The significance of health and disease on the preservation and management of endangered species has been recognized for many years (Scott, 1988; Thorne and Williams, 1988; Gilmartin et al., 1993; Lafferty and Gerber, 2002). The discipline of conservation medicine involves documenting, evaluating, monitoring, modifying, and/or preventing the impact of disease on wildlife. It also includes the study of the multiple two-way interactions between health and disease on the one hand and species and ecosystems on the other (Tabor, 2002). In large stable populations, disease is a normal part of population dynamics. However, when populations are extremely small or fragmented, stochastic events such as disease outbreaks and epizootics may have catastrophic effects. In some cases, veterinary intervention during a disease outbreak is essential to prevent extinction of the population (Thorne and Williams, 1988). The monitoring of occurrence and spread of disease is also a sensitive indicator of a change in the ecology of a species, often secondary to human intervention (Daszak, et al., 2000).

Randall E. Junge • Animal Health and Nutrition, Saint Louis Zoological Park, St. Louis, Missouri 63110 Michelle L. Sauther • Department of Anthropology, University of Colorado, Boulder, CO 80309

There are a variety of factors that are involved in the ecology of disease. For lemurs, a few of these have particular importance. Geographical isolation is an important factor in disease presence, absence, and significance. With regular migrational movement, species are exposed to a wider variety of pathogens and are given the opportunity to develop a degree of resistance or commensalism. Also with immigration, population numbers may be augmented after significant disease events. Likewise, overpopulation and its effects on stress, nutrition, and disease transmission may be less likely to occur when emigration is possible. Isolation does have positive effects as well. Novel pathogens are much less likely to be introduced (by natural means), and pressure from new competitors and predators is unlikely. However, such immunologically naive populations may be at higher risk from introduced pathogens. For example, it has been suggested that disease may have played a major role in the Pleistocene megafauna extinctions (McPhee and Marx, 1997). If humans or human associated animals carried disease agents of high virulence (so-called hyperdisease), first contact epidemics may have resulted in elimination of Madagascar megafauna shortly after human arrival (McPhee and Marx, 1997).

A disease is considered endemically stable when a balance is reached between host and pathogen. The disease, while present and exerting some morbidity and mortality, does not have overwhelming effects on the population. A disease becomes endemically unstable when the balance is disrupted. Disruption may occur with the introduction of stress, competition, decreased food availability, or new pathogens that may compromise the host. Invasive species may bring novel parasites, viruses, or disease to naive species, often with devestating effects. The introduction of canine distemper virus to African wild dogs from domestic dogs is an example of such a situation (Alexander and Appel, 1994). The anthropogenic introduction of novel pathogens (with or without their hosts) into naive populations has been referred to a pathogen pollution (Daszak et al., 2000). Pathogen pollution effects are especially strong when domestic animal hosts are introduced, as they provide a constant reservoir for pathogen introduction.

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