Blogs / The Loom

Thanks to PZ Myers for calling attention to this superb video of Corydceps, a parasitic fungus that drives its insect host up a plant before growing a spike out of its head. Leave it to David Attenborough, master of the nature documentary, to bring the beauty of this parasite to video. I’ve seen photographs of Cordyceps before, but I never knew it made such a graceful entrance.

What’s particularly cool about Cordyceps is that it is not alone. Other parasites drive their hosts to bizarre heights. Another fungus, called Entomophthora muscae, drives houseflies and other insects upwards, climbing screen doors in some cases, before springing out of its host’s body.

In the case of Entomophthora and Cordyceps, hosts go up so that parasites can come back down again–specifically, down on potential insect hosts living on the ground. But other parasites have another direction in mind. The lancet fluke drives its insect hosts up to the tops of plants so that grazing mammals may eat them. Only in the gut of a cow or some other grazer can the flukes mature and reproduce. These creatures are like the birds, bats, and pterosaurs of the parasitic world, hitting on the same brilliant solution again and again.

(Here’s the place where I write about these parasites in my book, Parasite Rex.) [Update: Excerpt link at Amazon link fixed.]

Well, the talk at Cornell last week went very well. Thanks to everyone who came. If you want to hear me wax rhapsodic about parasite manipulations (and explain how scientists study their evolution), you’re in luck. Cornell has put the video of the talk online. The image is pretty small on the screen, so I decided to post the slide show on my web site here. I suggest opening two screens and advancing the slides as the talk progresses.

At first the sound is a little scratchy on the video and the light balance takes a while to get properly adjusted. But don’t give up–it evens out. You may also hear a baby gurgling from time to time.

Near the end, when I talk about cuckoo birds as parasites, I refer to their host in one of the pictures as a cowbird. I should have said a reed warbler.

And if you are curious to find out more, check out my book, Parasite Rex.

Update: Apparently the video doesn’t work for some readers. I am at a loss.

Attention all Loom readers in the Cornell University area: I’m heading up to Ithaca to give a talk tomorrow on a subject near and dear to my heart–how parasites turn their hosts into puppets and slaves. I’ll be at the David Call auditorium in Kennedy Hall at 4 pm. The lecture is open to the public and will, of course, include a very creepy Powerpoint. Details here, map here.

Toxoplasma, that mind-altering, cell-manipulating, all-around awesome parasite that sits in the brains of billions of us, is back in the news. Infection with the parasite raises the chances a woman will have a boy from 51% to 72%. The average ratio of boys to girls at birth is 51%. Women with high levels of antibodies to Toxoplasma, scientists found, have a 72% chance of having a boy. While many effects of Toxoplasma probably have something to do with adaptations that allow the parasite to thrive and spread successfully, this one seems more like a side-effect, albeit a dramatic one.

Source: Guardian Unlimited | Science | Pregnant women infected by cat parasite more likely to give birth to boys, say researchers

Paper abstract here.

[Thanks to BC for the fact-check.

I’m heading out of blog contact for a couple days, so allow me to share one of my favorite posts, from last January–on wasps that perform brain surgery.

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Once again, I hear the siren song of Toxoplasma, the parasite that dwells in the brains of 50 million Americans.

Toxoplasma gondii is an extraordinary creature, whose exploits I’ve chronicled in previous posts , an article in the New York Times and my book Parasite Rex. This single-celled organism has a life cycle that takes it from cats to other mammals and birds and back to cats again. Studies have shown that the parasite can alter the behavior of rats, robbing them of their normal fear of cats–and presumably making it easier for the parasites to get into their next host.

Toxoplasma is astonishingly successful, able to live in thousands of species, including us. Billions of people are infected with Toxoplasma, which they pick up from the soil or from contaminated meat or water. In most people it remains dormant, but even in this quiet state it may also have affect human behavior. Some scientists have linked Toxoplasma to schizophrenia, while others have found personality differences between people with Toxoplasma and those who are Toxo-free. It’s possible that it uses its prey-altering strategy on our brains, too.

All well and good. But now Toxplasma is going big time. Today the Proceedings of the Royal Society of London is publishing a paper called, “Can the common brain parasite, Toxoplasma gondii, influence human culture?”

The paper’s answer? Quite possibly yes. Here’s why…

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Next time I go to the doctor, I think I’ll get him to give me a test for Toxoplasma. Fifty million Americans have the parasite, so I wouldn’t be the first. And if I was carrying it around in my head, that might explain why it’s so fascinating to me.

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If you keep a vegetable garden, there’s a fair chance you’ll encounter a grisly sight this summer. Some poor caterpillar will be clutching a leaf, with the pupae of parasitic wasps sprouting off its back. It has just died in a most grotesque way. A wasp has zeroed in on the caterpillar and injected eggs into its body. The eggs hatched, and the larvae devoured their hosts from within, keeping it alive until they were ready to emerge.

What makes this sight all the more grotesque is the fact that the plant the caterpillar is sitting on may have been an accomplice to the crime. When caterpillars nibble on plants, the plants sometimes respond by releasing a distinctive cocktail of chemicals. This odor then attracts parasitic wasps. The plants are not just releasing a sort of chemical scream. Wasps are very precise in the species of caterpillars they choose, and they can tell these odors apart.

But the caterpillars are not entirely helpless in this struggle. After all, they are in a sense parasites as well, and like other parasites, they have evolved ways of evading the defenses of their hosts. In the journal Public Library of Science Biology, Japanese scientists demonstrate that caterpillars are eavesdropping on the signals plants are sending and shifting their behavior to make it less likely they’ll become food for wasps.

