sphere of West Nile encephalitis, a potentially fatal disease caused by a virus that commonly infects birds and can be transmitted by mosquitoes, were documented in the New York City metropolitan area in late summer of 1999. The West Nile virus has now been detected in a number of states along the major migratory flyways of the eastern seaboard.

Many different determinants contribute to the emergence of new zoonotic agents, and it is rare that these factors act singly. Among the forces that shape their emergence are human demographics and behavior; technology, industry, and agriculture; economic development and land use; international travel, commerce, and military expeditions; microbial adaptation and change; and breakdown of public health measures. Indeed, social and environmental changes are accelerating, in both the developed and developing worlds. The developed world has the greatest travel and transport, providing particular risks for rapid spread. Ecological change is greatest in the developing world and biodiversity is greatest in the tropics, which makes these regions potentially productive breeding grounds for new pathogens. In the final analysis, it cannot be predicted which zoonotic pathogens are likely to emerge next or cause the biggest problem. Given the obvious link between human health and pathogens that circulate in domestic animals and wildlife, we must be alert to pathogen flow in any of these areas.

Among other issues, there is concern regarding the potential “bioweaponization” of zoonotic diseases, particularly by individuals or groups either acting alone or with sponsorship by a foreign government. Many observers now consider such terrorist attacks to be the major threat, embracing biological attack against U.S. forces in peacetime deployment, as well as against private citizens in major cities. Zoonotic agents have a number of attributes, including their potentially large impact when released into a human or animal community, that make them especially suitable for use as weapons. Ironically, continued advances in biotechnology, while offering great promise for improving human health, may concomitantly make it easier for terrorists to manufacture and deploy effective biological weapons.

In addition, a number of human activities undertaken with the best of intentions may have harmful potential. For example, the food and farming industries increasingly use antimicrobial agents and other types of drugs to boost the efficiency of food-producing animals and to prevent certain troublesome organisms from reaching consumers. Use of these chemicals probably enhances the proliferation of antibiotic-resistant microbes. Some observers also suggest that xenotransplantation, broadly defined as the use of nonhuman animal cells or tissues in humans for therapeutic purposes, may inadvertently introduce new zoonotic infections to recipients of such material. This risk raises the burden of preventive responsibility on scientists and research groups conducting such trials.

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