diseases are, in fact, a natural and necessary part of life. He writes that the fast pace of new infections are a direct result of the many changes we impose on ourselves and our surroundings and that currently humans and microbes are "…dancing faster than ever in order to survive each other."

West Nile Virus (WNV) is an example of an emerging disease in North America. Since first identified in New York City in 1999, this microbe's foothold in the United States has strengthened every year, with increasing numbers of illness cases from an ever-widening geographical area. As of September 30, 2003, the US Centers for Disease Control and Prevention (CDC) reported 5,722 cases of WNV this year, including 110 deaths. Evidence of WNV infection has been found in 45 statesⁱ.

West Nile Virus Emerges

First isolated in the West Nile province of Uganda in 1937, WNV is indigenous to Africa, Asia, Europe and Australia. The virus has recently caused large epidemics in Romania, Russia and Israel. How and when this microbe arrived in North America are not known, but it is thought that international travel or commerce, or perhaps the migration of infected birds, were factors.

Borne by Mosquitoes

A Culex quinquefasciatus mosquito on a human finger. The Culex quinquefasciatus mosquito is proven to be a vector associated with transmission of the West Nile Virus. (Photo credit: CDC/James Gathany)

WNV is one of many flaviviruses-a group of tick- and mosquito-borne viruses that includes the microbes responsible for yellow fever and St. Louis encephalitis. Birds are the virus' natural reservoir hosts-162 species of birds may be affected. The virus is maintained in nature in a mosquito-bird-mosquito transmission cycle in which the WNV thrives in birds, and relies on mosquitoes for transportation from one bird host to the next.

For all the stir the virus creates among humans, people are considered only "incidental hosts," meaning human infection is not central to the virus' survival. The virus is located in the salivary glands of 36 species of mosquitoes.ⁱⁱ When mosquitoes of species that feed on both humans and birds become infected, humans are at risk. Consequently, WNV cases peak during the mosquito season, from July to October.

West Nile Symptoms

Symptoms in humans may appear three to 14 days after being bitten. In 80% of cases there are no symptoms resulting from human infection with the WNV. Twenty percent of people show mild flu-like symptoms (called West Nile Fever). In rare cases the blood-brain barrier is breached and the virus infects the brain, interfering with normal central nervous system functioning. Meningitis and/or encephalitis develop in 1 out of 150 cases. Adults over 50 years old and the immunocompromised are most at risk for developing these potentially fatal conditions.

Tracking the Virus

WNV surveillance data are reported to CDC's ArboNet, a national electronic surveillance system used to track WNV activity in humans and certain animals. Birds, mosquitoes and horses are considered sentinel organisms whose infection could alert health officials to the occurrence of human disease. Armed with animal surveillance data, public health officials can direct timely, effective mosquito control and public education efforts to prevent widespread human and animal disease. Surveillance of human cases of WNV helps the CDC assess the public health impact of the disease and monitor national trends.

The CDC tracked the rapid expansion of WNV from its initial appearance in New York City to 44 states and the District of Columbia by the end of 2002. In addition, significant human disease was recorded in the Canadian provinces of Ontario and Quebec for the first time. WNV activity was documented in the Caribbean basin and Mexico, and there is evidence of its presence in Central America. The CDC reports that the 2002 WNV epidemic was the largest recognized insect-borne viral meningitis/encephalitis epidemic in the Western Hemisphere and the largest such epidemic ever recorded.ⁱⁱ

By the end of 2003, WNV is expected to complete its westward expansion to every one of the 48 contiguous United States.ii As of September 30, 2003, there were 5,722 reported cases of WNV in the U.S., representing 38% more than the total number of cases reported for 2002. Numbers of "cases" shown in Figure 1 include the full range of symptom severity, from West Nile Fever to West Nile meningitis or encephalitis. The majority of cases reported in 1999-2002 were among persons with meningitis/encephalitis. In contrast, the majority of cases reported as of September 30, 2003, (64%) are West Nile Fever.ⁱ

CDC Director, Dr. Julie Gerberding, cautioned that the higher number of infections this year may be the result of improved testing methods and earlier testing. She also said this year's reported cases tended to be less severe, in part because people with milder symptoms are more likely to be tested.