The scientists studied a species of moth (Mythimna separata) that eats corn. Previous research had shown that the catepillars were nocturnal, munching on corn at night and then slinking into leaves during the day. But the questioned remained what sort of cues the insects were responding to. Was it the rising and setting of the sun, or was it the scent released by injured corn plants–which they only release in the day?

The Japanese researchers set up an experiment to see what was driving the insects in and out of hiding. They put caterpillars into cups covered in nylon, and gave them a folded paper tent to hide in as they wished. The scientists then observed how the caterpillars responded to different combinations of signals. They exposed them to light and dark, to the scent of uninjured plants, and to the scent of plants that had been nibbled. If the caterpillars had a supply of artificial food, they didn’t respond much to light and dark on their own. The odors released by the corn plants had a much stronger effect, regardless of the lighting.

There’s a very good reason why a corn plant should release its distress calls during the day: that’s the only time when parasitic wasps are active. Manufacturing this fragrance at night would be a waste of effort. The caterpillars have responded, it appears, by evolving to hide when they smell the odor. The ones that respond this way are the ones that tend not to turn into inspiration for science fiction movies.

This new find is just the latest addition to a list of adaptations that have evolved in caterpillars against the threat of parasites. My all-time favorite is the way some caterpillars fire their droppings out of an anal cannon to avoid building up stinking piles that will give their location away. You can read about this hygienic howitzer here, in my book Parasite Rex, and in a recent scientific review here.

Update 5/16 9 am: Link to PLOS paper added. Plus, catepillars grow up to become caterpillars

At the Loom we believe that the path to wisdom runs through the Land of Gross.

We do not show you pictures of worms crawling out of frog noses merely to ruin your lunch. We do not urge you to check out these freaky videos of worms crawling out of frog mouths and fish gills merely to give you something to talk about at the high school cafeteria table tomorrow (Dude, you totally will not believe what I saw…) These images have something profound to say.

The worm in question is the gordian worm or horsehair worm, Paragordius tricuspidatus. It has become famous in recent months for its powers of manipulation. The gordian worm lives as an adult in the water, where they form orgiastic knots. They lay eggs at the edge of the water, which can only mature if they’re ingested by insects such as crickets. The worms feed on the inner juices of the crickets until they fill up the entire body cavity. In order to get back to the water, the gordian worms cause their hosts hurl themselves into ponds or streams. As the insects die, the worms slither out to find the nearest mating knot.

Gordian worms are not unique in their ability to manipulate hosts (see Parasite Rex for a heap of similar examples). But a paper in the current issue of Nature puts a very cool twist on the classic strategy of parasite mind control. It’s all very clever to send your insect host diving into the water, but there’s one very big flaw: it takes several minutes for the gordian worm to escape its host. The hapless host spends that time twitching and flailing in the water. In other words, it makes itself a perfect target for various predators. A Gordian worm that ends up as a frog’s lunch is a bit too clever for its own good.

French scientists have discovered that the gordian worm does not face certain death when its host gets eaten. They put infected crickets in front of frogs, bass, perch, and trout and watched what happened when the predator attacked its parasite-carrying prey. They watched 477 attacks. In many cases, the worm became part of the predator’s meal. But in a sizeable minority of case–18% for trout, 22% for perch, 26% for bass, and 35% for frogs–the worms escaped. After 8.6 minutes on average, the worm slipped out of the mouth or nose of the frogs, or the mouth or gills of the fish. In one particularly close shave, it took a worm 28 minutes to escape, as its predator repeatedly tried to swallow it again.

This past week has seen much discussion about how evolution struggles through seemingly impossible transitions–from fish walking toward land to receptors latching onto new hormones. I think the gordian worm earns a place in this hall of fame.

One of the most remarkable features of the parasitic world is the multi-host life cycle. Toxoplasma shuttled between cats and rats and other prey. Blood flukes reside in snails before infecting humans and causing schistosomiasis. Mosquitoes spread malaria to humans. In some cases, a parasite may have to live in three or four hosts before reaching maturity.

In their own creepy way, these life cycles pose an exciting challenge to biologists. How could they have evolved? These parasites must live in all their hosts in order to complete their life cycles. A tapeworm in a flour beetle must get into a bird to mature; otherwise it has reached an evolutionary dead end. It can be hard to conceive of how a complex life cycle could have evolved from a simpler one. If a parasite was adapted to live in a single host, finding itself accidentally inside another host ought to spell certain death.

The gordian worm shows why this is not so. Ending up in a frog or a fish is indeed fatal to most gordian worms. Like their hosts, they get digested in the acids of the predator’s stomach. But some gordian worms can survive. If they carry genes that help their chances of survival, natural selection may favor them. Ending up in an alien host is not always a dead end. Given enough time and enough evolution, it may even became an essential part of a parasite’s life cycle.

Hey…do I see something moving in your nose…?

Even I have my limits.

Loyal readers need no introduction to this bit of entomological “Faces of Death.” Others who think this must be some sort of hoax, read this (or this).

Courtesy of Dr. Fred Liebersat, oracle of the emerald cockroach wasp.

The Discovery Channel picked up my cockroach zombie story and interviewed Dr. Fred Liebersat on his research. They included some cool footage of the roach and its sinister wasp brain surgeon. To watch, go to their archives and scroll to “Roach-o-rama.”

Unfortunately, there’s no footage of young wasps poking their heads out of their cockroach hosts, but perhaps that was too hard to catch. Or maybe just too disturbing…