Navigating parasite webs and parasite flow: Emerging and re-emerging parasitic zoonoses of wildlife origin

Abstract

Wildlife are now recognised as an important source of emerging human pathogens, including parasites. This paper discusses the linkages between wildlife, people, zoonotic parasites and the ecosystems in which they co-exist, revisits definitions for ‘emerging’ and ‘re-emerging’, and lists zoonotic parasites that can be acquired from wildlife including, for some, estimates of the associated global human health burdens. The paper also introduces the concepts of ‘parasite webs’ and ‘parasite flow’, provides a context for parasites, relative to other infectious agents, as causes of emerging human disease, and discusses drivers of disease emergence and re-emergence, especially changes in biodiversity and climate. Angiostrongylus cantonensis in the Caribbean and the southern United States, Baylisascaris procyonis in California and Georgia, Plasmodium knowlesi in Sarawak, Malaysia, Human African Trypanosomiasis, Sarcoptes scabiei in carnivores, and Cryptosporidium, Giardia and Toxoplasma in marine ecosystems are presented as examples of wildlife-derived zoonotic parasites of particular recent interest. An ecological approach to disease is promoted, as is a need for an increased profile for this approach in undergraduate and graduate education in the health sciences. Synergy among scientists and disciplines is identified as critical for the study of parasites and parasitic disease in wildlife populations. Recent advances in techniques for the investigation of parasite fauna of wildlife are presented and monitoring and surveillance systems for wildlife disease are discussed. Some of the limitations inherent in predictions for the emergence and re-emergence of infection and disease associated with zoonotic parasites of wildlife are identified. The importance of public awareness and public education in the prevention and control of emerging and re-emerging zoonotic infection and disease are emphasised. Finally, some thoughts for the future are presented.

Introduction

Historically, appreciation of the ubiquity and significance of links between infectious diseases in human and animal hosts developed slowly, and domestic animals were naturally the initial focus of interest as sources of zoonoses (Hardy, 2003). More recently, however, with ongoing discoveries of new infectious agents and diseases, it has become clear that free-ranging animals and birds are a major source of emerging human pathogens (Daszak et al., 2000).

Among the emerging and re-emerging infectious agents that people can acquire from wildlife, viruses and bacteria are the subject of intensive study because of their frequently severe clinical effects in people and because of the complexities of treatment and control. For parasites, other than major pathogens, including Trypanosoma, Leishmania and Cryptosporidium, there seems less global concern. This may be because while some of these parasites have high regional prevalence, their effects are less dramatic, with fewer sudden outbreaks involving substantial human mortality. Interestingly, of two recently published overviews of wildlife and emerging zoonoses, one included no parasites (Bengis et al., 2004), and the second mentioned only Echinococcus multilocularis (Kruse et al., 2004).

The aims of this paper are: (i) to discuss definitions of ‘emerging’ and ‘re-emerging’ as they apply to zoonotic parasites derived from wildlife; (ii) to identify parasites for which wildlife have been reported as sources of human infection; (iii) to review new information on interesting wildlife-derived emerging and re-emerging zoonoses not included elsewhere in this issue: and (iv) to review recent literature on fundamental themes for the associations between emerging and re-emerging zoonotic parasites, their human and wildlife hosts, and the ecosystems in which they co-exist.