Trail of parasites is a brainy pathParasites may not be synonymous with hypnotists, but they do a great job at controlling other species’ brain activities.
Consider a microscopic parasite named Toxoplasma gondii. An Oxford University research team has been studying the behavior of rats infected with this crescent-shaped, single-cell organism, which takes shelter mainly in cats and rats.
The parasite infests every part of a rat’s body ― including its brain ― once the rodent contracts it by eating cat feces.
Normal rats are highly attuned to be wary of predators, but the parasite is believed to block this defense instinct in rats.
The study, led by Manuel Berody, suggests that the parasite controls the rodent’s brain so that it can more easily make it to its final host, the cat. The parasite can return to the cat only if the feline ingests the rodent. Therefore, T. gondii is believed to inhibit the rat’s fear response, which kicks in when a cat is near. However, rats’ other brain functions are not affected, the study notes.
Robert Sapolsky, professor of biological sciences and neuroscience at Stanford University, wrote in a Scientific American article that these parasites are so effective at manipulating neural activity that they make humans seem stupid by comparison.
Parasites like T. gondii alter their host’s natural behavior by controlling their brain function, he said, but do so in a way that helps them spread. For example, rabies could be spread like the common cold, by inducing sneezing. But instead, it exists in the brain, where it can stimulate aggression, and by inspiring their hosts to nip and bite, aid in its own transmission.
The lancet fluke (Dicrocelium dendriticum) lays its eggs in the bodies of herbivores such as cows. Its eggs come out through cow’s feces, and the larvae enter the bodies of ants when eggs hatch. To lay another egg the dormant parasite must return to the herbivore’s body.
It is believed to control the ant’s brain, so the insect remains still at the edge of a leaf, later to be consumed by the cow. Since the ant may die if it remains motionless under a scorching sun, the parasite is believed to release the ants to their normal state at times.
Parasites infest even the smallest insects like the mayfly, by laying their eggs underwater. The parasite’s spawn penetrates the body of the mayfly’s larvae, which are also underwater at this stage. When larvae become adults and mate, the parasites move into a female mayfly’s body, and the male mayfly dies after mating. The parasites leave the female mayfly when she lays eggs in water. Once free in the water, the parasite produces its own eggs and cycle restarts.
When a parasite accidentally enters a male mayfly while mating, it cannot reproduce. But it can survive by making the male mayfly’s appearance and behavior more feminine. The male mayfly’s reproductive organs wither away, and believing it is female, the host male mayfly eventually moves to water although it cannot lay eggs. The parasite returns to the water after penetrating the mayfly’s body.
Although it can’t control a human’s brain, the guinea worm does cause problems in Homo sapiens. When it’s time to return to water and reproduce, the guinea worm creates a blister on the human skin, from where it penetrates the body to re-enter the water. Before medicine was developed, humans infected by guinea worm would soak their feet in cold water to abate the soreness.
by Kwon Hyeok-joo
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