News from NIAID - November 1, 1993
National Institute of Allergy and Infectious Diseases, National Institutes of Health

"While such parasitic diseases as malaria, amebiasis and schistosomiasis particularly affect people living in tropical climates, the importance of other parasites in the United States is generally not appreciated," says Theodore Nash, M.D., of NIAID's Laboratory of Parasitic Diseases. "Not only do parasitic diseases affect thousands, if not millions, in the United States, they also disproportionally affect people with compromised immune systems, such as people with AIDS."

Dr. Nash will chair a session on parasitic diseases in the United States scheduled for Monday, Nov. 1, during the joint meeting of the American Society of Tropical Medicine and Hygiene and the American Society of Parasitologists in Atlanta, Ga. During the session, scientists will discuss the latest investigations of several parasites that affect people the world over: Cryptosporidium parvum, Giardia lamblia, Cyclospora, Trichomonas vaginalis, Microsporidium and Toxoplasma gondii.

These parasites are organisms that live inside or on the body of an animal host, receiving nourishment and reproducing while causing diarrhea, gastrointestinal upset, vaginal irritation, swelling of lymph glands, fever, adverse outcomes of pregnancy or damage to the brain or nervous system. These single-cell organisms or protozoa differ from bacteria and viruses and can undergo complex changes while in their host.

"Up to 40 percent of U.S. adults are infected with T. gondii and up to three million women have acquired sexually transmitted T. vaginalis," says Dr. Nash. "Many parasitic diseases such as giardiasis and cryptosporidiosis are not always reported to health authorities, so that we suspect that the extent and impact of parasitic diseases in the United States is underestimated."

Accurately defining U.S. populations affected by these diseases would greatly help in the development of strategies to prevent, control and treat parasitic infections not only in the United States, but in developing countries as well, he adds.

At NIAID laboratories in Bethesda, Md., Dr. Nash and his colleagues examine how parasites cause illness and the response of the body's immune system. These experiments will contribute to the development of vaccines. In addition, NIAID's International Collaborations in Infectious Disease Research support studies of organisms causing diarrheal disease in developing countries, where they are more common than in the United States. Such studies will lead to a better understanding of the disease process and to improved diagnostic tests, enhanced treatments and vaccines that will be useful worldwide. The centers are based at the University of Virginia in Charlottesville and Columbia University in New York. The Centers for Disease Control and Prevention's (CDC) National Centers for Infectious Diseases keeps track of the parasites that lead to diarrhea and other diseases in the United States.

In the United States, Dr. Nash notes, the most recent parasitic infection to garner public attention was the Milwaukee, Wisc., epidemic of diarrhea and related gastrointestinal disorders that involved more than 400,000 people this last spring, stemming from water tainted with the Cryptosporidium parvum parasite.

More frequent, but less in the public eye, are Giardia lamblia infections. This diarrhea- causing parasite is frequently associated with poor hygiene practices or contaminated water. The CDC estimates between 100,000 and one million U.S. cases of Giardia occur annually. Both Giardia and Cryptosporidia infections are common problems in daycare centers. Hikers and campers drinking from contaminated streams also frequently become infected with Giardia.

Not all parasitic infections routinely cause debilitating illnesses. For example, healthy people who become infected with T. gondii usually do not develop toxoplasmosis, which damages the brain and nervous system and sometimes kills. The disease frequently develops in those with weakened immune systems and affects 3 to 10 percent of AIDS patients. The disease also can occur among newborns if their mothers became infected during pregnancy. T. gondii naturally infects cats and can spread to humans if they handle dirt where cats have defecated, drink contaminated water or eat raw or undercooked meat.

People with AIDS or other immunocompromising conditions also are susceptible to Cryptosporidium infections. Cryptosporidiosis affects 3 to 15 percent of AIDS patients, who develop incurable diarrhea that can lead to death. Also, the parasite Microsporidium causes severe diarrhea in 20 to 25 percent of people with AIDS suffering from chronic diarrhea. Recently, scientists have identified another parasite, Cyclospora, that can bring on diarrhea in AIDS patients and healthy individuals.

NIAID, a component of the National Institutes of Health, also supports research on AIDS, tuberculosis and other infectious diseases as well as allergies and immunology. NIH and CDC are agencies of the U.S. Public Health Service, Department of Health and Human Services.