The roles free-living amebae and the parasitic protozoa Entamoeba histolytica and
Balantidium coli play as agents of waterborne zoonotic diseases are examined. The
free-living soil and water amebae Naegleria fowleri, Acanthamoeba spp., and Balamuthia
mandrillaris are recognized etiologic agents of mostly fatal amebic encephalitides
in humans and other animals, with immunocompromised and immunocompetent hosts among
the victims. Acanthamoeba spp. are also agents of amebic keratitis. Infection is through
the respiratory tract, breaks in the skin, or by uptake of water into the nostrils,
with spread to the central nervous system. E. histolytica and B. coli are parasitic
protozoa that cause amebic dysentery and balantidiasis, respectively. Both intestinal
infections are spread via a fecal-oral route, with cysts as the infective stage. Although
the amebic encephalitides can be acquired by contact with water, they are not, strictly
speaking, waterborne diseases and are not transmitted to humans from animals. Non-human
primates and swine are reservoirs for E. histolytica and B. coli, and the diseases
they cause are acquired from cysts, usually in sewage-contaminated water. Amebic dysentery
and balantidiasis are examples of zoonotic waterborne infections, though human-to-human
transmission can occur. The epidemiology of the diseases is examined, as are diagnostic
procedures, anti-microbial interventions, and the influence of globalization, climate
change, and technological advances on their spread.