Intestinal parasitism in the United States: update on a continuing problem.

To document patterns of intestinal parasitism in the United States, we analyzed results of 216,275 stool specimens examined by the state diagnostic laboratories in 1987; parasites were found in 20.0%. Percentages were highest for protozoans: Giardia lamblia (7.2%), Entamoeba coli and Endolimax nana (4.2% each), Blastocystis hominis (2.6%), and Entamoeba histolytica (0.9%). The most commonly identified helminths were nematodes: hookworm (1.5%), Trichuris trichiura (1.2%), and Ascaris lumbricoides (0.8%). Identifications of G. lamblia increased broadly from the 4.0% average found in 1979, with 40 states reporting increases and seven reporting decreases. Seasonally, Giardia identifications increased in the summer and fall, especially in the Midwest. Nine states reported hookworms in more than 2% of specimens; none were states with indigenous transmission. We analyzed similar, but abbreviated, data for 1991; parasites were found in 19.7% of the 178,786 specimens and Giardia was found in 5.6%. States reporting percentages of Giardia identification in the highest quartile for both 1987 and 1991 were located in the Midwest or in the Northwest. Cryptosporidium was identified in both the 1987 and 1991 surveys; it had not been identified in a previous survey. For each year, Cryptosporidium was reported from 25 states across the country (for both years in 17 states). We conclude that intestinal parasitism should not be overlooked as a cause of gastrointestinal illness in the United States and that the prevalence of Giardia may be increasing.