It Wants to Suck your Blood

10/9/2006

Science World

By Jeanna Bryner

Mark Siddall takes off his shoes and socks. Then he begins to slog through a rain forest in Madagascar. A steady rain pelts down on the leafy vegetation, malting for a slippery, humid hike. Siddall looks down at his feet: A handful of leeches have latched on to his bare skin.

At this point, most people would run home--but not Siddall. These blood-sucking worms are his passion, which is why he's in a rain forest in the first place. "Our best chances of finding leeches are when it's pouring rain and we're wandering through a wet jungle," explains Siddall, a scientist at the American Museum of Natural History in New York.

Most leeches are parasites, so they live and feed on another organism, such as turtles, fish, frogs--and humans. That makes leeches easy research subjects: "You can just wait for them to find you," Siddall says.

Leech hunting takes Siddall all over the world, from hippo-infested waters in Africa to Bangladesh, where Siddall will search for a leech that likes to feed inside nostrils. Yikes!

His unusual research is worth it, says Siddall. By studying leeches, he hopes to better understand the more than 600 leech species, including those that live in marine, freshwater, and land habitats. For instance, leeches could have medicinal value. That's because leeches have compounds in their saliva that keep their hosts' blood flowing. Doctors use these compounds to improve a patient's blood flow after surgery.

STUCK ON YOU

While in Madagascar, Siddall and his student assistants "got attacked like you wouldn't believe." Siddall says, "We started picking leeches off our ankles and legs."

So how do these annelids, or segmented worms, attack? Once a leech finds a feeding spot, it secures itself to the victim's skin with its rear sucker. To start feeding, some leeches pierce the skin with three teeth-lined jaws. Other leeches use a tiny flexible tube. After slicing into the skin, the leech attaches its oral sucker to the wound and begins pumping blood into its stomach. Normally, blood would clot to seal off the wound, so leeches inject an anti-coagulant to keep their blood meal in its liquid form.

In a single 30-minute sitting, leeches can slurp up five times their body weight in blood. Rather than prying a leech from his skin, Siddall politely waits for it to finish feeding. Once full, a leech releases its suction hold and falls to the ground--or into Siddall's collection bag.

LEECH LAB

Back at the lab, Siddall will look at each worm under a microscope and identify the species from its body markings.

For instance, the North American medicinal leech has orange-and-black polka dots. Also, he will study each leech's DNA. This genetic information will tell him how each species is related to others.

"There are so many types of leeches, and each is unique, so I just can't help but be fascinated by them," says Siddall.

* Leeches are classified as a type of worm. Do you know the name for a worm specialist? Answer: Vermiculturist.

* Most leeches are sanguivorous, meaning they feed on blood. They rely on anti-clotting chemicals in their saliva to help them receive a full meal. How do these chemicals help a leech eat?

* A leech digests its blood meal very slowly. This allows it to survive several months without eating. How much blood do you think a leech can eat per meal?

CRITICAL THINKING

* In ancient times, doctors used leeches for a therapy called bloodletting. They used the bloodsucking worms to drain blood from patients with varying illnesses, including headaches. Today, medicinal leeches are malting a comeback. Some doctors use leeches to keep blood from clotting during surgeries.

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