Next time you count your blessings, consider the humble sewer. Without a sewage system to whisk away fecal matter, you'd be home to even more human parasites than you already are, according to biologist Roger Knutson.

Yes, note the "already are."

Although few of us like to think about it, most people are host to at least one critter (not always microscopic, either) that swims around inside us, drinks our blood, eats our cells or reproduces with abandon.

"The world is filled with parasites that are more than happy to call us home," said Knutson, the scientist-turned-author of "Flattened Fauna," the now-classic field guide to identifying road kill.

On the squeamish scale, Knutson's newest literary effort, "Fearsome Fauna: A Field Guide to the Creatures that Live In You," manages to trump even flattened animals. An overview of human parasites both good and bad, the recently released book details the worms, protozoans, flatworms and nematodes that find humans more hospitable than anyone would care to think.

"We provide a multitude of potential habitats, some with abundant oxygen and some without, some acidic and some alkaline, some large and some spacious and some that have to be squirmed through," Knutson said in an interview. "We are a wondrous continent to our tiniest residents" -- or, as Knutson likes to call them, our "internal companions."

If the retired Luther College professor seems downright enthusiastic about human parasites, well, he is. For the rest of us, the term may conjure up nightmarish images of 12-foot worms burrowing through tissue. But Knutson sees it differently.

From his biologist's perspective, human parasites are intriguing creatures, and the reasons are many. For one, they are among the most abundant forms of life on earth; there are more kinds of parasites than insects.

Human parasites are also ubiquitous. They're found everywhere and live in almost every major organ of the body -- including the eyes, brain, blood and, most commonly, the murky backwaters of the gastrointestinal system. Perhaps most important, most manage to live with their hosts for years without harming them, Knutson said. Considering the damage that humans wreak upon the Earth, parasites are distinguished by a skill and courtesy that we should be so lucky to emulate.

All too common?  

One human parasite that Knutson admires in particular is the fish tapeworm -- so named because humans get it from handling fresh fish, eating undercooked fresh fish or accidentally ingesting a microscopic organism containing the young tapeworm (such as when swallowing a mouthful of lake water).

The fish tapeworm is among the largest of its kind and one of the most common parasites among humans, in whose guts it can live for years.

How large is large? Anywhere from 3 to 12 yards long. "If a large one were dangling outside a fourth-floor window, it would dangle all the way to the ground," Knutson said.

How common is common? Minnesotans may not want to know. In the north-central part of the United States, or in areas where freshwater fish is a frequent part of the diet, up to 100 percent of the human population may harbor a fish tapeworm.

"People get some interesting looks on their faces when I tell them that," said Knutson, who has lived in northern Michigan since retiring from Luther College, in Decorah, Iowa. "Sometimes I think I shouldn't tell them."

Despite their size, fish tapeworms usually aren't a health problem, said Knutson. Instead, they could be considered a poster parasite for long-term, harmonious relationships.

Unlike other parasites, which can wander around the body or overwhelm it, the fish tapeworm stays put once it's attached to the intestines. There, it lives on glucose, which is something our bodies derive from the food we eat and which most of us have plenty of.

Although the adult tapeworm produces billions of eggs, none of them hatch inside the body. Instead, they're ejected through the same exit taken by other unused solids.

Why this happens is uncertain, according to Knutson. Scientists believe that chemicals produced by the adult tapeworm inhibit embryo development. Doing so is in the best interest of humans and, therefore, of the tapeworm. Without this adaptive mechanism, the human host would be "tapeworms from the neck down," said Knutson, and the tapeworm would be crowded out of its home, which itself would have likely deteriorated and become less hospitable.

Because fish tapeworms are so common in our part of the country, Knutson advises Minnesotans to think of fish tapeworms as a kind of pet. Like a dog or cat, a fish tapeworm lives quietly and requires few resources to sustain itself.

Still, this is not a pet that we'd actively choose to own. Although it's always best to avoid uncooked fish of any type, "should you slip up once or twice and be unlucky enough to acquire a fish tapeworm, you will probably live out your life comfortably and never know it's there," Knutson said. "With a healthy diet and sufficient vitamins, a fish tapeworm is just a theoretical problem."

Learning opportunity  

The same, however, can't be said for all human parasites. Some, such as the mosquito-borne parasite that causes malaria, can be deadly. One parasite carried by an African fly can blind people in days. Others, such as hookworms in sufficient numbers, can suck enough blood out of its host to cause anemia.

Fortunately, we can "be grateful for public health measures," Knutson said. Sewage systems, which carry away parasites' favorite mode of transportation for eggs, as well as other sanitation strategies, have minimized the problems that these parasites cause humans simply because it becomes less likely we'll come in contact with them. However, potentially harmful parasites remain a part of everyday life in less developed countries.

But even the so-called "bad" parasites might actually help humans indirectly, Knutson says. The reason? Understanding how they live, act and evolve could help unlock medically useful secrets of nature.

Consider the guinea worm, which is acquired by drinking infected water in tropical parts of Asia and Africa. Scientists have discovered that the worm takes molecule-sized samples of human tissue from the areas of the host that it's traveling through. The tissue samples are then attached to its body surface. Because the host's immune system only recognizes the human body's own cells that are on the worm, it isn't recognized as an intruder and therefore is not attacked and destroyed.

Learning more about this phenomenon could have numerous medical implications, Knutson said. That's particularly true for organ transplants, which now force patients to remain on antirejection drugs for a lifetime. If the body was tricked into recognizing the new organs as its own, patients wouldn't need to take such drugs, he suggested.

Thus parasites are often the classic example of things not being what they seem -- or, if you will, of turning a lemon into lemonade.

"Parasites have an undeserved bad reputation," he said. "In reality, they're the greatest biological success story in the world," he said, and they can offer a great deal to the advancement of human knowledge and well-being.

Provided, of course, we don't get so grossed out that we fail to give these critters their proper respect.




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