In an effort to better predict future outbreaks, the United States Geological Survey is collaborating with the CDC on a three-year project to determine the spatial extent of WNV and how it moves among birds, mosquitoes and humans. One study will sample migratory and local wild birds to detect the virus and identify possible bird carriers. Another study will analyze mosquito distribution and species in relation to land cover, weather conditions and bird deaths.ⁱⁱⁱ

Preventing Infection

According to the CDC, the best way to reduce the risk of WNV infection, is to avoid mosquito bites. The recommended strategy includes applying an insect repellent containing DEET (N,N-diethyl-meta-toluamide) to exposed skin or clothing before going outdoors (the insecticide Permethrin is only recommended for use on clothing, not on skin); wearing long-sleeves, long pants and socks when outdoors; and being aware of peak mosquito hours (dusk to dawn). The CDC recommends mosquito-proofing homes and communities by draining standing water where mosquitoes lay their eggs and maintaining well-fitting screens on windows and doors.^iv

Human Trash Creates Breeding Sites. Mosquito larvae float in stagnant water collecting in a discarded jar. (Photo credit: CDC)

Responding to the public health need for a vaccine to protect against WNV, scientists at the National Institutes of Health recently reported significant progress. Researchers at the National Institute of Allergy and Infectious Diseases (NIAID) have created a vaccine by retrofitting parts of a dengue virus--a virus distantly related to WNV--with proteins from the WNV. According to an August 18 NIAID news release, the hybrid virus vaccine, a live, weakened virus, protects rhesus monkeys from the WNV infection. Human clinical trials of the vaccine are expected to begin before the end of 2003 and if successful, could provide life-long immunity against the WNV. Meanwhile, the Department of Agriculture has fully licensed a vaccine for horses that is already used widely.

Protecting the Blood Supply

WNV was shown to be transmitted through blood transfusions in 2002 when 23 people who received transfusions became ill. As a result, the blood banking industry, the Food and Drug Administration and the biotech industry worked to develop tests to detect WNV in donated blood. From mid-July to early August, all civilian blood donations (amounting to more than 1,000,000) collected in the US have been screened, revealing 160 infected donations. Virus-tainted blood from donors in Colorado, Florida, Louisiana, Mississippi, New Mexico, South Dakota and Texas was promptly removed from circulation. The donors were virtually symptom-free.^v

Conclusion

As Karlen writes in Man and Microbes: Disease and Plagues in History and Modern Times, most human infectious disease agents come from other species and thrive in ecological niches that people create. WNV, predominantly an infection of birds, appears to be spreading rapidly to birds throughout North America, and in doing so, infecting increasing numbers of North American residents through the mosquito vector. As late summer slips into autumn, public health officials will tally the peak outbreak of WNV and advance strategies to combat this emerging virus.

ⁱU.S. Dept. of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention , Division of Vector-Borne Infectious Diseases: West Nile Virus: Statistics, Surveillance, and Control. On-line. Available: http://www.cdc.gov/ncidod/dvbid/westnile/surv&control.htm (accessed 10-1-03).

ⁱⁱU.S. Dept. of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention. Epidemic/Epizootic West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control.

ⁱⁱⁱDocampo, R. (2003). Conference Summary: New and reemerging infectious diseases. On-line. Available: http://www.cdc.gov/ncidod/eid/vol9no8/03-0324.htm (accessed 9-10-03).

^ivU.S. Dept. of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, Division of Vector-Borne Infectious Diseases: West Nile Virus: Fight the bite! Avoid mosquito bites to avoid infection. On-line. Available: http://www.cdc.gov/ncidod/dvbid/westnile/prevention_info.htm (accessed 9-9-03).

^vU.S. Dept. of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention. Office of Communication (August 14, 2003). Telebriefing Transcript: West Nile virus in the United States. On-Line. Available: http://www.cdc.gov/od/oc/media/transcripts/t030814.htm (accessed 9-11-03).