Transmission also occurs sexually by oral-anal contact. Infection is common in areas with poor sanitation, in mental institutions and in homosexual males. Infection rates are highest in areas where sanitation is poor. Cases are most common in tropical areas and in institutions where it is difficult to maintain high standards of hygiene.
Homosexual males are also at high risk.
Most infections do not result in any symptoms. Clinical disease can range in severity from mild stomach discomfort with alternating bouts of diarrhea and constipation to severe diarrhea. In some cases, the parasites may invade the blood stream, infecting the liver, lung or brain. Ulceration of the skin surrounding the anus can pose a risk to homosexual males who may acquire penile lesions from contact with infected skin. Humans are the natural reservoir of the parasite, and persons who are excreting cysts can be a source of infection for others.