Parasitism is an interaction in which a predator (a parasite) lives on or in the prey (its host) and consumes only certain tissues. Many parasite–host relationships are symbiotic. Effects of parasite–host interactions can vary dramatically in magnitude, from having weak effects to causing the death of the host. While parasites typically feed on only one or a few host species, hosts typically house many species of parasites (Figure 55.7). Parasites can also have their own parasites. This specialization helps explain why there are so many parasite species; it is estimated that roughly 50 percent of the species on Earth are parasites.

Parasites can be classified as microparasites, which include viruses, bacteria, and protists, or macroparasites, which are larger and include such groups as worms and insects.

MICROPARASITES Microparasites are many orders of magnitude smaller than their hosts and generally live and reproduce inside their hosts. Multiple generations may reside within a single individual, and a host may harbor thousands or millions of them. Many microparasites, in the process of acquiring nutrients at the expense of their host, cause symptoms of disease and thus are considered pathogens. Host mechanisms that can limit pathogen proliferation include the immune system in animals (see Chapter 41) and biochemical defenses in plants (see Key Concept 38.3).

Just as with other predator–prey interactions, hosts and pathogens can place strong selective pressure on one another. If a pathogen strain is to persist in a host population, the pathogens must continually infect new host individuals. A less deadly strain that kills a smaller proportion of host individuals may be able to infect a larger number of new hosts. Thus pathogen and host may reach a state of coexistence as increased host resistance (ability to withstand the effects of a pathogen) and decreased pathogen virulence (ability to cause disease) evolve. Yet new virulent strains may also arise, reminding us that evolution is constantly occurring.

An example of the complex interactions between host and pathogen comes from Australia, where the myxoma virus was introduced to control the exploding population of European rabbits (Oryctolagus cuniculus), which were devastating the rangeland vegetation used by cattle and sheep. The virus, which is transmitted by mosquitos, causes infections and blindness in rabbits, eventually leading to starvation and death. After the introduction of the virus in 1950, 99.8 percent of infected rabbits died. Millions of rabbits died over the following decades, but the rabbits slowly evolved resistance to the virus. The virus is still used to control rabbit populations, but this requires introducing new, more lethal strains of the virus to ultimately be effective.

MACROPARASITES While microparasites generally live and reproduce inside their hosts, macroparasites are not always intimately associated with their hosts. Macroparasites rarely cause the same kinds of disease symptoms that pathogenic microparasites cause, but they may nevertheless affect host survival and reproduction and can thereby act as agents of selection on their hosts. Ectoparasites are macroparasites that live outside the bodies of their hosts. Endoparasites, such as the tapeworms described in Key Concept 30.4, are macroparasites that spend at least part of their life cycle inside the bodies of their hosts.

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Some ectoparasites, such as leeches and mosquitoes, are only casually associated with their hosts, interacting with them just long enough to eat their fill and then moving on. Ectoparasites that spend their entire lives on their hosts have several attributes that keep them attached to their hosts. Whale lice, which are generally found in skin lesions, nostrils, and eyes of marine mammals, have claws on the tips of their legs that pierce the skin of the whale or dolphin host (Figure 55.8A). Most whale lice are associated with a single species of whale and spend their entire lives on one individual whale. The lice feed on algae or flaking skin on the whale’s body, causing minor skin damage but no significant health risks.

Most hosts actively work to rid themselves of their ectoparasites. Grooming behavior—an important component of the social interactions of many primates—may have evolved in response to ectoparasites. The Japanese macaque (Macaca fuscata), for example, is prone to infestation by two species of lice, which tend to lay their multitudinous eggs on the outer surfaces of their host’s back, arms, and legs. To keep louse populations in check, macaques form and maintain social bonds that ensure the consistent presence of grooming partners (Figure 55.8B). Some biologists believe that hairlessness in humans is an evolutionary response to ectoparasites.