Exploitation and Transmission Strategies

Adopting a parasitic life style cannot come without a series of modifications from the ancestral nonparasitic form. The success of a parasitic infection depends on two key steps: the ability of the parasite to establish within the host (to grow and reproduce) and the likelihood of propagules to be transmitted to a novel host. These two steps are associated with a number of adaptations aiming at maximizing parasite fitness (see Life-History Patterns). The strategies adopted to exploit the host and to transmit to other hosts can involve different traits. We will briefly discuss some of the exploitation and transmission strategies encountered in parasitic species.

Compared to most free-living organisms, parasites experience a relatively constant environment: the host. In most cases, the host provides a relatively constant food supply, constant temperature (for endotherms), shelter, and protection toward predators. Parasites have adapted to this particular milieu with an array of adaptations ranging from special structures to attach to host

Upward incorporation

Host 2

Host 2

Host 1

Host 1

Propagules

Host 1

Propagules

Downward incorporation (d) Host 1 (e)

Host 1

Propagules

Propagules

Host 2

Host 2

Propagules

Propagules

Figure 2 Transition from a one- to a two-host cycle by upward incorporation of a definitive host ((a)-(c)) or by downward incorporation of an intermediate host ((d)-(f)). (a and d) The initial life cycle involves one host. (b) Host 1 is frequently ingested by a predator (potential host 2), resulting in a flexible two-host cycle (reproduction of the parasite in both hosts). (c) Reproduction in host 1 becomes suppressed, leading to a two-host cycle in which host 1 has become an intermediate host. (e) Propagules sometimes enter potential host 2, which can be ingested by host 1: host 1 is directly infected or indirectly via host 2. (f) Direct transmission to host 1 may later be lost. The gray area indicates adult parasites in definitive hosts; black areas indicate immature parasites in intermediate hosts. Redrawn from Parker GA, Chubb JC, Ball MA, and Roberts GN (2003) Evolution of complex life cycles in helminth parasites. Nature 425: 480-484.

structures, to regression of organs no longer needed for a life within a host. The view that the host provides a benign environment for the parasites is, however, too simplistic. Although living in the blood vessels of the host certainly provides shelter and protection, it exposes the parasite to the attack of a particular form of predators: the cells and molecules of the immune system. The immune system is probably the most sophisticated, although not unique, defense mechanism that the hosts have evolved. Recent theoretical models have analyzed the impact of host immunity on parasite life histories, using the framework that has been developed to model the interactions between prey (here the parasite) and predators (here cytotoxic lymphocytes). These models have shown how pervasive the effect of host immunity can be on the dynamics and the virulence of the infection. It is not surprising therefore that parasites have adopted a series of strategies aiming at escaping the host immune response. Virus and bacteria, and also macroparasites such as helminths, can escape the immune response by hiding from or suppressing it. Antigenic variation is one of such strategies, where the same strain of a given microparasite expresses different antigenic epitopes. Antigenic variation allows, therefore, the parasite to escape the immune response, since epitopes that are first recognized by the immune system are gradually replaced as long as the infection progresses.

As mentioned above, a successful parasite is a parasite that transmits its progeny to other hosts. Transmission is, therefore, tightly linked to fitness in parasitic species. Transmission is not an easy stage of the life cycle. A propagule has to face the hostile external environment during a period that can be quite long, it has to encounter the appropriate host, and finally enter it. During each of these steps there can be intense mortality and strong selection to succeed. Everything else being equal, large fecundities can compensate for the high risk of mortality incurred by propagules during their free-living stage. Several helminth species are well known for their fecundity records with several millions of eggs produced during a lifetime. Asexual multiplication of microparasites within the host is also thought to be tightly linked to their transmission efficiency. As multiplication within the host usually determines the intensity of the cost of infection (parasite virulence), transmission is a key parameter to understand disease severity. The classical wisdom of parasite virulence assumed that, over time, host-parasite interactions should evolve toward benign associations, because virulence can be costly for both partners. This view, however, neglected the fact that disease severity can be a side effect of parasite exploitation strategies, selected to maximize transmission rate. Theoretical and empirical work has shown that virulence can, indeed, evolve up and down depending on the relative benefits (increased transmission rate) and costs (host mortality) of disease severity.

Another way to improve transmission efficiency is to reduce the hazard of external life (when the parasite has to pass from one host to the other). This can be achieved by incorporating an intermediate host in the life cycle (or using a vector to reach the definitive host). This kind of upward incorporation of hosts has been suggested to be at the origin of complex parasite life cycles (see above). Parasites with heteroxenous life cycles, however, face another dilemma. Their cycle is completed only when the parasite is transferred from the intermediate to the appropriate definitive host. For instance, any mechanisms that make the intermediate host more susceptible to predation directly favor parasite transmission. This process of parasite manipulation (the parasite modifies the morphology and/or behavior of its intermediate host to increase the chance that it will be preyed upon) is very widespread (it has been reported in several species of protozoan and metazoan parasites) and can take unsuspected forms. Classical examples of parasite manipulation of host behavior include the effect of the digenean Dicrocoelium dendriticum on its ant intermediate host. Dicrocoelium causes infected ants to climb to the tip of grass blades and stay there waiting for a grazing sheep. As one might expect, sheeps are the definitive hosts of Dicrocoelium. Other digenean parasites, such as species of the genus Leucochloridium, are known to alter the shape, size, and coloration of the tentacles of the snail they exploit. Modified tentacles strikingly resemble caterpillars which are likely to be more easily detected by birds, definitive hosts of the parasite.

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