The wonderful world of Parasites
One of the most disturbing movies I have seen was in 1975 – a virus comes through the water system of a building and turns everyone into sex crazed monsters before they die in agony. I have still not forgotten it. An entire horror movie genre is devoted to parasites – the extremely popular vampire movies are one example.
The word parasite means, "one who eats at the table of another" Parasitism — a survival strategy that involves hijacking a host's nutrients for one's own benefit — has emerged in the animal kingdom in at least 223 species, according to a study published July 19 in the journal Biology Letters. Those inside their host are known as endoparasites, whereas those found outside their hosts are ectoparasites. The majority of parasites in the animal kingdom are invertebrates - virus, fungi, bacteria, amoebae, flagellates and sporozoa (such as the malarial parasites). Worms and flukes are very important parasites of man and mammals. 1.4 billion people have roundworms. In fact, 342 kinds of worm parasites affect all people. Endoparasites include ticks, mites, lice and fleas.
If parasitism was simply the sucking of food/blood from the host, that would be bad enough. But some parasites also capture the mind and make the host change its behaviour to benefit them – in effect creating a zombie.
The Ophiocordyceps unilateralis fungus prefers the undersides of leaves of plants growing on the forest floor as temperature, humidity and sunlight are ideal for it to grow and reproduce. The parasite invades ants and compels them to go to a low leaf, bite a vein on the underside and die hanging upside down. The fungus erupts a long stalk from the ant’s head with which it sprinkles its spores onto other ants.
The chest-bursting alien in Ridley Scott's "Alien" was inspired by parasitoid wasps. These wasps lay their eggs in caterpillars, beetles and other insects. When the eggs hatch, they crunch their way out of their living incubators. One wasp species, Dinocampus coccinellae, target ladybugs. A single egg is laid in the abdomen. The larva eats its way out and spins a cocoon underneath the ladybug who lives on as one of the undead, partially paralyzed on top of the cocoon. The wasp larva provides resources to keep the ladybug alive, while the ladybug provides protection from bugs that might otherwise eat the larva.
The female Hymenoepimecis argyraphaga wasp hunts down the Plesiometa argyra spider and paralyzes it for about 10 minutes with a sting. In that time, she lays an egg, attaches it to the spider's belly and departs. When the spider recovers it goes about its business as though nothing has happened. All the while the wasp egg is growing. Some days later it hatches, stabs a hole in the spider's stomach and begins feeding off it. The infant wasp injects some kind of psychoactive substance that convinces the spider to spin a new kind of web designed to provide shelter from the weather and predators. When it's done, the wasp kills the spider, eats it and then uses the freshly spun web to wind itself into a cocoon. Two weeks later a fully grown wasp emerges.
Glypatapanteles wasps lay up to 80 eggs on a live caterpillar. When these hatch the larvae snack on the host’s body and then attach themselves to a nearby plant and form a cocoon. The caterpillar who is still alive, strangely, turns bodyguard. It starves itself while standing guard over the cocoons, violently swinging its head to beat away predators. How the parasites cause the caterpillars to turn into faithful guardians is unknown, but unparasitized caterpillars don’t behave like this.
Horsehair worms are parasites of crickets. The larvae live in water and are eaten by mosquitoes who are then eaten by crickets. The worms burrow into the gut and grow to adulthood. They need to get back into the water to lay eggs. They cause the cricket to become suicidal. Normally crickets stay away from water, but infected hosts run into it and drown. The worms emerge from the dead body.
The parasitic barnacle, Sacculina carcini, invades crabs. It sprouts tendrils that reach throughout the victim's body, even coiling around its eyestalks. Living off nutrients from the crab’s blood, this parasite grows a bulge on the host’s underside where it keeps its eggs. It alters the mind of the crab so that the female crab nurtures this knob as she would her own fertilized eggs. Even the male crab grows a wide female abdomen to accommodate the barnacle's knob and protects the eggs just as a female would. Then the parasite directs its host out to deeper waters where it can avoid predators and competition for food.
Toxoplasma gondii, a parasitic protozoan, can grow inside any animal, but it has to complete the sexual phase of its life cycle inside a cat. An infected mouse loses its fear of cats, is eaten easily and the parasite continues its life cycle. Humans can fall victim to T. gondii, too, and researchers suspect that the parasite might have links to human mental health, including schizophrenia and brain cancer. Scientists say a high level of parasite infection in a society could change entire cultural behaviour.
Some parasites decide the sex of the host. The Wolbachia bacteria infests 70 percent of the world's invertebrates, and has evolved strategies to keep spreading. In female hosts, the bacteria attaches itself to the eggs. Since males are useless for the bacteria's survival, the parasite often eliminates the male embryos to increase the number of females, or turns them into females.
The female crustacean, Cymothoa exigua, is the only parasite known to replace an organ. It enters through the gills of the spotted rose snapper and attaches itself to the base of the fish’s tongue, where it drinks its blood. The bloodsucking causes the tongue to wither away. The crustacean attaches itself to the tongue stub and acts as the fish's tongue. A male will crawl up from the gills and mate with the female inside the fish’s mouth. She then releases her babies through the fish’s mouth who go on to other victims.
The guinea worm Dracununculus medinensis larvae are ingested by water fleas who enter humans when they drink untreated water. The stomach acid dissolves the fleas, releasing the larvae who burrow through the intestinal wall. Male and female mate with as many as 3 million embryos inside the female at one time. The female slithers down to the human foot. She pierces the skin. Because it burns, the human dunks the foot in water which is exactly what the worm wants. The female then pokes her head out and vomits embryos from her mouth into the water. Those are eaten by fleas and the cycle starts again.
The parasitic fly, known as Apocephalus borealis, injects its eggs into a honeybee's abdomen, where the fly larvae mature. The parasitized bees abandon their hives and walk in circles till they die. The mature larvae burst out from the dead bees' bodies.
A species of baculovirus infects gypsy moth caterpillars and makes them climb up treetops to die. When the caterpillar's body liquefies, the ooze drips down onto other caterpillars — creating more robots.
In the California salt marshes, the fluke Euhaplorchis californiensis reduces the ability of its killifish host to avoid predators. It makes the fish swim close to the water surface where it can easily be swallowed by egrets, a bird the parasite needs to complete its life cycle. Uninfected fish swim far below the surface.
Why do parasites modify host behaviour? Many require several hosts of different species to complete their life cycles and have to rely on predator-prey interactions to get from one host to another. So it is vital that they change the behaviour of infected hosts, to make transmission to other hosts more likely to occur.
Believe it or not, some people intentionally infect themselves with parasites. Patients with diseases like ulcerative colitis swallow worms in an attempt to have them clean up their insides. Could they have had their behaviour changed by parasites who already exist in their bodies?
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