The War of the Parasites

Hey all: Firstly, sorry for the large absence since my last post. Interestingly, I had my own parasite (pneumonia), which put me out of action for a while.

Anyway, to the post!

Hosts often have multiple parasites at a time, and what is good for one parasite may not be good for another parasite.

A real-life example.

The cestode parasite Schistocephalus solidus has a three host life cycle. It first infects copepods (Macrocyclops albidus). Then, three-spined sticklebacks (a fish) acquire the parasite through eating infected copepods. Finally, birds acquire the parasite by eating infected fish. The parasite reproduces inside the bird, releasing further parasites into the environment, which will infect copepods. The circle of life.

life cycle

Such a transmission cycle, in which the parasite moves through consumption of one species by another, is referred to as trophic transmission.

The predictions.

Trophically-transmitted parasites often manipulate the behavior of one of their earlier hosts to facilitate transmission to the final host (see, for example, Moore 2013, Poulin 2010). Normally, we think of these behavioral alterations as making hosts more susceptible to predation by the next host in the life cycle (e.g., Lafferty 1992). Parasites, however, often have development times inside their hosts. By development times, I mean that parasites are not typically infective to the next host in the life cycle immediately upon infecting the current host. Once in a host (say, the copepod), a parasite needs some time before it can successfully infect the next host (the fish). If the parasite’s host were to be eaten before the parasite was infective to its next host, then the parasite would die, even it were eaten by the correct next host. For this reason, many authors have hypothesized that before the parasite is infective, it should reduce its host’s susceptibility to predation (e.g., by increasing host hiding behaviors). Other authors have further noted that if a host is infected by parasites at different developmental stages (parasitized by both infective and uninfective stages), then the different parasites may attempt to sabotage the manipulation of the other. For example, the infective parasite may prevent the uninfective parasite from reducing the host’s susceptibility to predation. Crazy (cool), right?

Hypothesis Tested

Hafer and Milinski (2015) tested this idea with S. solidus in its copepod host. Just ignoring all the details about statistics and experimental design (who cares about that anyway?), here are their results: Hafer and Milinski found that copepods infected with different stages of parasite experienced sabotage of behavioral manipulation. When copepods were infected with two parasites, which were both infective, then the copepods displayed reduced anti-predator behaviors. Likewise, when both parasites were uninfective, the copepods exhibited increased anti-predator behaviors. But when copepods were infected with one infective and one uninfective parasite, then the copepods still demonstrated reduced anti-predator behaviors. The uninfective parasites lost out! The manipulation by the infective parasites prevented that by the uninfective parasites.

only one


Hafer, N. and M. Milinski (2015). “When parasites disagree: Evidence for parasite‐induced sabotage of host manipulation.” Evolution.

Lafferty, K. D. (1992). Foraging on prey that are modified by parasites. American Naturalist.

Moore, J. 2013. An overview of parasite-induced behavioral alterations – and some lessons from bats. Journal of Experimental Biology.

Poulin, R. 2010. Parasite manipulation of host behavior: an update and frequently asked questions. Advances in the Study of Behavior.