Science Made Cool

Friday Parasite: Beautiful Nematodes

Nikon announced the winners of their annual Small World photomicroscopy contest yesterday, and slide shows of the top 20 images are bouncing around the blogosphere. But buried deep in the contest's "Images of Distinction" section is J. Claire Hoving's beautiful picture of a female Nippostrongylus brasiliensis – a nematode worm found in the intestines of rats. Her body wall glows blue; her body cavity is stuffed with eggs that may, in time, find a new rat host to cozy up to.

Photo by J. Claire Hoving, Institute of Infectious Disease and Molecular Medicine, University Of Cape Town, South Africa
Section of female Nippostrongylus brasiliensis (nematode) with eggs (200x)
 

Friday Parasite: Biting Bats

Look carefully. Don’t let the flat body and lack of wings fool you – it’s not a spider. It’s a bat fly.

Bat flies don’t look much like horse flies or house flies, but they’re part of the insect order Diptera, just like their more familiar (if distant) cousins. But instead of scavenging garbage, these flies eat just one thing: bat blood. And they’ve evolved a specialized body shape that helps them cling to the fur or wing membranes of their hosts.

But living on a host that eats insects is dangerous when you're a fly: bats aren't at all squeamish about munching the bat flies they find when they're grooming their fur. Adult bat flies can scuttle quickly out of the way of an approaching tongue, but baby flies (ie: maggots) aren't exactly known for being nimble. Enter one of the strangest things about these flies: they're viviparous. A female bat fly has only one maggot at a time, and it grows inside her body until it's ready to pupate. At that point, the female deposits the larvae onto a wall near the bats' roosting area. The larvae immediately pupates, and stays stuck to the wall while the fly metamorphoses. After the adult fly emerges it crawls off to find another bat, and a lifetime of meals.

References: Dick, C.W. & B.D. Patterson. 2006. Bat flies - obligate ectoparasites of bats. Pp. 179-194 In Micromammals and macroparasites: from evolutionary ecology to management (S. Morand, B. Krasnov, and R. Poulin, eds.). Springer-Verlag, Tokyo

Katharina Dittmar de la Cruz (SUNY Buffalo) is leading a comprehensive study of these insects: a blog highlighting her research is in development.

Photo by Carl W. Dick.

Friday Parasite: Unsanitary, But Effective

(Third in a series on emergent parasitic diseases in the US).

Another disturbing bit of the Wall Street Journal article that inspired this series was the report that people are turning up in US doctors’ offices with cases of Chagas disease. To be fair, in many cases they contracted it in the South American countries where it’s more common. But it’s a nasty, sneaky, medical condition -- a carrier can feel fine for years until his inflamed, damaged heart fails. And treatment is tricky, thanks to the nature of the infection: Chagas disease is caused by a single-celled protozoan parasite that can hide inside human cells.

The protozoan in question, Trypanosoma cruzi, is a member of the same genus as the protists that cause sleeping sickness in Africa. Like them, T. cruzi depends on an insect vector to move from one host to another. But its insect vector isn’t a fly or a mosquito – it’s a big bloodsucking bug. 

Friday Parasite: Why My Mother Always Overcooked Pork Chops

(second in a series on parasites that cause human disease that are becoming more common in the United States)

Before I met my New Orleans bred, meat-braising husband, I thought that cooked pork was naturally dry and flaky. After we started dating, I learned that my mother had just been overcooking it for years (sorry, Mom). My mother wasn’t really a bad cook -- she was just worried about accidentally infecting us kids with pork tapeworms. And now that I know more about this parasite, I can hardly blame her for trying to cook the tref out of those chops.

Unlike the parasitic roundworms featured last week where a human infection is a reproductive dead end for the worms, the tapeworm Taenia solium is a human parasite. Adult worms live in human intestines, where they use the hooks on their head to attach to the intestinal wall and shed egg-filled segments of their bodies, which travel to the outside world in feces.

If an unsuspecting pig is rooting around the privy, it might swallow a few thousand tapeworm eggs. Inside the pig, the worm larvae drill through the intestinal wall into the pig’s bloodstream, then get swept to muscle or nerve tissue where they can encyst themselves and wait for the slaughter. After a person eats the infected meat, the immature worms break out of their cysts to mature in a new intestine.

Despite their alarming size (they can grow to be seven meters long), a mature worm in a human intestine causes little more than a bit of indigestion or constipation. The real human health problems start when the immature form gets in a person instead of its proper porcine intermediate host. Because worms in this stage are going to encyst themselves  – and the cysts can interfere with the normal function of the human body. Cysts in muscles can cause swelling and fever. Cysts in the eyes can cause blindness. And cysts in the brain can cause epilepsy or strokes.

Prevention? Simple public health measures like keeping sewage away from people and food animals can break the transmission chain. High heat kills worms that are encycted in meat, so cook that ham thoroughly. And as always, wash, wash, wash those hands.

Image: immature Taenia solium encysted in a human brain. (Centers for Disease Control and Preventionl)

Friday Parasite: Emerging Infections

Last Saturday the Wall Street Journal reported that a growing number of people in the United States are showing up in doctors’ offices with parasitic infections. They’re not infected with relatively harmless parasites like pinworm -- they’re carrying potentially debilitating species that were once found mainly in tropical countries.  Some people are bringing hitchhikers home from their Costa Rican vacation, but the vast majority of infections are in people who are poor and living in either the South or the mountains of Appalachia. And that means these parasites aren’t accidental visitors to our shores – they’re living among us now.

