Let's read through Parasite Rex (2001), chapter by chapter.
Prologue
Parasites are much more prevalent and important than people give credit for. A majority of species of multicelluar life are parasites.
The biodiversity of parasites is astonishing and there is so much to learn, especially since biologists have long considered parasites as a freakish side-show to the spectacle of life.
Chap 1. Early history of parasitology.
Caduceus is probably based on the primitive therapy for guinea worm.
Renaissance doctors think that parasites are internally generated tissues/spontaneously generated organisms, symptoms, not causes, of diseases.
Spontaneous generation is falling out of favor in the 18th century thanks to experiments on rotten meat (disproving spontaneous generation of maggots) and meat soup experiment of Pasteur (disproving spontaneous generation of microorganisms).
Parasites posed theological problems:
1. If parasites are truly spontaneously generated, then it'd be accrediting bodies with the power to create life, which is playing God.
2. Theodicy. Darwin's quote about parasitoid is relevant:
I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars...
The familiar animals (fish, cats, humans...) have their young looking very similar to their adults. This is not so for parasites. Most of them have complex life-histories, with vastly different body shapes. Japetus Steenstrup was the first to propose that they travelled from host to host, changing their body shapes as they do.
Friedrich Küchenmeister proposed that bladder worms (little worm hiding in cysts in animal muscles) would develop into tapeworms in the digestive tract after being swallowed (usually when the host is eaten).
In 1854, then 1859, he proved his theory by feeding death row inmates with bladder worms (hidden in food), then dissecting the corpses and finding developed tapeworms.
Parasitology languished for many reasons.
One is that it got cut-off from the big-picture of biology. Parasites were mainly studied by "tropical disease doctors", who worked to help European imperialism.
Another is the error of orthogenesis, thinking that evolution has a definite direction. Parasites were thought of as criminals who disobeyed the progress of evolution. Darwin wrote about them only to illustrate the problem of evil in nature.
Ray Lankester, for example, wrote "Degeneration: A Chapter in Darwinism", an essay about how parasites are degenerates, lifeforms that gave up the good fight after they've found a comfortable hole to hide in.
Socialists also used this analogy to decry the super-rich as parasites. As late as 1955, parasitologist Horace Stunkard used this analogy to decry the welfare state as allowing people to become lazy parasites.
This idea has been applied to social darwinism, too. Lazy people are called parasites (they still are). Nazis, for example, called Jews a race of parasites.
This undeserved bad publicity haunts parasitology to this day. Even in 1989, Konrad Lorenz, the great pioneer in animal behavior, was writing about the "retrograde evolution" of parasites.
If one judges the adapted forms of the parasites according to the amounts of retrogressed information, one finds a loss of information that coincides with and completely confirms the low estimation we have of them and how we feel about them. The mature Sacculina carcini has no information about any of the particularities and singularities of its habitat; the only thing it knows anything about is its host.
Chap 2. The body, as felt by a parasite. That is, the umwelt of a parasite.
I wrote a whole post just on this topic.
The life of a fluke is complex. It has to go from the stomach to the intestine, then somehow to the liver. It took Michael Sukhdeo many years to figure it out.
Turns out the fluke is a purely reflexive animal and relies on discrete events to navigate. Free-living animals search by gradients (smelling the molecules), but parasites like fluke navigate by following a strict script: start thrashing out of the cyst when the pepsin is sensed (indicating the stomach), start burrowing when the bile is sensed (indicating the intestine), start climbing when no intestine is sensed (indicating it's in the abdominal wall), and climb until it reaches the liver.
Since parasites are so specialized to a predictable environment, they rely on intricately evolved scripts, executed reflexively.
Their umwelt is very simple and discrete.
A long list of parasite lifecycles follows. The most impressive ones are that of parasitoid wasps.
There are several new tricks I learned:
- Injecting virus to compromise the immune system (basically caterpillar AIDS)
- Controlling the caterpillar behavior, to make it protect the wasp larvae like a bodyguard.
- And whatever this is!
