General Information: Protozoans are single-celled, free-living, animal-like organisms ranging in size from 0.005 mm to 5 mm. All life-sustaining processes occur in one cell. Except for some in wet soil, all protozoans occur in the aquatic environment. Pathogenic protozoans comprise approximately 10,000 of the 35,000 species of protozoans known, and cause some of the worst diseases (Mitchell et al., 1988).

Numerical Categories:

Designated Use

Human Consumption: The maximum contaminant level goal (not enforced) for protozoans is zero cysts per 100 ml sample of drinking water. The maximum contaminant level (MCL) is simply whatever level the best available technology can achieve (Kubek et al., 1990).

Health Effects: Protozoans pose a hazard primarily in areas lacking sanitary conditions. Pathogenic protozoans may cause serious health problems, including gastrointestinal disease (Giardia lamblia, Cryptosporidium), dysentary and ulceration of the colon and the liver (Entamoeba hystolytica), and amoebic meningoencephalitis ( Naegleria fowleri). Ingestion of and primary contact with water are the primary methods of transmission.

Environmental Effects: Giardia and Cryptosporidium can both infect mammals if the oocysts are ingested. Infected animals experience symptoms similar to those experienced by humans and act as vectors to enhance transmission to humans(Kubek et al., 1990).


  1. Nonpoint:

  2. Point source: Sewered communities may not have enough capacity to treat the extremely large volume of water sometimes resulting from large rainfalls. Periodically, treatment facilities may find it necessary to bypass treatment of their wastewater. In this case, water containing protozoan cysts is discharged directly into the surface water body.

    Mode of Transport: Transport in water: Water carries protozoan cysts to surface water systems in overland flow (runoff), unsaturated flow, and saturated flow. Cysts in overland flow can be transported freely and within organic particles. Overland flow is the most direct route for protozoan transportation. Underground flow is less direct because water flow and cyst passage are impeded by soil permeability and porosity constraints.

Analytical Techniques: Current methods for protozoan detection are poorly standardized.

Isolation and identification of protozoa is difficult because they are relatively few in number, even in polluted water. Instead, other more plentiful organisms such as total coliforms, fecal coliform, and fecal streptococci are often used as indicator organisms. Detection of the indicator organism suggests that protozoa might also be present. Standard tests for coliforms are performed to assess probable presence of protozoa (Tchobanoglous 1991).

Although methods are poorly standardized, to test for protozoans, take your water quality sample to a qualified laboratory for inspection.