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Parasite Rex

The World of Nature's Most Dangerous Creatures

Carl Zimmer

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Every living thing has at least one parasite living on it, or with it. Many, like humans, have many more. There's a parrot that has 30 different mites just in its feathers. Andprasites have parasites of their own, and some of those have parasites as well.

We have no idea how many pecies of parasites there are, but they are the most numerous species on Earth. They probbably outnumber fre-living species by at least four to one.

German doctor named Friedrich Kuchenmeister was the first to recognize that tapeworms life cycle started as bladder worms in prey (mice pigs and cows) and ended in predators (cats, dogs, humans).

He demonstarted this in sheep and dogs, then in pigs and humans. He got permission to feed bladder worms to a prisoner who was due to be executed, and then dissected his body and found tapeworms dveveloping.

The inside of our body is a tough place to survive. Parasites have to be able to navigate through a murky labrynth, cut through skin and gristle, survive the cauldron of acids in the stomach. They can find a home in every part of our body, rebuilding parts to suit their needs. They can feed on almost anything, and they can make their host deliver the food.

Plasmodium, the parasite that causes malaria, lives in red blood cells, where it feeds on the haemoglobin. It then divides into 16 new parasites, each of which goes looking for another red blood cell.

Red blood cells travel about 300 miles in their lifetime, getting squeezed through tiny capillaries where it has to compress to one fifth its normal size, then bounce back once its through. To do this it has a membrane which is set up like a mesh bag, able to be crumpled and compressed. But eventually this memebrane becomes too stiff to work properly. As it passes through the spleen, it gets carefully inspected. Only young red blood cells make it out of the spleen; the rest get destroyed.

Plasmodium evades the spleen filter by lodging in capillaries in the brain, liver or other organs. But this leads to another problem for the parasite. It attaches to organ with latches which snag onto the side of the capillary. Up until now it has been camouflaged from the immune system bc red blood cells don't make MHC molecules which signal an invader. When they appear, the immune system sees the latches as an invader.

But plasmodium uses only a single gene to make each type of latch. It has over a hundred different genes which can make the, and every so often, as the parasite divides, it turns on a different gene which makes a different kind of latch. So by the time the immune system has recognized one latch and mounted resources to attack, the parasite is busy making cells with a new type of latch. An endless and exhausting game.

Parasites are not passive consumers. They typically hijack their host's cells to make a secure and well-resourced home. Trichinella invades muscle cells, rebuilding them to attract new capillaries to feed them blood, and make collagen to form a tough protective shell.

Root-knot nematodes reverse the function of root cells in infested plants. Root cells normally draw in water and nutrients from surrounding soil. But nematodes rewire cell so it sucks nutrients back from the plant. As the cell fills with food, it threatens to burst under pressure. But the nematode prevents that by rewiring surrounding cells to form a tough protective shell. Just as trichinella speaks the mammal genetic language, so nematodes have learned the language of plants.

Leishmania actually hijacks the immune system. It allows the macrophages, which usually swallow and chop up invading cells, to ingest it, but stops the destructive stage. The immune system has two types of T cells which help attack invaders - inflammatory and antigen producers. If host can make enough inflammatory T cells, it can fight off the infection. But leishmania rewires its host macrophage to releasesignals calling for more antiden T cells, which are useless bc they can't reach the parasite tucked away inside the macrophage.

Millions of humans are infected with toxiplasma gondii, but they are not the intended host. It normally cycles between cats and their prey. Toxiplasma needs to keep its host alive long enough to get eaten by a cat. So it too hijacks the immune system, but uses the opposite strategy to leishmania. It gets the immune system to produce huge numbers of inflammatory T cells, keeping animal healthy while the parasite is safe within protective cysts.

Normally, this has little to do with humans. But two groups with little or no immune system - fetuses and AIDS victims - are uniquely vulnerable, bc the parasite reproduces unchecked without the T cells to rein it in. HIV (human immunodeficiency virus) infects T cells, using them to reproduce and killing them in the process. When toxiplasma pops out of its cyst, it has no T cell opposition, reproduces madly, often causing extensive brain damage and killing its host.

A fungus that lives inside houseflies spreads through its body, sucking up all its nutrients. The fly carries on as normal for a few days. But when fungus matures and is ready to spore, it forces the fly up to a high place, drops its front legs so its abdomen is sticking up, locks its wings in an upright position, and uses its probiscus to stick itself in place. Then, always at sunset when other flies are returning to the ground below, the fly dies and the fungus catapults its spores out of its body, showering down on healthy flies.

The tapeworm Hymenolepis live and mate in the bowels of rats. Their eggs end in rat droppings which are eaten by beetles. The tapeworm produces a chemical which makes the rat droppings irresistable to the beetles, so they preferentially devour the egg laden ones. The eggs grow into a juvenile form within the beetle, which it sterilizes, stopping the beetle diverting resources to egg production. When it matures, worm releases an opiate which makes beetle sluggish and careless about hiding, so an easy prey for a rat. And while healthy beetles produce a foul-tasting chemical which causes the rat to spit itout, the worm blocks the production of this, guranteeing that the rat will swallow it.

Why do animals have sex? Takes a lot more resources than cloning. The two fvored explanations called The Tangled Bank and the Lottery. Lottery suggests that sex produces variety that allows adaption to changing environments. The Tangled Bank says that need to be able to exploit multiple environments in case one food source dries up. But a third possibility, the Red Queen race, came out of NZ research. Idea that constant arms race against parasites.

It seems that evo works fastest on species that are under heavy predation by parasites. Not only by encouraging the survival of helpful mutations, but by directly affecting host DNA. A parasite invariably hijacks host's genes to reshape the organism for the parasites benefit. So it cuts into DNA chains, and this disruption can lead to new proteins being made by the host,and so expressing genes differently. The survivors of attacks are different from their ancestors and cousins, and well on the way to becoming a separate species.

Colitis and Crohn's disease occur when the person's own immune system violently attacks part of their digestive system. There's no cure, and victims are tormented ofr lifetime. But these diseases were unknown before the 1930's. And their pattern is most unusual - it appears first in well-to-do people. It was NY jews in 1930's, then other whites, then blacks in America. It is not found in very poor countries at all. But Japan and Korea are now facing epidemics ofr the first time.

Suggestion is that the causal factor was the eradication of intestinal worms. Parasitic worms nudge the immune system into a gentler form which still controls viruses and bacteria, but doesn't harm too many worms. This truce makes sense for the host as well, bc in areas where worms plentiful, the body wd be forced to wage a non-stop battle against a constantly reinforced invader.

Scientists tested this idea by feeding patients with pig worm eggs (which don't cause disease in human gut) and most went into remission.

Possibility that parasites have tamed aggression in monkeys. Howler monkeys are predated by a vicious parasite, the primary screwworm fly. It can find the smallest wound in a mammal, into which it lays its eggs. When the larvae hatch they start eating the animal's flesh, causing so much damage that the animal can die. So howler monkeys notably avoid physical fights, bc the smallest wound cd be fatal. So they howl and they slap, but they don't scartch or bite. Evo may have selected for the less aggressive animals.

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