Watching a zombie show like The Walking Dead, it is difficult to not wonder how aware the zombified humans are of what they are doing. Are they essentially dead, or are they locked in their heads screaming but without the ability to control their actions? Arguably, the scariest thing about zombies isn’t that they bite; it’s the fear of what will happen as the contagion takes hold.
One of the creepiest topics in parasitology is that of parasites that alter the behavior of their hosts, essentially leading to suicidal behavior. While this might sound counter-intuitive at first, the parasites that do this benefit from orchestrating the death of their hosts under the right conditions. In the most common case, parasites need their hosts to be eaten so that they can infect the predator and complete their complex lifecycle. When the parasites are ready to be transmitted to a new, predatory host, they induce hosts to engage in risky behavior that increases the chances of being eaten. For instance, many acanthocephalan parasites cause their amphipod crustacean hosts to swim near the top of the water column and cling to objects at the water’s surface. This makes it much easier for birds to find and eat the amphipods, transmitting the parasite in the process. This phenomenon also occurs in vertebrate hosts. There is a trematode brain parasite of California killifish that causes the fish to surface, jerk, shimmy, and contort. This behavior makes it 10-30 times more likely that infected fish will be eaten by birds, the final host of the parasite. The freakiest example I know of is when Toxoplasma gondii causes infected mice to be sexually attracted to the smell of cat urine, so the mice actually seek out cats that might eat them.
Other parasites use their hosts as disposable transportation. Hairworms have a life-stage that lives in water, but they also infect terrestrial crickets. When the hairworms are ready to reproduce, they cause their hosts to take a suicide dive into water. The cricket typically drowns, and the hairworm emerges from the dead insect. A video is here, but it’s a bit graphic.
One of the more bizarre and disgusting parasites, the green-banded brood sac, burrows into the eye-stalks of snails and turns them into nasty, pulsating baby-sacs (see video here). Infected snails spend more time in exposed locations, but the end-game isn’t necessarily for a bird to eat the snail. The brood sacs can squirm out of the snail once in a good location and will continue to gyrate invitingly for up to an hour. It is thought that when a bird eats an infected snail’s face off it’s just collateral damage. No hard feelings, right?
Finally, some parasites force their victims to perform services for them. Often these are parasitoid wasps. Basically, these wasps are the real-life inspiration for the diner scene in Alien when the baby alien gestates inside one of the human characters and finally burrows out of his chest and kills him. Imagine if that human was compelled to build a nursery for the alien parasite before the alien springs out; that is essentially what one species of wasp larvae does to orb spiders. The wasp larvae growing inside the spider cause it to create a different type of web, the sole purpose of which is to support the cocoon that the larvae will make after eating their way out of the spider. If that wasn’t bad enough, parasitoid wasps in the Glyptapanteles genus turn their caterpillar victims into devoted slaves. The caterpillar hosts actually survive when the wasp larvae burrow their way out of it (see video here). Once the larvae are out they begin to spin cocoons. The caterpillar lays down a protective layer of web over the wasp cocoons, and guards the developing wasps from predators and other parasitoids. The caterpillar is so consumed by this task that it doesn’t eat and eventually starves to death.
Fun fact: The risk taking behavior incited by acanthocephalan parasites in their amphipod hosts can be weakened by a second type of parasite, a microsporidian. The microsporidia are transmitted from parent to offspring, so their survival and transmission depends on the host not being eaten before it has a chance to reproduce. When parasites fight, sometimes the host wins.
The featured image is of a caterpillar spinning a web around Glyptapanteles wasp cocoons (from Grosman et al. 2008).
Ataev, G. L., Zhukova, A. A., Tokmakova, А. S., & Prokhorova, Е. E. 2016. Multiple infection of amber Succinea putris snails with sporocysts of Leucochloridium spp. (Trematoda). Parasitology research, 115(8): 3203-3208.
Bates, M. 2014. Meet 5 “zombie” parasites that mind-control their hosts. National Geographic. Available at: http://news.nationalgeographic.com/news/2014/10/141031-zombies-parasites-animals-science-halloween/.
Bauer, A., Haine, E. R., Perrot‐Minnot, M. J., & Rigaud, T. 2005. The acanthocephalan parasite Polymorphus minutus alters the geotactic and clinging behaviours of two sympatric amphipod hosts: the native Gammarus pulex and the invasive Gammarus roeseli. Journal of Zoology, 267(1): 39-43.
Grosman, A. H., Janssen, A., De Brito, E. F., Cordeiro, E. G., Colares, F., Fonseca, J. O., Lima, E. R., Pallini, A., & Sabelis, M. W. 2008. Parasitoid increases survival of its pupae by inducing hosts to fight predators. PLoS One, 3(6): e2276.
Haine, E. R., Boucansaud, K., & Rigaud, T. 2005. Conflict between parasites with different transmission strategies infecting an amphipod host. Proceedings of the Royal Society of London B: Biological Sciences, 272(1580): 2505-2510.
Jones, L. 2015. Ten sinister parasites that control their hosts’ minds. BBC. Available at: http://www.bbc.com/earth/story/20150316-ten-parasites-that-control-minds.
Kloss, T. G., Gonzaga, M. O., Roxinol, J. A. M., & Sperber, C. F. 2016. Host behavioural manipulation of two orb-weaver spiders by parasitoid wasps. Animal Behaviour, 111: 289-296.
Lafferty, K. D., & Morris, A. K. 1996. Altered behavior of parasitized killifish increases susceptibility to predation by bird final hosts. Ecology, 77(5): 1390-1397.
Leung, T. 2016. Leucochloridium paradoxum (revisited). Parasite of the day. Available at: http://dailyparasite.blogspot.com/2016/12/leucochloridium-paradoxum-revisited.html.
Shaw, J.C., Korzan, W.J., Carpenter, R.E., Kuris, A.M., Lafferty, K.D., Summers, C.H., & Øverli, Ø. 2009. Parasite manipulation of brain monoamines in California killifish (Fundulus parvipinnis) by the trematode Euhaplorchis californiensis. Proc. R. Soc. B, 276: 1137-1146.
Simon, M. 2014. Absurd Creature of the Week: The Wasp That Lays Eggs Inside Caterpillars and Turns Them Into Slaves. Wired. Available at: https://www.wired.com/2014/10/absurd-creature-week-glyptapanteles-wasp-caterpillar-bodyguard/.