Sexually transmitted parasitic diseases. Sexual activity is the primary method of transmission for several important parasitic diseases and has resulted in a significant prevalence of enteric parasitic infection among male homosexuals. The majority of parasitic sexually transmitted diseases involve protozoan pathogens; however, nematode and arthropod illnesses are also included in this group. Trichomoniasis, caused by Trichomonas vaginalis, is the most common parasitic STD. Infection with this organism typically results in the signs and symptoms of vaginitis. Trichomoniasis can be diagnosed in the office setting by performing a microscopic evaluation of infected vaginal secretions and can be successfully treated with metronidazole. Both pediculosis pubis, caused by the crab louse Pthirus pubis, and scabies, caused by the itch mite Sarcoptes scabiei, present with severe pruritus. A papular or vesicular rash and linear burrows seen in the finger webs and genital area are characteristic of scabies. Pediculosis pubis is diagnosed by observing adult lice or their nits in areas that bear coarse hair. The diagnosis of scabies is confirmed by scraping suspicious burrows and viewing the mite or its byproducts under the microscope. Lindane, 1% used in treating scabies, is also very effective for treating pediculosis pubis. Synthetic pyrethrins, also applied as a cream or lotion, are less toxic alternatives for the treatment of either condition. Oral-anal and oral-genital sexual practices predispose male homosexuals to infection with many enteric pathogens, including parasitic protozoans and helminths. The most common of these parasitic infections are amebiasis, caused by Entamoeba histolytica, and giardiasis caused by Giardia lamblia. Both entities may cause acute or chronic diarrhea, as well as other abdominal symptoms. Most gay men with amebiasis are asymptomatic, and invasive disease in this group is extremely rare. Both amebiasis and giardiasis can be diagnosed on the basis of microscopic examination of stool specimens, although duodenal aspiration is occasionally necessary to confirm a diagnosis of giardiasis. Multiple treatment regimens exist for amebiasis. Iodoquinol is a good choice for asymptomatic cyst carriers, whereas the combination of metronidazole plus iodoquinol is used for symptomatic patients. Quinacrine and metronidazole are both efficacious in the treatment of giardiasis. 