Typically, Copidosoma lays only two eggs in its host, one male and one female. As with any egg, each begins as a single cell and divides, but then it veers away from the normal path of development most animals follow. The cluster of wasp cells divides itself up into hundreds of smaller clusters, each of which then develops into separate wasps. Suddenly, a single egg gives rise to twelve hundred clones. Some of the clusters develop much faster than the rest, becoming fully formed larvae only four days after their original egg was laid. These two hundred larvae, known as soldiers, are long and slender females, with tapered tails and sharp mandibles. They roam through the caterpillar, seeking out one of the tubes the caterpillar uses to breathe. They wrap their tails around a breathing tube, and like sea horses anchored to a coral reef, they rock in the flow of caterpillar blood.
The task for these soldiers is simple: they live only to kill other wasps. Any wasp larva that passes by, whether other Copidosoma floridanum or another species, prompts a soldier to lash out from its tube, snagging the larva in its mandibles, sucking out its guts, and letting the emptied corpse float away. As this slaughter goes on, the rest of the Copidosoma embryos slowly develop and finally grow into a thousand more wasp larvae. These larvae, called reproductives, look very different from the soldiers. They have only a siphon for a mouth, and they're so tubby and sluggish that they can move only by being carried by the flow of the caterpillar's blood. Reproductives would be helpless against any attack, but thanks to the soldiers, they can just drink the caterpillar's juices as the shriveled corpses of their rivals float past.
After a while, the soldiers turn on their siblings— more specifically, on their brothers. A mother Copidosoma lays one male egg and one female egg; after they've both multiplied, they produce a fifty-fifty split betwen the sexes. But the soldiers selectively kill the males so that the vast majority of survivors are females. Entomologists once documented two thousand sisters and a single brother Copidosoma emerging from a caterpillar.
As ruthless as soldiers may be, they're also selfless. They are born without the equipment for escaping the caterpillar themselves. While their reproductive siblings drill out of the host and build themselves cocoons, the soldiers are trapped inside. When their host dies, they die with it.
Chap 3. Parasite vs immune system.
Trypanosomes follow a genetically hard-coded script in changing their surface antigens, to keep the immune system off the trail.
Toxoplasma in the body might have bursts of reproductions to deliberately provokes inflammation reactions, before retreating into cysts. This can be deadly in pregnant babies, who get all their immunity from the mother. The mother can only send antibodies into the baby through the placenta, not the immune cells themselves (else the immune cells could attack the baby as a giant tumor). Toxoplasma would replicate exponentially in the fetus (congenital toxoplasmosis). The same problem happens for AIDS patients, causing brain damage.
Cotesia congregata parasitoid wasps have a bizarre genetic weapon. As they develop an egg, a bunch of sections in the genome are cut out, assembled, and it constructs a plasmid and another DNA to put inside, essentially constructing a virus as a phenotype. This virus is then replicated rapidly in a liquid that the egg floats in. This whole concoction of egg and virus is injected into the caterpillar. The immune system of the caterpillar is so distracted by the virus blend that the wasp egg can develop in peace.
Chap 4.
Parasites can manipulate host behavior precisely by release of neurotransmitters.
Guinea worms reproduce in a cool way.
The guinea worm spends its early life curled up inside a copepod swimming in water. A person drinking that water swallows the copepod, and when it dissolves away in stomach acid, the guinea worm escapes. It slips into the intestines and burrows out into the abdominal cavity. From there it wanders through the connective tissue until it finds a mate. The two-inch male and the two-foot female have sex, and then the male looks for a place to die. The female slithers through the skin until she reaches a leg. As she travels, her fertilized eggs begin to develop, and by the time she has reached her destination the eggs have hatched and become a crowd of bustling juveniles in her uterus. These juveniles need to get into a copepod if they are to become adults themselves, and so they drive their human host to water. They press against their mother's uterus so hard that they force it partially out of her body, letting some of the larvae spill out. Adult guinea worms tame the human immune system so that they can travel through our bodies unharmed, but the juveniles do just the opposite. They draw a quick reaction that brings immune cells rushing to them, making the skin around them swell and blister. The easiest way for a victim to get some relief from the hot pain of the wound is to pour cool water on it or just stick the leg in a pond. The juveniles that have already escaped their mother inside the blister respond to the splash by swimming free. The mother responds to the water as well by getting rid of more of her young. She doesn't herniate herself the way she did before; this time she lets her babies escape through an even stranger route: her mouth. For every splash, half a million baby guinea worms come heaving up through her esophagous. The contractions pull her out of the wound bit by bit until she and her young have all left the host— the mother to die, the young to search the water for a new copepod to curl up inside.
