When you first start learning about parasite ecology—and even decades later, when you’re an expert—parasite life cycles can be confusing! Parasite life stages have a plethora of fancy names, like L3 larva and miracidium, which can be difficult to remember. Furthermore, every parasite’s trajectory from immature to adult stages seems different than the last, and we don’t even know all the life cycle details for most parasite species. So, if you’re feeling confused by parasite life cycles, you’re in good company! You might never memorize all the complex life cycles that exist, but you can understand the general ecology and evolution of complex life cycles. We’ll cover the basics in this post.
Simple versus Complex Life Cycles:
Let’s start with simple life cycles, which are sometimes called direct life cycles or one-host life cycles. Monoxenic or homoxenous parasite species with simple life cycles only use a single host species in their life. The single host species is entered by infective stages of the parasite, and then the parasite grows and develops in the host before switching to reproducing within the host.
Direct life cycle parasites include parasites with fecal-oral transmission, like Ascaris lumbricoides, a roundworm that infects humans. Adult male or female roundworms live in the small intestine, where the female releases fertilized eggs into the environment through human feces. Eggs are ingested by people when they (usually accidentally) consume fecal matter. The parasite grows and develops in a series of larval stages in multiple tissues. The larvae eventually make their way to the respiratory system, where they are then coughed up and swallowed, which allows them to find their way to the small intestine to mature. This entire life cycle uses just a single host (humans), even as the parasite goes through many developmental stages inside and outside host.
Now let’s move on to complex life cycles, which are sometimes called indirect life cycles or multi-host life cycles. Parasites with complex life cycles are indirectly transmitted from one host species to the next. These heteroxenic parasites need to use multiple host species in sequence to successfully develop and reproduce. Reproduction occurs in the final host in the life cycle, which is called the definitive host. The one or more hosts where parasite growth or development occur (but no reproduction occurs) are called intermediate hosts. Sometimes a parasite will not complete any life stages within a host, and instead only use the host for transportation; those transportation hosts are called paratenic hosts. The number of hosts needed to complete a life cycle is the life cycle length, and it must be at least two hosts long.
Ready for some examples? Schistosomiasis is caused by a parasite with a two-host life cycle, which uses humans as the definitive human host and snails as the intermediate host. Euhaplorchis californiensis is an example of a parasite with a three-host life cycle, which uses birds like herons as the definitive host, snails as the first intermediate host, and killifish as the second intermediate host. Life cycle lengths appear to have an upward limit, because most life cycles require four or fewer host species. Why do you think that is?
To get from one host to the next, parasites with complex life cycles can use a few different modes of horizontal transmission. Passive transmission occurs when the parasite just waits around for the next host, like when E. californiensis eggs in heron feces wait to be consumed by a salt marsh snail. Active transmission occurs when the parasite is free-living in the environment and moves around to seek out the next host, like when E. californiensis cercariae leave their snail host and swim around looking for a killifish to infect. And finally, complex life cycles often involve trophic transmission, where the parasite is consumed along with its intermediate host by the next host in the life cycle. Trophic transmission is used by all cestodes and acanthocephalans and many nematode and trematode species. Trophic transmission is probably so common in complex life cycles because predator–prey interactions are one of the most common ways that two (host) species might interact.
Three ways to add hosts to a life cycle
You might have noticed that we need to make an important distinction between whether a parasite uses multiple host species in sequence, in parallel, or both. For example, E. californiensis does both: it must infect birds, snails, and killifish in that order to complete its life cycle, so it uses multiple host species in sequence. But it can also use multiple host species at a given life stage; in particular, E. californiensis can successfully infect and reproduce in more than one bird species. The number of host species that a parasite can successfully use for any given life stage is quantified as host specificity. Many complex life cycle parasites have high host specificity for some parts of their life cycle (e.g., they can only infect a single snail species as a first intermediate host) and low specificity for other parts of their life cycle (e.g., they can infect many bird species as definitive hosts).
This brings up an important question: how do host species get “added” to a parasite’s life cycle? We assume that parasite species start by infecting just one host species and then complex life cycles evolve from those simple life cycles. There are two ways that a host species is thought to be added in sequence, thereby increasing the life cycle length: upward incorporation and downward incorporation.
In upward incorporation, a new definitive host that is a predator of the original definitive host is added to the life cycle. Parasites that can infect the new predator without being digested are selected for because they have avoided a source of mortality. They also likely have higher adult body sizes, life spans, and fecundity inside the new definitive host, because the new definitive host species should be larger and longer-lived, on average, than the old definitive host species. After the new definitive host species is added to the life cycle, the parasite then represses reproduction in the old definitive host species, which is now used as an intermediate host. There is an upper limit to how long the life cycle can be made using upward incorporation, because there are only so many trophic levels in a given food web.
In downward incorporation, a new intermediate host that consumes free-living stages of the parasite and is consumed by the definitive host is added to the life cycle. (Yes, parasites are often eaten by predators!) This again reduces parasite mortality, because the parasites can now infect the new intermediate host instead of being digested. Since fewer parasite stages are lost to mortality and more make it to the definitive host via trophic transmission, overall transmission rates increase. Both downward incorporation and upward incorporation are thought to have led to the evolution of complex life cycles for some parasite species.
There is also lateral incorporation, where host species are added in parallel, making the parasite species less host specific (more of a generalist) for a given life stage. Parasites can benefit from infecting more host species in a given life stage whenever that makes them more likely to be able to find a host that they can successfully infect that can continue their life cycle. However, there are also some likely costs associated with being a generalist, instead of specializing on just one host resource.
In summary, some parasite species have complex life cycles and some have simple life cycles. Some parasite species are highly host specific at every life stage and others are host generalists that seem to infect nearly everything. There’s a lot of variability in parasite life cycles, but in general, they can be described by their life cycle length and the transmission modes that parasites use to get from one host to the next. Beyond that, each life cycle diagram is just a bunch of fancy terminology.
If you’re new to parasite ecology and thinking about life cycles for the first time, I have a question to leave you with: how do you think scientists have figured out all these life cycles? If you found a new species of larval trematode in a fish, how would you figure out its life cycle?
Parker, G.A., Ball, M.A. & Chubb, J.C. (2015). Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent. J. Evol. Biol., 28, 267–291.