We’ve all seen movies or read books featuring zombies—mindless creatures driven by a singular, relentless urge. The reassuring thought is that these undead beings are purely fictional… or so we’ve convinced ourselves. In the animal kingdom, a kind of ‘zombification’ can exist in the unique relationship that occurs between a parasite and its unfortunate host.

Parasites are organisms that require a host for survival and ultimately benefit from this relationship while the host is harmed. The main goal of a parasite is to maintain their own existence and they have evolved to do this in clever ways.  This can include manipulating their host, which can lead the host to death. Here are some examples of this extreme parasite induced death seeking behavior:

One of the major reasons why parasites lead their hosts to death is to complete their own life cycle, which can take place in a secondary host. Certain species of parasitic flatworms (Euhaplorchis californiensis) can enter the gills and migrate to the brains of California killifish (Fundulus parvipinnis) causing them to swim to the surface of the water. This exposes the fish to flying predators such as birds, allowing them to get eaten and happily reproduce in their new host’s small intestine. Similarly, a parasitic fluke (Dicrocoelium dendriticum) can take over the navigational behavior of ants. If an unsuspecting ant eats a slimeball secreted by infected snails, the ingested parasite will cause the ant to climb to the tip of a blade of grass instead of back to the safety of its own nest during the night. There, the ant is susceptible to being eaten by a grazing predator, allowing the parasite to continue its life cycle. Crickets are another example of this parasite-induced brain control. Once infected with hairworms (Paragordius tricuspidatus), crickets will voluntarily enter the water and drown, which allows the parasite to exit the cricket and reproduce. Clearly, parasites have become very effective at manipulating host behavior in a way that allows them to complete their own life cycle.

Parasite-induced death seeking behaviors can also occur the mammalian world. One example is the parasite Toxoplasma gondii (T. gondii), which typically infects cats and rodents but can also be detected in humans. In fact, it is estimated that 1-4 in 10 Canadians have been infected with this parasite. This parasite uses rodents as an intermediary host and is thought to form cysts in the brains of infected rats. These cysts occur in areas that regulate fear and can lead to a loss of fear of predatory odours that make the rodents more susceptible to being preyed upon by cats, allowing the parasite to enter its final host. Researchers are uncovering that T. gondii can alter the behavior of other warm-blooded animals to reach their final feline hosts, as well. Hyena cubs infected with this parasite show excessive bold behavior, causing them to be more susceptible to lion attacks. Thankfully, T. gondii infection doesn’t cause substantial harm to adult humans, although it can cause complications in the fetus if this infection is contracted by the mother during pregnancy.

These are just some examples of how parasites can turn their hosts into mindless zombies – many more exist that we are still learning about today. Thankfully, this interesting natural phenomenon doesn’t pose as much of threat to humans as the fictional zombies we entertain ourselves with.


References 

1.  Shaw, J. c, W. j Korzan, R. e Carpenter, A. m Kuris, K. d Lafferty, C. h Summers, and Ø. Øverli. 2008. Parasite manipulation of brain monoamines in California killifish (Fundulus parvipinnis) by the trematode Euhaplorchis californiensis. Proceedings of the Royal Society B: Biological Sciences 276: 1137–1146. 

2. Libersat, F., M. Kaiser, and S. Emanuel. 2018. Mind Control: How Parasites Manipulate Cognitive Functions in Their Insect Hosts. Front. Psychol. 9. 

3. Naeem, M., M. Younus2, U. Nisa3, T. Akhtar, R. Kausar5, H. Muzaffar, and Irza. 2024. Zoonotic Infertility Due to Toxoplasma Gondii. In International Journal of Agriculture and Biosciences 412–422. 

4. Gering, E., Laubach, Z.M., Weber, P.S.D. et al. 2024. Toxoplasma gondii infections are associated with costly boldness toward felids in a wild host. Nat Commun12, 3842. https://doi.org/10.1038/s41467-021-24092-x 

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Carmen Ucciferri

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