Parasite Virulence

Some parasites exact a terrible price from their hosts, causing severe pathology and reducing the host’s fitness, whereas other parasites are essentially benign. Several kinds of comparisons highlight this observation. Least interesting are comparisons of parasites with very different life histories or types of host tissues invaded (compare human immunodeficiency virus (HIV) and rhinovirus infection in humans). In other cases, the same parasite causes great harm in one species of host, but is tolerable to another, such as the rabies virus, which kills canid hosts but can reside in mustelid populations as non-lethal infections (Kaplan, 1985). Again, this may result from different types of tissues invaded by the parasite. Most intriguing are examples of very different levels of pathology caused to the same host species by closely related parasite species or even different genetic strains within a parasite species. Examples of this last situation are abundant. Strains of Trypanosoma brucei, the causative agent of African sleeping sickness in humans, differ in the severity of the pathology they cause, so much so that they were long classified as different species, based on symptomatology (Toft and Aeschlimann, 1991). Malaria has the reputation as the malignant ‘million-murdering death’, but only Plasmodium falciparum kills a significant number of victims outright and, within each species, the morbidity and mortality associated with infection vary geographically (Arnot, 1998). The rabies virus, so notorious for its lethality for humans and extremely high mortality for dogs, has evolved an African strain that produces non-fatal oulou fato in dogs (Kaplan, 1985). Entamoeba histolytica and Giardia lamblia, widespread and important intestinal parasites of humans, vary in their pathology by genotype, which led to different species names for polyphyletic clusters of strains (Mehlotra, 1998;