SAN FRANCISCODifferent intestinal parasites frequently cause clinically identical diarrheal illnesses, so doctors must identify the pathogen to determine the most appropriate treatment or even if treatment is warranted, according to a pediatric infectious disease specialist.
-- Oocysts of Cryptosporidium (in red) as revealed by acid-fast staining. (Courtesy Wanda M. Wenman, MD)
The specialist, Wanda M. Wenman, MD, urged physicians to be aware of the microscopy techniques that laboratories use to identify parasites. The methods laboratories use to stain microbes before examination are particularly important. For example, laboratories that do not test stool samples using acid-fast stains may fail to identify the protozoan parasites Cyclospora and Cryptosporidium, said Wenman, of the University of California, Davis, where she is Professor of Pediatrics and Director of the Pediatric Infectious Diseases Division. Not distinguishing the two microbes could have therapeutic consequences. Cyclospora infection responds well to drug therapy, whereas Cryptosporidium infection does not.
Wenman spoke about the diagnosis and treatment of protozoan diarrheal illness at the recent annual meeting here of the American Academy of Pediatrics.
Cyclospora organisms, which until recently were considered blue-green algae, were discovered at the turn of the century. The first human cases of Cyclospora infection were reported in the 1970s, Wenman said. In the early 1980s, Cyclospora was recognized as a pathogen in patients with AIDS. It is known now that Cyclospora is endemic in many parts of the world, and it appears to be an important cause of traveler's diarrhea. Cyclospora are two to three times larger than Cryptosporidium, but otherwise have similar features. Cyclospora diarrheal illness in patients with healthy immune systems can be cured with a week of therapy with trimethoprim-sulfamethoxazole (TMP-SMX). People who are immunosuppressed may require longer courses of TMP-SMX therapy.
In people with healthy immune systems, cryptosporidial diarrhea resolves on its own, Wenman said. In people who are immunosuppressed, such as those with HIV infection who have CD4 counts of fewer than 150 cells/mm3, Cryptosporidium can establish persistent infection and cause chronic diarrhea that leads to dehydration, wasting, and death. The best agent available for treating cryptosporidial infection is paromomycin, which causes a moderate decrease in Cryptosporidium oocysts the parasite's reproductive formand a decrease in stool frequency and stool weight. Posttreatment relapses are common. Some researchers have been experimenting with a combination of paromomycin and macrolides.
The first human cases of cryptosporidial diarrhea were not reported until the mid-1970s. At the time, people thought of cryptosporidial infections as exotic. Between 1976 and 1982, fewer than a dozen cases of cryptosporidial diarrhea were reported in the literature, she said. They occurred mostly in immunocompromised patients, such as in children with leukemia.
During the 1980s, cryptosporidial diarrhea emerged as a major problem in people with AIDS. Water-borne and food-borne outbreaks of diarrhea due to Cryptosporidium in recent years have shown that healthy people can contract cryptosporidial diarrhea and that Cryptosporidium is an important parasitic cause of diarrhea worldwide. The largest documented outbreak occurred in the Milwaukee area in 1993. In that outbreak, which investigators traced to the municipal water system, more than 400,000 people contracted cryptosporidial diarrhea.
Wenman also discussed Giardia. She noted that this protozoan can be transmitted by food and water, but children most commonly acquire the organism by the fecal-oral route. As many as 50% of children in day care carry Giardia. Giardia cysts are extremely resistant to chlorine, so they are found in municipal drinking water. A small number of organisms is required to establish infection, she said.
Despite the availability of antigen tests for Giardia, diagnosis is based primarily on microscopic examination of stool specimens, she said, and infection with Giardia is treatable. A 5 to 7-day course of metronidazole, the drug she uses most frequently to treat giardiasis, is effective in 90% of cases, Wenman said. "It's a drug with a very long and safe track record in all ages, including children."
Quinacrine is more effective than metronidazole, but it is more toxic and tastes "horrible," she said. Also, quinacrine has not been produced in the United States since 1992, and it is going to become increasingly difficult to obtain, she said. Another agent, furazolidone, is the least effective, and it must be given longerusually 7 to 10 days. Furazolidone is available in a suspension, so it may be the drug of choice for children, especially those younger than 5 years, Wenman said.
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