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| Preventing and Controlling Emerging Infectious Disease |
The prevention and control of emerging infectious diseases requires a global perspective that accounts for biocomplexity, all the interrelated factors that contribute to the evolution and survival of infectious agents. Individuals from many disciplines - biologists, chemists, statisticians, atmospheric scientists, and ecologists - must work together. Effective surveillance is essential. Multiple control measures will often be appropriate. New genomic and proteomic techniques may provide not only more effective detection but also prevention by novel vaccines.
The effective interaction between public health officials and individuals from a variety of disciplines was exemplified during the West Nile virus outbreak that occurred in the New York City area during the summer of 1999. By mid-October forty-eight people had demonstrated an unusual illness characterized by fever, extreme muscle weakness, and pneumonia-like symptoms. Four had died. Encephalitis or meningitis was present in a few of the more serious cases. West Nile virus was identified as the causative agents using antibody-based tests and DNA comparisons.
West Nile virus hails from Africa, Australia, and the Middle East, and had never been seen in the Western Hemisphere. At the time city wildlife officials and veterinarians at the nearby Bronx zoo were struggling with a peculiar infection among crows and the zoo's collection of exotic birds. Brain hemorrhages and heart lesions were observed in dissected birds; DNA analysis showed the presence of West Nile virus. The discovery of the virus in wild bird populations, which could potentially serve as a reservoir for human disease, spawned surveillance of birds throughout the United States. Concerns that migratory birds would spread the virus rose. Flocks of chickens were used to monitor viral spread. In the meantime New York City began spraying for mosquitoes. By September 2002 the virus had infected a woman in Los Angeles. Continued surveillance of the bird population and continuing communication between wildlife experts, public health workers, and others will be instrumental in curtailing this infection in the United States.
Effective surveillance is a critical step in preventing the spread of emerging diseases. For example, the new influenza vaccine available each year is the result of constant vigilance. The World Health Organization and others identify the strains of influenza most likely to cause infection in the coming year and define the vaccine based on their findings. In this case an understanding of the animal reservoirs of the disease is important to the surveillance effort. The emergence of novel strains is most likely where poultry, pigs, and humans come in close contact. As a result, monitoring is conducted where such conditions abound.
It is often necessary to take multiple measures to control disease. In the case of malaria the first steps to prevention are as simple as the use of bed nets for reducing bites from mosquitoes and more frequent draining of flooded environments (such as rice fields) where mosquitoes thrive. In the end, DNA-based vaccines, founded on an understanding of the complex life cycle of the protozoal parasite, may be the answer.
Daniel Carucci of the U.S. Naval Medical Research Center and others have identified various proteins that are expressed by the malarial parasite during different stages of its life cycle. Some of these proteins should be recognized as foreign by the immune system and might serve as vaccines. The goal is to stimulate the production of not only antibodies but also cellular immunity specific for various stages of the parasite. (See the HIV and AIDS unit for an introduction to the immune system.) Rather than injecting the proteins into individuals, Carucci is evaluating the use of DNA vaccines. Such vaccines usually comprise DNA, encoding the protein(s) of interest, adsorbed onto gold particles and injected with an air gun into muscle tissue. The expression of malarial proteins by recipient cells and the subsequent immune response to the proteins is being evaluated. If successful, DNA-based vaccines might offer advantages over traditional vaccines; they are less expensive to prepare and easier to store than protein-based vaccines. However, DNA can serve as an immunogen itself; it is thought that diseases such as lupus result from an immune reaction to DNA. As vaccine development continues, the importance of traditional public health measures to prevent and treat malaria remains essential.
The threat from established and evolving disease organisms remains with us. Given high reproductive rates and mechanisms for lateral gene transfer, microbes can adapt to and rapidly circumvent the best treatments scientists develop. We have seen how new diseases arise and spread when humans interact with each other or with the environment in new ways. The anthrax attacks in the fall of 2001 remind us that the threat of bioterrorism continues. This ancient form of warfare dates as far back as 1346 when the Tartar army catapulted the bodies of plague victims into the city of Kaffa.
The journalist Laurie Garrett has suggested that because human behavior influences the emergence of disease, we have significant control over our struggle with microbes. Certainly, our understanding of the factors that contribute to the evolution of new pathogens is continuing to increase, and experience with such outbreaks as West Nile have helped hone surveillance and control measures. However, the global nature of disease means that public health strategies must be global as well.
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| ||End Notes |
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- Garrett, L. 1994. The coming plague - Newly emerging diseases in a sorld out of balance, 11. New York: Penguin Books.
- Smolinski, M. S., M. A. Hamburg, and J. Lederberg, eds. 2003. Committee on Emerging Microbial Threats to Health in the 21st Century. Microbial threats to health: Emergence, detection, and response. National Academies Press. http://www.nap.edu/catalog/10636.html.