West Niles Virus and Public Health

With the emergence of West Nile Virus, this international conference focused on the science of the virus and its implications for public heath.

Published June 1, 2001

By Fred Moreno and Jill Stolarik
Academy Contributors

Since the West Nile Virus first appeared in the United States in 1999, it has dispersed from its original hot zone in the Queens section of New York City to the mid-Atlantic states and as far south as North Carolina. The mosquito-borne disease, which can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord), has infected humans, other mammals and birds worldwide. As the virus has leaped into public attention and concern, the scientific community has expanded its efforts to develop strategies for preventing and controlling future outbreaks

This spring, more than 200 medical, veterinary, laboratory and public health practitioners presented the latest research on the virus at an international conference sponsored by The New York Academy of Sciences (the Academy), the New York State Department of Health, the New York City Department of Health, and the Centers for Disease Control and Prevention. Representatives of ten nations discussed their work, which covered topics such as epidemiology, surveillance mechanisms, human and veterinary pathology, viral and vector biology, antiviral and vaccine interventions, and strategies for dealing with the virus.

Will West Nile Virus Spread This Year?

In 1999, the virus killed seven people in the New York metropolitan area and devastated the state’s bird population, especially crows. Birds are the natural hosts for this virus, which can be transmitted from infected birds to humans and other animals through bites of infected mosquitoes. While investigators believe that West Nile Virus (WNV) does not spread by person-to-person contact, or directly from birds to people, there is some evidence of bird-to-bird transmission.

Experts at the conference agree that the West Nile Virus is likely to continue to spread over a larger geographical area this year. Where the virus will appear next, however, is unknown. Migratory birds, suspected of being the natural hosts that introduced the virus to the U.S., may potentially disseminate the disease to other areas. “Infectious diseases know no boundaries. It is imperative that our efforts span the borders between states, countries and continents as we work toward the common goal of protecting public health,” said Dr. Antonia C. Novello, New York State Health Commissioner.

Weather conditions were cited as a factor in contributing to the spread of West Nile Virus. “Expect the unexpected,” commented Dr. Neil Cohen, Commissioner of New York State’s Department of Health. “Environmental changes may affect what happens this year as compared to the last. We need to be prepared.”

Recent research shows that mosquitoes successfully overwintered in New York City after 1999 and epizootic activity may continue to affect U.S. residents in the summer of 2001 and for many summers in the future. “If we have a hot and dry summer this year, it is possible we could get intense WNV activity in many areas,” predicted Dr. James Hadler from Connecticut’s Department of Public Health, Infectious Diseases Division.

Lessons from the Past

According to Duane J. Gubler, Director of the Division of Vector-Borne Infectious Diseases at the Centers for Disease Control and Prevention, there has been a dramatic global resurgence of arboviruses in the last 20-30 years. He attributes this increase in virus transmission to “increased population growth, air travel, changing agricultural practices, changing lifestyles, and deterioration of public health infrastructure.”

West Nile Virus is a member of the flavivirus family and is related to other arthropod-borne viruses such as the St. Louis Encephalitis and Kunjin Virus. It was first isolated from a febrile woman in the West Nile District of Uganda in 1937. Historically, the virus hasn’t caused much human mortality.

Its ecology was characterized in Egypt in the 1950s and the virus caused severe huma meningoencephalitis (inflammation of the spinal cord and brain) in elderly patients during an outbreak in Israel in 1957. In the early 1960s, Egypt and France experienced outbreaks of West Nile Virus among horses. The NY outbreak represents the first cases of West Nile virus in the Western Hemisphere and the first arboviral infection in NYC since yellow fever in the 19th century.

International Studies of WNV

Outbreaks of WN virus encephalitis in humans are known to have occurred in Algeria in 1994, Romania in 1996-1997, the Czech Republic in 1997, the Democratic Republic of the Congo in 1998, Russia in 1999, the United States in 1999-2000, and Israel in 2000. Researchers from Romania, Russia, the Mediterranean Basin, Israel, South Africa and Australia discussed their experiences with the virus. Poor urban conditions in Romania exacerbated that nation’s 1996 epidemic, in which 17 people died and more than 400 cases of meningoencephalitis occurred. Romania is at risk for another outbreak, given its poor mosquito control programs.

