Sex and the single schistosome: once thought to pair for life, infective flatworms may lose their mates in battle

Natural History, June, 2008 by Patrick J. Skelly

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CALL ME NAIVE, BUT I WAS A LITTLE surprised that the trip to the ancient temple of the pharaohs in Luxor, Egypt, did not require a couple of days' ride into the desert on a camel. I had visions of heat and dust and sandstorms, with the temple emerging like a mirage, magnificent in the distance. Nothing like it: the temple (magnificent indeed) sits in downtown Luxor, not far from the post office and the train station. A little farther along the road, keeping the Nile River on your left, you will find the great temple of the god Amun at Karnak. And across the river from Luxor on the west bank lies the Valley of the Kings, where, from about 1500 to 1000 B.C., Egyptians buried their pharaohs.

It was a tourist treat. But I was in Egypt for something the guidebooks warn you against--parasitic flatworms. (I was there to meet with an Egyptian pharmaceutical company to discuss development of a vaccine to ward off the worms.) Parasitologists have long known that Egyptians, ancient and modern, pharaohs and commoners, have been engaged in an ongoing battle with small, primitive worms called schistosomes, which inhabit tropical freshwater such as the Nile. Although it may look inviting on a hot day, such water often harbors tiny, tadpole-like forms that can latch onto your skin, burrow inside, and stake a claim. Of the twenty-one species of Schistosoma currently recognized, five infect humans. Livestock and wildlife also contract disease from such worms. For instance, Schistosoma japonicum can infect humans, cattle, and other mammals; S. margrebowiei and S. leiperi infect antelope, and S. leiperi has been found in zebras as well; S. edwardiense and the imaginatively named S. hippopotami both infect the hippopotamus.

Many millions of Egyptians are infected today with schistosomes. In their time, the pharaohs too were infected. Schistosome eggs have been detected in royal mummies thousands of years old. In addition, X-ray examination of mummies has revealed the pathological calcifications typical of schistosome infection, and worm proteins have been identified in rehydrated ancient tissue. If they have prevailed across time, schistosomes have also been undaunted by space: they are endemic in rural and suburban areas of seventy-four countries in Africa, Asia, and Latin America (where they first arrived in the bodies of slaves). Globally, about one in thirty people has them living in his or her bloodstream right now.

ONCE THEY GET INSIDE YOU, schistosomes are remarkably persistent. A German medical journal, Deutsche Medizinische Wochenschrift, had a report in its May 2005 issue entitled, "Surprising findings in the colon 15 years after a holiday in Africa." The patient, a sixty-five-year-old woman who went to the hospital with abdominal discomfort learned she had schistosome eggs in her intestine. She had picked up schistosomes at some point during two stays in Mwanza, Tanzania, fifteen and seventeen years earlier--and the same worms had remained in her body ever since. As most everyone does, she likely became infected by venturing into contaminated water.

Schistosomes start off as eggs--expelled into water by their human hosts via urine or feces--and hatch into free-swimming microscopic miracidia, which penetrate the tissues of certain freshwater snails and replicate there, developing into microscopic forms called cercariae that are capable of infecting a new human host. The cercariae have powerful thrashing tails, but no teeth or hooks with which to latch on to human skin [see top photograph on opposite page]. Instead, they more or less throw up their sticky secretions onto your skin. That vomit--the beachhead that allows for infection--makes it difficult to wash them off and gives the parasites time to probe your surface, searching for the edges where the scales of your skin overlap. In fact, since their mucus swells in water, this probably lifts some of the scales to allow the parasites to gain purchase and squeeze underneath your skin. Released enzymes help degrade the barrier, and within a few minutes they are in, beneath your epidermis. Mission accomplished.

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Once inside the subdermal tissues, the parasites jettison their tails and further outfit themselves for a new environment. No longer in fresh water, they alter their entire physiology and biochemistry: they shed their old surface coats and synthesize new ones, slipping into something more comfortable, more suited to the indoors. After a few days .just under the skin, the baby invaders push on, aiming to hit a blood vessel that will carry them to the lungs and, after several days of further maturation there, to the blood vessels of the liver, where males and females mature and mate. It is here that our drama unfolds.

SCHISTOSOMES ARE CLASSIFIED with the flatworms, Platyhelminthes, which are said to be the most primitive of all of the organisms that have heads. (The platyhelminth cheer: "We may be primitive, but we have heads!") Most of those flatworms are hermaphroditic, but not schistosomes--they have separate sexes. Individual male and female worms must locate each other within their host's circulatory system in order for the parasites to continue their life cycle. The adult male and female schistosomes are not only sexually distinct but visibly different-looking. Males are bulky and Schwarzeneggerian, weighing three or four times as much as the longer, slimmer females.