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Chemical trails and the parasites that follow them

Parasitic relationships constitute an important ecological association, responsible in large part for controlling the population of all living things. Parasitism spans the entire breadth and depth of the tree of life from viruses to 40-foot-long tapeworms living in the intestinal lumen of 40-foot-long whales. Infectious agents keep evolutionary pace with their hosts by natural selection, becoming increasingly more adapted to restricted (i.e., host species-specific) associations. Well known examples include medically important pathogens, such as the human malarias, down to our innocuous pinworm, Enterobius vermicularis. Although parasitism is commonplace, the precise mechanisms that permit a pathogen to successfully infect its host vary widely. Humans serve as the host for >350 different protozoan and helminth species (1). Although there are common features of their biology that unite some into related groups regarding their means of gaining entrance into our body (fecal–oral, skin penetration, or vector-borne, for example), the details of their lives as pathogens remain unique to each organism. That specific parasitic relationships arose independently many times throughout the last 3.5 billion years of Earth's history now is taken as fact. Hence, what we learn from one association at the molecular level may not apply even to closely related organisms that infect different hosts (e.g., human vs. dog). A number of them have been adapted for study in the laboratory, and a few medically relevant ones and their mechanisms of pathogenicity have been the subjects of intensive investigation for some time. This research activity has increased over the last few years as complete DNA sequences of some parasites have led to the study of their proteins (2). The first obvious step in any infection is that of contact between the two organisms. Just how parasites find their way in the world, both outside and inside the host, is largely unknown (3 …