In Russia, more than 600 people were infected and 46 died in 1999. Isolates from three regions affected by WN virus suggest that Russia’s outbreak is related to epidemics in New York, Romania, and Israel. In Israel, the virus infected geese and humans during the 1990s; last year, 29 people died of the disease. Weather conditions appeared to play a factor: in South Africa, Australia, Romania, Russia, and the U.S., outbreaks were preceded by unusually hot and dry weather conditions, or heavy rainfall.

A Vaccine for West Nile Virus

Two researchers reported on the development and production of safe and viable West Nile Virus vaccines. Dr. G. Jeffrey Chang from CDC’s Arbovirus Disease Branch, Division of Vector-Borne Infectious Diseases, discussed the potential of a DNA vaccine, citing advantages of this approach such as the low risk of infection, long term immunity, and its facility for “mixing and matching” individual vaccine components against geography-specific flaviviruses such as St. Louis Encephalitis virus as well as West Nile Virus.

Dr. Thomas P. Monath, Vice President Research and Medical Affairs at Acambis, Inc., compared the advantages and disadvantages of different approaches to developing a vaccine. He explained how the ChimeriVax vaccine offers an approach that is more effective and preferable to DNA vaccines and using a formal inactivated whole virus vaccine. The prototype live, attenuated vaccine (yellow fever 17 D), which has a long history of safe and efficacious use, is being used as a vector for genes encoding protective antigens of West Nile virus. To construct a vaccine, the envelope (prM and E) genes of yellow fever 17 D are replaced with the corresponding genes of West Nile Virus.

Dr. Monath said that a vaccine for horses would be ready later this year, while one for humans will be ready in 2002.

Strategies for Combating the Virus

To combat the virus this year, experts stress that expanding surveillance and mosquito control programs as well as increasing public education about the virus are crucial. Depending on a number of factors, two approaches to surveillance may be adopted: in “active” surveillance, public health departments solicit reports of West Nile Virus from doctors and in “passive’’ surveillance, physicians receive mailings to boost their awareness of the disease.

The density of dead crows sighted per square mile may provide an early warning of possible human cases to follow, as has happened in New York State. However, this will not be effective without a system in place to receive reports; if the public becomes complacent about West Nile Virus; or if crows are too few in number to serve as sentinels. There is also an additional concern regarding the development of immunity in the crow population.

Instead of widespread spraying in areas where mosquitoes have been found, experts now recommend extensive spraying only in areas that have been defined as high-risk to humans. “We now have in place a state-of-the art mosquito control program and we are now able to categorize the level of risk for humans,” said Dr. Cohen. Funding is needed for expanding larviciding efforts.

Public Participation

According to Dr. Stephen M. Ostroff, Associate Director for Epidemiologic Science at the Centers for Disease Control and Prevention, fewer human cases of WN encephalitis in 2000 (as compared with 1999) may be attributed to intense larviciding efforts by public health agencies in areas affected by WNV; the unusually mild summer weather in 2000; and the effectiveness of public education. “What we know about the virus can help the public protect themselves,” he said. “Our best defense against emerging infections is a robust scientific enterprise and a responsive infrastructure.”

Thus, the public’s participation and support for an effective mosquito control program is crucial. “We need the public’s assistance to report sightings of dead birds and potential mosquito breeding sites, such as areas with standing water,” noted Dr. James R. Miller of Vector-Borne Disease Surveillance and Control at New York City’s Department of Health.

“It does appear that adulticiding may be interrupting viral activity,” noted Dr. Millicent Eidson, of the Zoonoses Program at New York State’s Department of Health. “If spraying is stopped, viral activity may rebound.” Dr. David A. Dame, past President of the American Mosquito Control Association, said that six safe pesticides are currently on the market; he cautioned that public protest against spraying may end their manufacture. Several speakers emphasized, however, that public mistrust of pesticides must be addressed and is crucial to combating the West Nile Virus.

Dispelling Misinformation

Many of the speakers believe that misinformation by the media and the public’s negative perception of pesticide use can be addressed through publicizing scientific data showing the safety of pesticides and keeping the public informed on what the adverse health effects of pesticide use may be.

The conference was organized by Drs. Dennis J. White and Dale L. Morse of the New York State Department of Health.

Also read: The Rising Threat of Mosquito & Tick-Borne Illnesses