Why do so many parasites infect definitive and intermediate hosts rather than just one host?

Question

Many parasites infect multiple host species, with one host species being the definitive host (where the parasite reproduces), and the other host species being the intermediate host (where the parasite grows to maturity). (Further details described as "complex life cycle" at https://en.wikipedia.org/wiki/Parasitic_life_cycle ) Why is this complex life cycle so common? Why wouldn't more parasites adopt a simpler life cycle of just one host?

I knew that there were some parasites that infected more than one kind of host species, but now that I am looking into it, it seems like a majority of internal parasites have this kind of life cycle. Does anyone know why?

Answer 1

Several reasons have been proposed.

We suggest that complex cycles in helminths without penetrative infective stages evolve by two essentially different processes, depending on where in the cycle a new host is inserted. In ‘upward incorporation’, a new definitive host, typically higher up a food web and which preys on the original definitive host, is added. Advantages to the parasite are avoidance of mortality due to the predator, greater body size at maturity and higher fecundity. The original host typically becomes an intermediate host, in which reproduction is suppressed. In ‘downward incorporation’, a new intermediate host is added at a lower trophic level; this reduces mortality and facilitates transmission to the original definitive host.

--Evolution of complex life cycles in helminth parasites

Despite these advantages to omitting hosts, the majority of digenean trematodes have a life cycle with three hosts, suggesting strong counteracting forces that maintain hosts in the cycle. Such forces have been proposed to be higher growth and fecundity (Parker et al., 2003), an increased probability of finding a mating partner (Brown et al., 2001), and higher transmission rates (Morand et al., 1995; Choisy et al., 2003; Parker et al., 2003). ... we propose here that maintaining a second intermediate host in the life cycle can be advantageous for the individual parasite to increase the intermixture of different clones and therefore decrease the risk of matings between genetically identical individuals in the definitive host.

--How a complex life cycle can improve a parasite's sex life

Also see The evolutionary ecology of complex lifecycle parasites: linking phenomena with mechanisms.

Answer 2

Alot of lower eukaryotic parasites do infect just one group of hosts and are some, such as malaria, are host specific. Transmission from host to another is not trivial for a large infectious agent and no parasitic agent is associated with aerosol transmission.

In this case, malaria, leishmaniasis, African sleeping sickness amongst others, a vector is required.

Even schistosomiasis, may similarly be viewed as using vectors (snails), so the definitive host, whilst it certainly happens for example Toxoplasma gondii, it is less frequent than widespread than you are inferring in my opinion.

Answer 3

I assume that by internal parasites you mean animals like tapeworms etc?

They can infect several species simply because they are not picky. These parasites do not rely on precise molecular hijacking mechanisms like intracellular parasites (e.g. Apicomplexa).

All they need is a certain amount of heat and humidity (which any host provides) and food pre-digested by the host, which is essentially the same for host species that share the same feeding habits, so there is no reason a priori for them to have a narrow range of hosts.