Parasitic Invaders and the Reluctant Human Host

by Food and Drug Administration (FDA)

Thousands of Milwaukee area residents got an unwanted crash course last April in cryptosporidiosis, a disease most had never heard of until they — or friends or family — contracted it. The culprit was a parasite, Cryptosporidium, that had invaded the city's drinking water supply, causing people to become sick with diarrhea and other intestinal symptoms; several died.

Cryptosporidium lives in the intestines of cattle and other animals and is excreted in feces. Health officials suspect the water supply became contaminated from a high level of runoff into Lake Michigan from area dairy farms or slaughterhouses near the water plant's intake pipe. An inadequate filtration system allowed the parasites entry into the water supply.

Cryptosporidiosis is just one of several diseases caused by parasites, which are largely unfamiliar to Americans. But in much of the rest of the world, they are all too well-known. These tiny ravagers — many no larger than a single cell — claim the health and lives of millions of people around the globe. Parasites live in or on another organism, known as the host, from which they receive nourishment and protection. Some pass successive stages of maturity in hosts of different species, including humans. Parasites are of different types, including protozoa (one-celled animals) and helminths (worms) ranging in size from microscopic eggs to adults up to several feet long. The illnesses they cause range from mild discomfort of short duration to chronic, debilitating disease and death. People who live in areas where the disease is endemic (constantly present) suffer the devastation most keenly. Hardest hit are developing countries in the tropics, where poor sanitation fosters the parasites and the insects that transfer many of them from one host to another. A major killer among the parasitic diseases, and perhaps the one best known to Americans, is malaria. It is caused by the protozoa Plasmodium, transferred to humans by the bite of the Anopheles mosquito. Although malaria is not a significant health problem in the United States, more than 1 billion people worldwide live in areas where the disease is endemic, and between 125 million and 200 million people are infected at any given time. Each year in Africa alone, malaria claims the lives of 1 million children.

Millions more adults and children in Africa, South and Central America, Asia, and parts of Europe suffer from other devastating parasitic diseases as well. For example, African sleeping sickness, caused by the protozoan Trypanosoma brucei, is one of the most lethal of all human diseases. It produces fever, enlarged lymph glands, skin lesions, and painful swelling. Neurological symptoms, including tremors, headache, apathy, and convulsions, predominate later in the disease, which can end in coma and death. Schistosomiasis, a helminthic disease affecting approximately 200 million people between the tropics of Cancer and Capricorn, can produce bladder, intestinal or liver disease that may lead to death. Onchocerciasis (river blindness), found in Mexico, South and Central America, and Africa, results from infection with larvae of the Onchocerca volvulus worm, transmitted by flies that breed along fast-moving streams. It causes a skin rash, often with severe and constant itching. Eye lesions lead to blindness in about 5 percent of people infected. "It is true that parasitic diseases are a much greater problem in other parts of the world," says George Jackson, Ph.D., a microbiologist in FDA's Center for Food Safety and Applied Nutrition, "but they seem to be on the increase in industrialized temperate zone countries, and there are even certain parasitic infections among Native Alaskans."

Travelers Play Host

The rising incidence of parasitic diseases in the United States is due in part to increasing international travel.

Approximately 8 million Americans travel to the developing world annually, according to the national Centers for Disease Control and Prevention, and the speed of jet transport permits travelers to return home within the incubation period of every infectious disease. As a result, the agency says, an increasing number of parasitic infections are being diagnosed in business travelers and tourists and are causing considerable disease and occasional death.

One of these diseases is malaria. Plasmodium parasites infect red blood cells, causing a spiking fever, possible immune problems, damage to internal organs, and, potentially, death. Certain species of the parasite may lie dormant in the organs for years, until something — perhaps another infection — triggers the disease.

Although once endemic in the U.S. Southeast, malaria was declared eradicated in this country in the 1940s; the last case originating here was reported to CDC in 1957. Yet from 1969 to 1980, the number of cases in civilians reported to that agency rose from 151 to 1,864, and since 1980, the number of cases in travelers has averaged 1,000 annually. Compounding the problem is the fact that malaria has become resistant to the drugs taken to prevent contracting the disease. Once brought in, local transmission is rare, but does occur, since the Anopheles mosquito does exist in this country. Other previously rare and potentially fatal parasitic infections, such as leishmaniasis (which may affect the skin, mucous membranes, or internal organs), schistosomiasis, and onchocerciasis, are also increasing among returning U.S. travelers.

While those particular parasitic diseases are still uncommon in the United States, others are seen here much more often, especially in specific groups of people.

Parasitic Disease and Weakened Immunity

"There are certain parasites we're all exposed to that don't cause us much trouble unless we're particularly vulnerable to them," explains Randolph Wykoff, M.D. "The protozoa that cause pneumocystis pneumonia and toxoplasmosis can be fairly common in the population. Most people have been exposed to them with limited illness, if any."

Wykoff, who is a specialist in tropical medicine and heads FDA's Office of AIDS Coordination, explains that little more than a decade ago these two diseases were seen infrequently — almost exclusively in people with immune systems weakened by cancer chemotherapy, or in malnourished, chronically ill, and pre-term infants.

That changed with the appearance and spread of HIV (human immune deficiency virus) infection. Immune suppression is the hallmark of this infection, which leads to AIDS. HIV-infected patients are vulnerable to many opportunistic infections (infections that would not cause illness in someone with a healthy immune system), including those caused by parasites. Pneumocystis pneumonia, toxoplasmosis, and cryptosporidiosis are responsible for much of the illness and death suffered by people with AIDS.

About the Author

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