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Topic Last Updated on 11-07-2024
Post on topic: Parasites Manipulators.
Kicked out and Crucified
The setting: a rainforest. Starring: the carpenter ant, Camponotus rufipes, and the fungus, Ophiocordyceps camponoti-rufipedis. Some species of Cordyceps parasitize on caterpillars, flies, and other insects, but Ophiocordyceps camponoti-rufipedis is unique in that it can change the behavior of its victims. This fungus infects the tropical ant Camponotus rufipes: when infected, the social insect leaves the anthill, finds a leaf low off the ground, and picks a place on the vein on the underside of the leaf. The fungus has achieved its purpose and grows through its victim — soon there’s only the chitinous silhouette left on the bottom of the leaf. Then, the fruit body ripens and releases spores that infect other ants, and the cycle repeats.
How the fungus controls the ant
If relatives of the zombifier fungus get by without complex manipulations, why does Ophiocordyceps camponoti-rufipedis need it? Because the inhabitants of the anthill are worried about order and sanitation: they take the dead out of the house and bury them outside. Scientists from the US and Brazil decided to find out what would happen if the bodies of infected ants ended up in an anthill. As expected, the corpses were promptly removed from inhabited anthills. But even when the carcass of an infected ant was placed inside an abandoned anthill, the fungus would not sprout. The zoologists concluded that the conditions inside might be unsuitable. So they began to monitor the movement of ants relative to anthills and leaves with germinated fungus.
It took them as many as 20 months of observations to notice a curious feature of the parasite fungus. Steered the victims to a relatively close location: most of the leaves with dead ants were located near the foraging paths leading to the anthill. It turns out that the fungus “aims” not to get inside the anthill, but to increase its chances of infecting new victims.
Discomfort or Manipulation?
The ergot fungus Ophiocordyceps camponoti-rufipedis exclusively uses ants as hosts. In some parasites, the life cycle is more complicated: eggs of one species get infected and host the parasite while it is in the larva state. However, the “guest” reaches sexual maturity in another host. That is exactly the lifestyle of the beef tapeworm from zoology textbooks, Taenia saginata. This tapeworm does not control the behavior of its hosts to multiply, but its relative, Ligula (Ligula intestinalis) has been caught red-handed manipulating its intermediate carrier.
The life cycle of Ligula is in no way connected with either humans or cattle. It is even more complicated than that of the beef tapeworm. Once in the water, Ligula eggs turn into small floating larvae. They are swallowed by small crustaceans, and the larva loses cilia and the ability to swim. At the next stage, a fish consumes the crustaceans, and within 10 to 12 months, the microscopic larva becomes a worm that can reach up to 3 ft in length.
Such a parasite causes a lot of inconvenience to its host. The fish infected with Ligula begins to swim closer to the surface. This significantly increases their chances of being eaten by the final host, a seagull, or another bird of prey. Once inside the seagull, Ligula begins to produce lots of eggs, which get back into the water with bird’s feces, thus closing the cycle. The worm dies inside the bird without causing it any harm.
Life cycle of ordinary Ligula
What makes a fish stay near the surface of the water? Is it internal discomfort and loss of balance due to an overgrown parasite inside? Or does the worm “purposefully” affect its intermediate host? There is no consensus here. However, a recent study at the University of Bergen can be considered a weighty argument in favor of the second theory.
Researchers caught fish from different depths and evaluated them by the degree of infestation with Ligula: uninfected; infected, but the worm has not reached puberty; infected, and the worm is ready to “switch” hosts. It turned out that, although at early stages the parasite is comparable in size to mature individuals. The infection does not affect the depth at which the fish lives. So, the point is not the discomfort caused by the pressure the parasite exerts on the internal organs, but rather the intentional manipulation by the worm. What mechanisms the Ligula uses to control the behavior of its intermediate host is still a mystery.
If You Smell a Cat, Go Chase It
In fact, changing the behavior of the intermediate host to get to the final one is a fairly common strategy in parasites. Perhaps Toxoplasma may be the most well-studied such manipulator. Toxoplasma gondii is a single-celled Apicomplexan eukaryote, a close relative of the malaria parasite. The final host of Toxoplasma is always a feline, while the range of intermediate ones is unusually broad. They can be birds, rodents, or primates (including humans), and all of them become the subjects of manipulation.
It’s relatively easy to explain how a parasite manipulates rodents: typical feline food is mice and rats. Toxoplasma seeks to increase the chance of an infected rodent bumping into a cat and “forcing” the former to be attracted to the smell of a cat’s urine. It’s a bit more complicated with primates. Big cats, such as leopards, hunt primates. Therefore, monkeys infected with Toxoplasma also become bolder and go after feline smell, disregarding their own safety.
In the modern world, it is difficult to imagine a cat eating a human being. However, vulnerability to Toxoplasma was most likely inherited from our primate ancestors. Most healthy people do not experience any symptoms…except for one — behavioral change. The tendency to take risks, a passion for bloody meat, and an immense love for cats and their smell — toxoplasmosis may be the reason for such symptoms. According to various estimates, anywhere from 10% to 90% of the world’s population is infected. Some scientists believe that Toxoplasmosis has determined the culture of whole nations. It is associated with the cult of the cat goddess Bastet in ancient Egypt and the French love for undercooked steaks.
