Whether they are infectious or parasitic, disease agents of wildlife share a common evolutionary pattern: all disease - causing organisms extract nutrients from their hosts. However, if they extract too much too quickly they jeopardize their own survival and ability to reproduce. For their progeny to survive, the parasite has to mature, reproduce, and either be transmitted to another host or be put in an environment where they (or their progeny) can likely fi nd another host. Natural selection favors organisms that are successful in this pursuit; they are most fit.
When disease becomes unusually abundant, this is usually called an epizooti c , epidemic , or outbreak. An exceptionally widespread epidemic is called a pandemic . In contrast, when the disease is at a low or normal level (not readily observable) it is said to be enzootic or endemic . Unfortunately, conservation biologists and biogeographers often use the term ‘ endemic ’ to refer to organisms that are native to an area, so this term has more than one meaning, a confusing situation. Native organisms are better referred to as indigenous . When diseases are capable of spreading from one individual to another, they are said to be contagious . Diseases that are new or increasing in prevalence are called emerging pathogens . For example, West Nile virus (see also discussion of West Nile virus, Chapter 8 ) fi rst attained the western hemisphere in 1999. Because it was new, it was viewed as an emerging pathogen. Initially, it caused epizootics among some forms of wildlife, particularly some avifauna. In most areas where it has occurred for a few years it has fallen to an enzootic state due to mortality among the most susceptible hosts and development of resistance among others. However, it will certainly cause an epizootic again at some time in the future as resistance diminishes, a new more virulent form of the virus evolves, or as the number of vectors increases. West Nile virus may be viewed as an emerging pathogen as it gains access to areas where it has not occurred previously, but it will never be an indigenous species in the western hemisphere.
Disease is often considered to consist of the effects of infective and parasitic organisms, but in fact disease is much more encompassing than that. The defi nition of disease can be expanded to include nearly anything that causes an impairment of the host animal. This includes (in addition to parasites) environmental factors like nutrition, toxicants, weather, inherited abnormalities, and combinations of these factors. Thus, when assessing disease in wildlife, it is important to consider that:
• disease is not measured by deaths of individuals, but
by impairment of performance. Wildlife populations
can go into decline due to decreased reproduction and
longevity, but with no noticeable increase in
mortality.
• disease is caused not only by extrinsic factors such as parasites, but also by intrinsic factors such as inherited physiological processes.
• disease may be the result of factors acting individually, or in concert with other factors. Indeed, several functions may be impaired simultaneously, none very noticeably, but collectively very important to fi tness of the organisms. Often, parasitic organisms are held in check by immune responses or general vigor of the host, but when stressed by lack of food or cold weather, the animals will succumb to the parasite.
In addition, it is useful to understand that disease can be brought about by the host ’ s physiological response to a disease agent, not simply by the direct effect of the disease agent. For example, invasion of a host by a microbial parasite may eventually cause injury, but disease develops initially as the host ’ s immune system recognizes the presence of the foreign bodies and responds with elevated body temperatures and increased numbers of white blood cells. The host animal may be less alert and unable to feed or hunt while responding metabolically to this invasion. Thus, although many animals have immune systems designed to thwart invasion or minimize feeding, it comes at a physiological cost.
