Perspectives in Biology and Medicine

EMERGING PATHOGENS: THREAT AND OPPORTUNITY DAVID M. FORREST and BRIAN GUSHULAK* Recently, there has been growing interest in the natural history and impact of emerging pathogens as a major public health problem. Public awareness of these diseases has been heightened by their often devastating effects (as with Hantavirus Pulmonary Syndrome, Ebola infection, and ?G? disease) and the considerable media and entertainment industry attention paid them. Resultant public anxiety has exerted pressure on the medical system to enhance early diagnosis and improve treatment and on the public health system to be better prepared for these diseases—and to prevent them. Emerging pathogens have been defined as clinically distinct infectious diseases whose incidence in humans has increased [1, 2]. Yet most are not actually "new" diseases, but may be "reemergent" [2]. Many simply have not been identified before, while others have existed but have been sequestered from a population such that they have not caused significant, widespread disease. This is often the case for reemergent diseases. Usually, "those agents best adapted to human transmission are likely to be those that will emerge" [2]. Thus, they are characterized by rapid increase in incidence or in geographic range. An example of such a disease is Ebola virus: likely the virus causes occasional sporadic episodes of infection, but onlywhen itwas transmitted more broadly to a larger population (i.e., when it emerged) in 1995, resulting in many cases, did it become epidemic and a public health concern. Hence, "most emerging infections appear to be caused by pathogens already present in the environment, brought out of obscurity or given a selective advantage by changing conditions and afforded an opportunity to infect new host populations" [1] . They may originate in (or emerge from) one geographic location and then disseminate to others (as in the case of plague), or common factors may support simul- * Critical Care Medicine and Infectious Diseases, University of British Columbia, Echelon 360, 855 W. 12th Avenue, Vancouver, B. C, Canada.© 1997 by The University of Chicago. All rights reserved. 0031-5982/9"7/4002-0989$01.00 118 David M. Forrest and Brian Gushulak ¦ Emerging Pathogens taneous emergence at multiple sites (as has occurred with multi-drug resistant tuberculosis). Most emerging infections are zoonoses, thus being acquired from animal (usually vertebrate) reservoir hosts. Indeed, given the vast array of animal species with which humans interact and the large pool of infecting agents which animals can carry, non-human animal species represent a rich source of known and previously unrecognized emerging diseases. Although study of emerging pathogens has been equated to the study of zoonoses [3], many emerging diseases are not zoonotic. For example, a recent outbreak of cholera in South America (which had not been reported in the region in this century) is likely related to contamination of ballast water in a freighter from Asia [I]. The emergence of other Vibrio species has been related to alterations in algae [4]. The recent fascination with emerging pathogens has sparked considerable study of (and postulation about) the means by which they arise. Predominantly , this seems a facet of social, cultural, and behavioral human activities and interaction between humans and the environment. Thus, "environmental changes probably account for most emerging diseases" [2]. There are four major groups of factors which seem responsible, solely or in combination, for the emergence of new diseases. First, complex environmental factors can account for the emergence of disease by favoring conditions which influence the growth of pathogens, reservoirs of disease or vectors, or a combination of these. Geoclimatic conditions can potentiate the introduction and dissemination of infection [5] , but they may also mitigate against the emergence of certain diseases [3] . Changes in animal factors, influencing reservoirs and/or vectors, can also be important [3]. Changes in animal genetic characteristics and natural/acquired immunity influence animal susceptibility. Alterations in migratory patterns or introduction of new animal species to different habitats can also facilitate emergence of new diseases (e.g., newly urbanized raccoons and rabies) . A new or different interaction between domestic and wild animals or with arthropods can be important. In addition, changes in predators or their food sources can alter population growth patterns, negatively or positively, thus influencing susceptibility ofthe animal population...