Now, I can’t solve the problems of poverty or health care access on a blog. But the article mentioned that medical schools often give short shrift to these parasites, so many doctors don’t even know they exist. That, I can do something about. For the next three weeks our Friday Parasite will focus on the parasites discussed in the WSJ article, describing their life cycles, how they manage to get inside people, and what they can do once they’re there.

First up: Toxocara canis – a parasitic roundworm that doesn’t even want to be in you. As the name suggests, the worms are normally found inside of dogs, where they live in a kind of arrested development because they can only grow to maturity inside very young puppies.

Attack of the Tomato Killers

Normally at this time of year, we’re buried in tomatoes. Pyramids of them sit on our kitchen counter ¬– literal piles of tomatoes from our CSA share. We try to keep up by putting them into nearly every meal: sliced in salads and BLTs, pureed into gazpacho, or simply salted with oil and basil. But this year, there’s nothing but dead and dying vines in the fields. We’re living through the Great New England Tomato Famine.

OK, I’m being flip. But the parasite that’s attacking the tomatoes is a strain of the same fungus that caused the Irish potato famine 150 years ago. Phytophthora infestans, or “late blight” is native to central Mexico, where it infests many different members of the genus Solarium – a group that includes not just tomatoes and potatoes, but also nightshade and bittersweet. The fungus doesn’t survive well in our typical hot, dry summers, but this July was wet and cool, creating the perfect conditions for it to spread. When spores from mature P. infestans land in wet fields, they open and release zoospores. The zoospores are tiny, but their paired flagella help them swim over the moist leaves and drill into them. Once inside the plant, P. infestans acts like any other fungus, growing a network of threadlike structures called hyphae to digest the surrounding plant tissue. And as soon as the fungus is mature, it sends hyphae out of the leaves to release more spores into the wind.

Photo: P. infestans infects a leaf. (Cover image: Grenville-Briggs, L. J. et al. 2008. Cellulose Synthesis in Phytophthora infestans Is Required for Normal Appressorium Formation and Successful Infection of Potato. Plant Cell 20: 720-738.)

Parasites and the Red Queen's Race

The Red Queen is a character in Lewis Carroll's Through the Looking-Glass, who has to run as fast as she can just to stay in one place. Biologists use the term "Red Queen's Race" to refer to the process by which species must evolve and adapt just to cope with day-to-day threats which are evolving equally rapidly to overcome their countermeasures.

One of the most common sorts of Red Queen's Race is the battle between a host organism and its parasites. The host evolves new ways to repel or destroy the parasites, while the parasites come up with ways to evade the host's immune system. Of course, if the host devotes too much of its resources to stopping parasites, it becomes uncompetitive in other ways -- to change analogies, the host is like a country. Does it spend money coming up with weapons to fight its enemies, or spend it on economic development?

Here's a great example of one such race in action. Snails in New Zealand have to fight off parasites, which means they need to evolve as fast as the parasites do, even though the parasites probably have a faster reproductive cycle than the host snails do. But the snails can make use of a powerful ally:  SEX. (I did that to help our Google search results.)

Sexual reproduction is a great way to jazz up your genetic diversity, helping you evolve better. The snails which are afflicted with parasites are more interested in finding sex partners so they can perform sexual reproduction -- and thereby give their offspring a marginally better chance of fighting the parasites. Snails with no parasites aren't interested in the dating game, and reproduce asexually.

I don't know if this would help humans looking to attract a partner, but it might be worth a shot:  "Hey, sweetie! My genes may help your offspring avoid deadly parasites! Wanna go back to my place and watch Firefly reruns?" Try it and see!

Friday Parasite: Deadly Divisions

A Trypanosoma brucei cell divides in the bloodstream of its human host. (The two flagellae on the left show that this particular protist was caught in the middle of mitosis.) If untreated, the host will develop trypanosomiasis -- West African sleeping sickness.

SEM by the Gull Lab, Sir William Dunn School of Pathology, courtesy of the wonderful Wellcome Images collection.

Friday Parasite: Heads will Roll

A couple of years ago, I wrote about the life history of ant-decapitating phorid flies and how they might help control the fire ants that are invading the American South. This week, a wave of excitement has washed over the internet about the “zombie ants” these flies make, fueled by an announcement from Texas A&M University that they’ve released fire ants infected with a new South American species of phorid fly on a plot of land in east Texas. Despite the way it sounds, the fly isn’t ‘new’ in the sense of being “developed in a lab to take out ants;” it’s just ‘new’ to being part of these field experiments. In fact, evolution already did the job of turning these flies into fire ant scourges: the fly is a parasitoid from the ants’ original Argentinean home. Because the fly only parasitizes the imported fire ants it can’t run amok over our native ant species. And because this fly dive-bombs foraging ants  (some great photos of a related species of fly in action can be found over at Photo Synthesis), the hope is that they’ll disrupt the normal growth  of fire ant colonies – not just by killing workers in the course of making more flies, but by making the ants drop their food and run for their lives.

Photo by Sanford Porter (via USDA).

More on the project at:
The Texas Imported Fire Ant Research and Management Project
The University of Texas at Austin Fire Ant Project

150-Year-Old Friday Parasites From Japan

At the delightfully cephalopodian Pink Tentacle Web log, there's a wonderful post about a Japanese work from the 1860s called Kenbikyu Mushi No Zu, which features illustrations of insects as seen through an imported microscope. Think of it as Hooke's Micrographia, but in a kimono.