Warning: Yucky parasites are closer than you think

The high school student's lung disease was baffling. Previously healthy, the patient had become progressively short of breath over weeks. Her airways filled with so many immune cells that her doctors thought she had leukemia. Admitted to a Boston hospital, she underwent exhaustive tests, including various scans, biopsies, and blood tests.

After days, one test unexpectedly returned positive -- and the diagnosis was shocking. The teenager had a parasite called toxocara, a tiny worm that normally infects dogs. But, in one study of urban New York children whose blood was submitted for routine lead testing, about one in 20 had been infected by toxocara, though they showed no symptoms.

On a dare, the patient had eaten an earthworm several weeks earlier. Since up to a quarter of city soil samples contain toxocara eggs -- presumably from dog feces -- some eggs entered the patient's intestine and hatched. From there, numerous tiny larvae migrated into the bloodstream, and hitched rides to the girl's lungs. That's why she was short of breath: Her lungs were packed with microscopic worms.

Americans rarely worry about parasites such as worms and protozoa. But they're actually quite common, and infect people through extraordinarily devious means.

In the early 1900s, one in five Staten Island residents had malaria, one-quarter of pork sold in the United States was contaminated by parasites, and many Southern Americans labeled lazy and stupid were, in fact, seriously anemic due to hookworm infections.

Cleaner drinking water, meat inspection, and public sewer services have had a major beneficial impact on pediatric parasitic disease. But, in some rural American areas, almost one in four children still carries parasitic worms in their intestines, according to epidemiological studies.

Worldwide, parasites inflict tremendous suffering on children, causing anemia, poor growth, and numerous illnesses. In 1999, a report in the New England Journal of Medicine argued that most immigrants entering the United States from non-European countries should be "dewormed" with the drug, albendazole, to prevent them from getting sick and possibly from spreading the worms. More than a billion people worldwide, for example, carry hookworms in their guts, and the inchlong parasites bite into intestines and slowly suck blood to cause anemia. People catch it by walking in contaminated soil; the worm burrows through the skin and finds its way to the gut.

What's especially chilling is how manipulative some parasites can be. For example, pregnant women are advised never to change cat litter -- but few realize the reason is to avoid a mind-controlling parasite called toxoplasma sometimes found in rats. Normally rats instinctively run from cat urine, since cats are their predators. But like "Manchurian Candidate"-style mind-control devices, toxoplasmosis actually makes rats lose their fear of cat urine, thereby increasing the parasite's chance of getting into a cat's intestines, the only place it can reproduce. An infected cat excretes 10 million toxoplasma eggs daily.

If infected via cat feces, a pregnant woman fights off toxoplasmosis easily. But the fetus doesn't. The parasite crosses the placenta and enters the unborn child's brain and eyes, causing irreversible damage that can result in mental retardation and growth failure. (About 10 percent of raw beef also contains toxoplasma.)

Another example occurs in some Latin American countries, where almost half of all children with seizures are infected with the Taenia worm from contaminated pork. In these children, the worms burrow from the intestines into the bloodstream, and ultimately into the brain, causing chronic seizures.

Remarkably, Taenia found ways to infect even Americans whose religion forbids them from eating pork. In 1990, an epidemic of Taenia occurred in an Orthodox Jewish community in the Northeast. An investigation by the US Centers for Disease Control revealed that each affected person employed illegal Latin American immigrants for domestic assistance, and these workers had intestinal larvae that laid eggs, which passed in the stool. Because the workers practiced poor hand-washing, their hands were contaminated with these eggs, which found their way into their employers' foods and then their brains.

Parents and public health authorities can take steps to protect kids from parasites. Sharing bedclothes or underwear can spread pinworms, a common cause of anal itching in children; these practices should be avoided.

Giardia, the most common diarrhea-causing parasite in the United States, has been linked to numerous outbreaks from contaminated water. The parasite often disables the gut's ability to absorb nutrients, leading to malnutrition and weight loss. Because the parasite's eggs can survive for months in contaminated areas, parents using well or other unpurified water should boil it for a few minutes before drinking. Giardia outbreaks also arise from inadequately chlorinated swimming pools visited by infected children, since a single child produces enough cysts theoretically to infect millions of others.

The shrewdest way to fight parasites is to attack the organisms that harbor them. To fight Taenia, the seizure-causing worm, Peruvian researchers treat infected pigs with oxfendazole, a drug that kills the worm before it infect humans. Spraying DDT to kill malaria-carrying mosquitoes has saved tens of millions of African lives.

Not all parasite infections are deadly. But in "Parasite Rex," Carl Zimmer writes, "Parasites like hookworm [make] it hard for children to learn in school; all it takes is a dose of [antiworm] medicine to make some slow children bright again." This toll of parasites, from rural America to China, robs many children of their full potential -- but the cure costs only a few dollars.

Many of the nation's pharmaceutical companies have risen to the occasion, donating tens of millions of doses of antiparasitic drugs, like ivermectin, to developing countries. This drug treats filaria, the worms causing "elephantiasis," where a patient's limbs can swell to several feet in diameter.

The high school student seen at our hospital probably was one of the lucky ones. Her doctors made the right diagnosis, and she was able to get the most sophisticated antiparasitic drugs available. She will probably make a full recovery. Perhaps one day all children will be so fortunate.

Dr. Darshak Sanghavi is a clinical fellow at Children's Hospital and Harvard Medical School. His e-mail address is 

Copyright 2004 The New York Times Company