Parasites are sneaky critters. Some of them take ‘devious’ to a whole other level, as they can alter the behavior of their host. Puppet masters, I say.
Two recent studies have been published that look at behavioral changes due to a parasite.
The first one discusses a plant virus that changes the behavior of insects feeding on infected plants. After an aphid feeds on a plant infected with the Barley Yellow Dwarf virus, it changes its food selection. Non-infected aphids prefer plants with the virus (courtesy of organic compounds produced by the virus), but infected aphids prefer uninfected plants, thereby promoting the transmission of the virus to new, ‘virus-virgin’ plants.
All in all, an effective strategy of the virus. (You can almost hear the virus going “Dance, aphids, dance”.) The authors term it the ‘Vector Manipulation Hypothesis’ or (VMH):
Direct effects of virus acquisition on the vector host plant selection behavior in a manner that will promote the spread of the virus is consistent with an evolved strategy in the pathogen of manipulation of its vector. We propose the “Vector Manipulation Hypothesis” (VMH) to explain the evolution of strategies in plant pathogens that enhance their spread to new hosts through their effects on mobile vectors.
The second study looks at human behavior, more specifically at susceptibility for suicide. The parasite in question (Toxoplasma gondii) is also the one responsible for the rodents mentioned earlier, and is quite prevalent in human beings with an estimated 10-20% of the U.S. population carrying it (mostly dormant). However, this parasite can cause inflammation in the brain, which has previously been linked with suicidal tendencies and depression. The authors found that people infected with T. gondii have a seven times higher chance to attempt suicide (which still leaves a huge majority of the infected that are not suicidal, so don’t worry too much).
However, two brief thoughts on this second study:
- It’s a correlation, not a causal connection, which means that it’s too early to assert that the parasite induced changes are directly responsible for the behavioral findings. There could be an underlying common cause. To really assert a causal connection you should, for example, look at changes in people before and after infection. (But purposefully infecting people is probably not the most ethical study…)
- It’s probably not a cunning strategy by the parasite. Humans are accidental hosts here. And killing your host is not a good idea, unless it gets you transmitted to the next host (see the ants and rodents). But unless people get eaten by cats (a rather rare occasion, I’m happy to assert) inducing suicide isn’t the smartest thing to do. More likely (if it turns out to be causal) it’s an unintended side effect).
Nevertheless, many parasites are masters in manipulating the behavior of their host(s) or vector(s).
Perhaps they’re secretly ruling the world.
Just to be on the safe side: All hail the parasite overlords. (However, we’re not completely helpless…)
Ingwell, L.L., Eigenbrode, S.D., & Bosque-Pérez, N.A. (2012). Plant viruses alter insect behavior to enhance their spread. Scientific Reports, 2 DOI: 10.1038/srep00578
Zhang, Y., Träskman-Bendz, L., Janelidze, S., Langenberg, P., Saleh, A., Constantine, N., Okusaga, O., Bay-Ritcher, C., Brundin, L., & Postolache, T.T. (2012). Toxoplasma gondii Immunoglobulin G Antibodies and Nonfatal Suicidal Self-Directed Violence. Journal of Clinical Psychiatry, 73 (8), 1069-1076 DOI: 10.4088/JCP.11m07532