You're an excellent host

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http://archive.salon.com/health/books/2000/09/26/parasite/index.html

 

You're an excellent host

Parasites can slip into your body, rewrite your DNA and, sometimes,

change your mood.

 

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By Jill Wolfson

 

Sept. 26, 2000 | Parasites can castrate their hosts, take over their

minds and short out their DNA. They can turn healthy organisms into

the living dead. And they can be found anywhere -- in our legs, our

brains, our intestines, our kitty litter.

 

Science writer Carl Zimmer's new book, " Parasite Rex: Inside the

Bizarre World of Nature's Most Dangerous Creatures, " introduces

readers to some of nature's most sinister characters: nematodes that

cause blindness, worms that swell up a scrotum until it fills a

wheelbarrow, 60-foot-long tapeworms and deadly creatures so tiny they

hitchhike on the back of a fly.

 

 

 

Parasite Rex: The Secret and Bizarre World of Nature's Most Dangerous

Creatures

 

By Carl Zimmer

 

304 pages

Nonfiction

 

 

 

Zimmer, who lives in New York and is a contributing editor at Discover

Magazine, says that -- parasitically speaking -- there is an

embarrassment of riches all around us.

 

But it wasn't pure science that first piqued his fascination (dare we

say obsession?) with this world of bloodsuckers. As a kid, he always

got a kick out of parasite-type science fiction movies. So, in the

name of research, he not only traveled to far-off places but fired up

the VCR and watched a gruesome marathon of jelly-like aliens, hairless

bipeds and gut-devouring lizards vying -- literally -- for the heart

of man.

 

The result? He knows far more about parasites than even he bargained

for. Salon talked with Zimmer by phone about malaria, monster movies

and the possibility of parasitic world domination.

 

" Parasite " is such a loaded, metaphoric word. We think of welfare

mothers, ex-wives, dinner guests who don't return the invitation,

Hitler's label for the Jews. Technically though, what is a parasite?

 

Anything that thrives at the expense of what it's living on and living

in. In the broadest definition, you include viruses, a lot of bacteria

and things you don't normally think of as parasites. A fetus in the

womb actually behaves a lot like a parasite. It uses strategies to

extract nutrients and energy out of its mother. And the mother, to a

certain extent, has to defend herself against it.

 

In the scientific world, parasites have been cast as minor

hitchhikers, not a serious force of nature. Yet, you say that

parasites make the world go 'round.

 

Historically, they were viewed as agents of disease. Or they were seen

as nature's degenerates -- animals and organisms that had devolved and

lost all ability to live in the free world. Scientists thought you

could look at the world and ignore the parasites.

 

But in fact, they are everywhere. Open up any animal -- healthy or

sick -- and it's just loaded. No one really knows how many parasites

exist, but there are estimates of four species of parasites for every

nonparasite. The vast majority of species are parasites. They may, in

fact, have a very powerful evolutionary effect.

 

Does that mean that parasites drive everything from eating to mating

behavior?

 

Sex is a good example. Why did sex evolve? If you look at it, it

really doesn't make much sense compared to just cloning yourself. So

scientists consider factors that might make sex an advantage. One of

the best theories is that parasites make sex desirable because they

are always trying to adapt to their hosts, take advantage of and in

some cases, kill them. Anything an organism can do to defend itself is

going to give a good evolutionary advantage. The way that sex shuffles

up genes can help give a species an edge.

 

You said that if you open up most animals, you'll find basically a

parasite hotel. How often are we humans offering room service?

 

It depends on the person and where in the world you go. In the United

States, you might open up a person and maybe in their brain you would

find cysts of a single-celled parasite called toxoplasma. Somewhere

between 30 to 50 percent of the U.S. population carry it in their

brains. In France, the proportion goes up to 90 percent or more.

 

Toxoplasma normally lives inside cats and they shed it out in their

feces and then it moves to its next host, which is a rat or a bird or

some other prey of a cat. Once the parasite gets into the intermediate

host, it replicates and then builds a protective shell around itself.

Each of these cysts contains a few hundred toxoplasma. The cysts can

sit for years. The parasite is basically waiting for its host to be

eaten by another cat so that it can begin its life cycle again. People

pick up toxoplasma all the time, whether they are gardening or

handling kitty litter. It likes to be in the brain.

 

Cysts in the brain? Wouldn't we notice that?

 

The cysts will just sit there, waiting for us to be eaten by a cat.

(Toxoplasma is usually harmless in humans, except for pregnant women

and people with compromised immune systems.)

 

But there is some really interesting research. When toxoplasma cysts

are in a rat, the parasite somehow does things to the host that makes

it more likely to be eaten by a cat. For example, it takes away the

rat's healthy sense of fear. Rats with toxoplasma are less likely to

run away in terror at the smell of a cat.

 

Does that apply to humans? Once infected, do we get an urge to sign up

for a safari and put ourselves at the mercy of big game?

 

Some psychologists have actually done studies comparing people who

have toxoplasma and people who don't. They found very subtle

personality differences. People who have toxoplasma may be more

openhearted than people who don't.

 

So a parasite could be responsible for altering the personalities of

billions of people?

 

Hookworms are a definite example of that. Up until the mid-20th

century, they were rampant in the United States, especially the South.

They penetrate your skin, burrow into your body and live in the

intestines. The whole reputation of the lazy Southerner comes from the

fact that a lot of them had hookworms. They weren't lazy. They were

anemic. Up until recently, Americans had lots of parasites.

 

Confession time. I've had my share of parasites. Giardia, head lice

and, OK, pinworms. How about you?

