Parasites can influence the behaviour of their host to serve their own agenda. For example, to complete their life cycle a parasite will often need to find a way to change hosts. Copepod larva (pictured) can be infected by tapeworm parasites. At first the tapeworm is happy and even helps this aquatic crustacean to survive – ensuring that it behaves less actively than non-infected copepods to avoid being spotted by a bird. But when the parasite is ready to move on, the copepod becomes much more active and will eventually be eaten. In cases where the copepod is infected with more than one tapeworm, the parasites will either team up, or battle it out to gain control of their host. If the tapeworms have competing interests because some of them are too young to move on, the older tapeworms triumph. These insights could one day help tackle human parasite infections.
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