Chap 5. Tracing the family history of parasites.
The cysts of some tapeworms today get so big, and grow with such force, that they can split open a human skull.
Malaria contains genes from chloroblasts, indicating an ancient event of endocytosis.
Don't forget social parasites, like the cuckoo, or the ant that secretes the chemicals to fool other ants into treating it as a queen.
The embryo is basically a half-parasite in a competition with the mother. The placenta seems to borrow some genes from parasites to neutralize mother's immune system.
The conventional wisdom that an old pathogen is mild, is not true. Every parasite-host interaction has an optimal virulence that the pair towards. Sometimes, this optimal point is quite brutal.
Besides, malaria has been with humans for thousands of years, with no sign of getting less virulent.
Chap 6. Evolution and parasites
Parasites drove evolution. One reason for sex is as parasite defence: sexual reproduction increases offspring diversity, which makes them harder to exploit. Similarly, parasites use sex to make them harder to target. It's a classic case of arms race that doesn't really go anywhere (Red Queen hypothesis).
This is tested in Nigerian snails that can reproduce asexually and sexually. Turns out that the snails reproduce sexually mainly in March--June, the fluke season.
Also, nematodes that grew up in an immunocompromised rat would produce more asexual offsprings.
Hamilton and Zuk proposed that a heavy parasite load in a species would make sexual selection to focus on immune system strength, which would then favor costly signals for that. In short, parasite-infested sexual species tend to have flashy sexual displays.
Queen bees that mate with more males would produce broods that are more resistant to parasites, likely because they have a better ecological diversity.
Leaf-cutting ants carry little ants on tops of leaves, specifically to defend against parasitic flies that would eat the mandible muscles in the ant heads.
Mammals and birds spend a lot of precious time grooming.
Screwworm flies lie eggs in any kind of open wound, which hatch into flesh-eating maggots. This might have made howler monkeys less violent, since fights lead easily to death by screwworms.
Cannon ass leaf-rolling caterpillars, to throw parasitoid wasps off their scent.
Cows avoid lush grasses if they are fertilized with shit, because those likely contain parasite eggs.
Snails infected with fluke could use their final time to engage in as much sex as possible, or to rapidly mature if not yet mature.
Parasites tend to specialize in just one species, but dabble in other species. Specialization is favored because it can't deal with several Red Queen races at the same time.
Chap 7. Human parasites
Many human "diseases "are really side effects of parasite defences, like thalassemia, sickle cell, immunoglobulin E allergies.
Humans started getting really infected with parasites after the agricultural revolution. Toxoplasma follows cats and rats. Tapeworm follows pigs and other livestocks. Blood flukes breed in irrigation systems.
Chimps teach and learn plant medicine against parasites.
Parasite diseases are hard to eradicate because parasites are complex, has no vaccine, and evolves quickly. It's better to handle them by artificial selection taming. Also, lack of parasites could cause autoimmunal diseases (colitis, Crohn's disease), which could be cured by injecting harmless parasite-analogs.
Chap 8. Ecological engineering with parasites
Biological control worked sometimes: in the 1980s, African cassava plants were infested with mealy bugs, which were suppressed by introducing parasitoid wasps.
Other times, the introduced parasite would have bad side effects.
Parasites are a source of medicine. Fluke worms can avoid immunal reaction, with chemicals that could one day help organ transplants to not trigger autoimmunal reactions.
Parasites are vital in ecologies for several reasons. One, it is just so numerous. Most free-living species harbor many different species of parasites. Two, parasites often hop between species, and thus hold the species together like connective tissues of the ecology. Three, they are canards: disappearance of parasites often indicate ecological stress (Canadian eel example).
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