 

I'm basically parasite free. I've traveled a lot in the developing

world. So far -- knock on wood -- nothing serious. If it's any

consolation, pinworms are everywhere, and in everything.

 

If you have to have something, that's not so bad compared to what's

out there in the rest of the world. There's a fluke called schistosome

that lives in the bloodstream. It infects 200 million to 300 million

people a year in Africa and southern Asia. There are 1.2 billion cases

of hookworm in the world. Two or 3 million die of malaria a year. And

many of these parasites are on the rise, not the wane.

 

What parasite ranks highest on your personal yuck scale?

 

A little crustacean that lives in the ocean. It swims inside the mouth

of certain fish and devours their tongue and plants itself down in

their mouth and proceeds to act like their tongue. Quite disconcerting.

 

Is there a pinup parasite?

 

Actually, when you look at them awhile, some of them are downright

pretty. For instance, tapeworms have a little head that is adapted for

letting them hold on inside their host's intestine. Some heads look

like ram's horns. Some look like arrows. Some look like dandelions.

They are all different and quite beautiful.

 

Give us a mini-biography, from birth to death, of one of the more

sinister parasites.

 

The Guinea worm starts off its life cycle living inside a microscopic

copepod that swims around in fresh water. When a person drinks the

water, the stomach acid dissolves away the copepod. But the larva

manages to survive and burrows its way out of the intestinal wall. It

wanders around the abdominal cavity looking for a mate. The male

Guinea worms get up to 2 inches long. The females can get up to 2 feet

long.

 

When they mate, the male dies. The female starts traveling through the

connective tissue of a person's body, down into their leg. All the

while, its fertilized eggs are developing. It crawls to the leg and

starts releasing the larvae just under the skin. That creates a very

painful blister that people generally want to wash off in water.

 

Which is exactly what the parasite wants the person to do! The blister

pops and the larvae go into the water. When the Guinea worm senses the

water being poured on it, it will slowly start pushing itself out of a

person's body and releasing more larvae. Once in the water, they go

off looking for another copepod.

 

Assure me there's a cure.

 

There's no drug you can take if you get Guinea worms. The only " cure "

is one that's been around for thousands of years. As the Guinea worm

is pushing itself out of your body, you slowly wrap it around on a

stick. You don't want to grab it and pull it out because it will break

and die, and then you have a 2-foot-long parasite in your body that

will cause infection and might very well kill you.

 

For thousands of years, people have lain around for days just turning

these sticks. Some think that this procedure is the symbol of medicine

-- the snakes around the staff. There are references to fiery serpents

in the Bible when the Israelites are wandering in the desert. Guinea

worms. Fortunately, even though there's no vaccine, they may be

eradicated soon through public health awareness on how to avoid them

[for example, using cheesecloth to drink fresh water and learning to

not wash in freshwater sources].

 

Good riddance to Guinea worms -- and pinworms and head lice, too. But,

don't we have to consider the big picture? If we are living in a

parasitic world and they are such a dominant force, are we messing

things up by eradicating them?

 

Truthfully, total eradication is just a dream. They are so resourceful

and abundant. But in terms of medicine, there may be some unexpected

results when you get rid of parasites.

 

Scientists are looking into the possibility that eradication might be

responsible for allergies. There's a pretty clear correlation. Places

where parasites, such as intestinal worms, have been eradicated are

the places where you see the most allergies and disorders like Crohn's

disease (an autoimmune disease in the intestines).

 

In Venezuela, scientists found that affluent city dwellers had high

rates of allergies and very few parasites. Poor people living in the

cities, because their sanitation isn't as good, have more parasites

but their allergy rates are consistently lower. Same with Indians

dwelling in the rain forest.

 

Right now, these are tantalizing connections. You have to keep in mind

that our ancestors have been grappling with parasites for millions and

millions of years. In a sense, we have an uneasy truce with them. Part

of that involves the way the immune system holds them in check. Taking

away parasites all of a sudden may make the immune system prone to

overreacting to things like cat dander, peanuts or even your own body.

 

Attacked by parasites and attacked by our own bodies. Sounds like a

standard-issue horror story scenario. In fact, some of our deepest,

most universal fears are based on parasite images. " Alien. " " Invasion

of the Body Snatchers. " Is there any possibility of some kind of

catastrophic invasion?

 

There are certainly lots of parasites that we don't know about out

there, whether they are virus, bacteria, worm or fungi. There are lots

of things in the natural world that make their living by invading

other organisms.

 

A lot of them are living inside hosts that live in the rain forest and

remote parts of the world. The more we disturb these environments and

the more globalized human society becomes, the easier it is for

something to hop out of its previous host and try to infect humans. It

may find that humans are a perfectly good host.

 

If we hold up the mirror, what can we learn about ourselves from

parasites?

 

If you think about the relationship we have with the rest of nature,

the way we extract from the natural world, the way we use up resources

without restoring them, the way that we manipulate nature in order to

make it serve us better: These are all things that parasites do, and

do very well.

 

After learning so much about them, I don't think that it is

necessarily a bad thing to be called a parasite. They are quite

clearly the most successful organisms on the planet.

 

But it might be useful for us to get an idea of what they are doing

right. Because if we humans are parasites, I don't think we are very

good ones. A parasite that kills off its hosts has got nowhere to

live. Human beings only have one host. We have to treat it accordingly.

 

salon.com | Sept. 26, 2000

 

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About the writer

Jill Wolfson is a co-author of " Somebody Else's Children: The Courts,

the Kids, and the Struggle to Save America's Troubled Families, " and

she reviews books for the San Jose Mercury News. She lives in Northern

California.