Disease emergence in birds: Challenges for the twenty-first century

The paper by Hartup et al. (2001) on House Finch (Carpodacus mexicanus) conjunctivitis is an example of the rapid geographic spread that can result from disease emergence in naïve populations. That event was neither novel nor transient relative to its occurrence or effects. Disease emergence and reemergence are hallmarks of the latter part of the twentieth century (Center for Disease Control 1994, Levins et al. 1994, DaSilva and Laccarino 1999, Gratz 1999). Current examples involving domestic animals include the problems in Europe with bovine spongiform encephalopathy (BSE, or “mad cow disease”) (Brown 2001) and foot-and-mouth disease (FMD) (Kitching 1999). Human health has been affected by diseases caused by an array of viruses (Morse 1993, Nichol et al. 1993, Murphy and Nathanson 1994), bacteria (Dennis 1998, DaSilva and Laccarino 1999), rickettsia (Walker and Dumier 1996, Azad et al. 1997), protozoans (Tuerrant 1997, Saini et al. 2000), and metazoan parasites (Hildreth et al. 1991, Gubler 1998), as well as other causes. Acquired immune deficiency syndrome (AIDS) has received the most notoriety of those diseases (Hahn et al. 2000, Schwartlander et al. 2000). A similar pattern exists on a global scale for free-ranging wildlife populations (Table 1) (Friend 1994, 1995; Epstein et al. 1998, Daszak et al. 2000). However, in comparison to disease emergence affecting humans and domestic animals, response to emerging diseases of wildlife is generally superficial. We present concepts and data to support our contention that failure to adequately address disease emergence in free-ranging wildlife is resulting in a diminished capability to achieve and sustain desired geographic distributions and population abundance for species of wild birds, including some threatened and endangered avifauna.

For clarity, we define disease and disease emergence in the context of our use of those terms because they are the focus of our comments. Disease is any departure from health (Guralnik 1982); that is, dysfunction contributing to physiological, physical, reproductive, behavioral, or other impairment that reduces the probability of survival of individuals. If enough individuals are affected, the collective effects can reduce the sustainability of the population. Although disease can result from exposure to a wide variety of physical, chemical, and biological agents and other conditions, we focus this paper on microbes and parasites and to overt mortality caused by them. Thus, disease effects presented only represent the proverbial “tip of the iceberg” relative to the challenges wild avifauna face from disease. Our perspective of disease emergence expands the earlier definitions of emerging diseases by others (Centers for Disease Control and Prevention 1994, Morse 1995) to include all species. Our comments are defined by the context of disease occurrences that have increased within the past three decades, or threaten to increase in the near future relative to populations affected, geographic distribution, or magnitude of effects.