A common strategy of parasites is to manipulate the behavior of their hosts to help with their transmission to another host. The zombie ant fungus, for example, causes the insect to die while biting down on a plant, which gives the fungus a higher platform for shooting out spores. In the wild, hosts can harbor multiple parasites, and sometimes they work together to alter the behavior of the host even more dramatically. But what if the parasites have competing interests? According to a study published in Evolution last week, one parasite could sabotage the other, suppressing its effect on their shared host.

Schistocephalus solidus is a parasitic tapeworm with a complicated life cycle involving three separate hosts: small crustaceans called copepods, then fish, then birds. After emerging from its egg, the young, free-swimming parasite infects a copepod. After an infected copepod is eaten by a fish, the parasite continues to develop until the fish, in turn, is ingested by a bird. Here, in its final bird host, the worm grows into an adult and produces eggs. These tapeworms are known to manipulate their hosts depending on what developmental stage they’re at. Previous experiments revealed how infecting copepods with younger tapeworms (who aren’t ready to move on to fish yet) results in copepods that are less active than uninfected copepods, Nature reports, making them less likely to be eaten by fish. When they’re ready, the parasites make the copepods become more active and conspicuous to fish. 

However, few studies have examined the effect of multi-parasite interactions on host manipulation. So, Nina Hafer and Manfred Milinski from the Max Planck Institute for Evolutionary Biology in Germany infected copepods with Schistocephalus solidus to see if there’s any experimental evidence for sabotage. When the co-infecting parasites have the same goal—making the infected copepod more active, for example—they enhance each other's manipulation, making the copepod even more active than it would be if it only had one parasite. 

However, if the co-infecting parasites are of different ages, they disagree over how they should manipulate the host. In that case, the infective, older parasite wins the fight, suppressing the manipulation of its non-infective, younger competitor. The younger parasite ends up having no effect whatsoever on the host’s behavior. This suggests that the older parasite is sabotaging the younger one's activity, Hafer tells Nature, because “we don’t expect the non-infective parasite to stop what it’s doing.” The findings support both cooperation in and sabotage of host manipulation, the authors write, and show one parasite can neutralize manipulation by another.