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Protozoa Parasites

Protozoa Parasites

All parasitic protozoa are complex single celled organisms.

Many, including Cryptosporidium, Giardia lamblia, and Entamoeba histolytica, transform from free living forms into dormant, but highly infectious cysts when expelled in the host’s faeces.

Cysts are usually round or oval shaped and have tough shells allowing them to survive long periods in water sources while they await ingestion by a suitable host in which they can reactivate into free living organisms and reproduce.

Compared to viruses, and even most bacteria, parasitic protozoa are veritable microscopic giants.

Cryptosporidium parvum oocycts (the cycst form of Crypto) are 2–6 microns in diameter, while Giardia lamblia trophoites (the mobile form) are 10–20 microns in length with cysts 8–14 microns in diameter.

The gigantic Balantidium coli, the largest protozoan, measures in at 45 microns by 60 microns, which is just on the edge of being visible to the naked eye.

Given their relatively large size, parasitic protozoa are easily seen with normal light microscopes.


Microbiologists divide parasitic protozoa into four groups—Sarcodina (amoebas), Mastigophora (flagellates), Ciliata or Ciliophora, and Sporoza—based on shape, means of locomotion, and other physical characteristics.

Within these groups are numerous parasitic protozoa, but only a few infect humans through the faecal–oral route in food and drinking water.

Sarcodina (amoebas), such as Entamoeba histolytica, the cause of amoebic dysentery, are the classics of high school biology. Amoebas look like translucent microscopic bags of jelly. They move by projecting a pseudopod and then flowing into it.
Mastigophora (flagellates), such as Giardia lamblia, have more defined shapes and move by means of one or more whip-like flagella.
Ciliophora move by wiggling rows of small cilia projecting from their sides. The only human parasite in this phyla is the large Balantidium coli, which infects the intestines of humans and pigs.

Sporoza have no special means of moving but have complex lifecycles that often involve alternating sexual and asexual reproduction in different hosts. Cryptosporidium parum is the most important waterborne member of this group.

Common Pathogenic Protozoa Worldwide
Protozoan Illness Treatment
Giardia lamblia Giardiasis (gastroenteritis) Fluid and electrolyte replacement and anti-microbial drugs
Cryptosporidium parvum Gastroenteritis Fluid and electrolyte replacement
Other Pathogenic Protozoa Worldwide, Uncommon in North America and Europe
Protozoan Illness Treatment
Entamoeba histolytica Amoebic dysentery and severe dehydration Fluid and electrolyte replacement and anti-microbial drugs
Isospora belli Dysentery Fluid and electrolyte replacement and anti-microbial drugs
Balantidium coli Dysentery Fluid and electrolyte replacement and anti-microbial drugs
Entamoeba polecki Gastroenteritis, possibly dysentery Fluid and electrolyte replacement and anti-microbial drugs


Depending on the species, parasitic protozoa reproduce either sexually, asexually, or using both methods. As with bacteria, rapid asexual division among protozoa produces staggering numbers of organisms in a short period of time.


Waterborne parasitic protozoa are transmitted by the faecal–oral route meaning that the organisms or cysts are ingested in water or food contaminated with infected human faeces. People, or animals, with heavy Giardia infections can pass 106 cysts per gram of faeces.

Infective Dose

While most bacteria and viruses require relatively high numbers to cause infection, because of their acid resistant shells, most protozoa cysts can infect humans in numbers of ten or less.

Given the high volumes of cysts in the faeces of infected people and animals, the potential for easy waterborne transmission and the low infectious dose, the potential for infection is massive.

As with other pathogens, the severity of infections caused by protozoa are usually related to the number of organisms ingested. Low doses may cause only asymptomatic infection, while heavy doses cause the worst illness.

Distribution and Reservoirs

All important waterborne parasitic protozoa are naturally present in the intestinal tracts of humans and other animals. Global infection rates with Giardia and Cryptosporidium are enormous.

For example, Cryptosporidium has historically infected the water supply of St. Petersburg, Russia. Estimates in the 1990s were that 90% of the city’s population was infected with the protozoan.

In North America, Giardia and Cryptosporidium are the most prevalent protozoa. Giardia have been isolated from domestic cats and dogs, bears, deer, bighorn sheep, rodents and, of course, beavers, which lend giardiasis the popular name ‘Beaver Fever.’

Cryptosporidium have also been isolated from herd animals including domestic sheep, cattle, goats and wild deer and elk. Given the high volumes of cysts in infected faeces, the huge natural reservoirs and the ability of cycsts to persist in the environment for weeks or months, it is obvious why Giardia and Cryptosporidium are the most common of all waterborne pathogens in North America and Europe.

Studies have found Giardia cysts in 97% of all surface water in North America, including high quality mountain streams and lakes even in remote areas of the Yukon. A study of 10,000 waterways found Cryptosporidium in about 75% of all rivers and lakes in the western US. Currently, but probably not for long, Cryptosporidium oocysts may only be absent in the most remote lakes of northern Canada and Alaska while all European waterways are likely infected.

Concentrations of cysts vary depending on the water source and the exact location of faeces in the water. On average, concentrations in water in wilderness areas of North America are usually below one cyst per litre of water. However, water in a beaver pond can average well over 100 Giardia cysts per litre.

Raw sewage or run-off from land populated by cattle infected with Cryptosporidium can exceed 5,000 oocysts per litre.

In 1996, spring run-off in Cranbrook, British Columbia, Canada flushed infected cattle faeces into the town’s reservoir, which caused a massive outbreak of cryptosporidiosis that involved several thousand people.

In 1993, in the largest ever North American outbreak, 400,000 people became infected with Cryptosporidium in Milwaukee, Wisconsin from contaminated drinking water. Adding to the potential for severe outbreaks is the strong resistance of Giardia cysts and total immunity of Cryptosporidium oocysts to the chlorine used to treat urban drinking water.


As with bacteria and viruses, intestinal protozoa cause ailments ranging from asymptomatic infection to gastroenteritis, dysentery, and, in some cases, tissue invasion and organ damage. Treatment is usually in the form of hydration and electrolyte replacement. Drug therapy might be available and necessary depending on the pathogen.

High Risk Groups

No treatment currently exists for Cryptosporidium infection, making it an important health risk for the elderly, children and other people with weak immune systems, especially AIDS patients.

Normally, the human immune system can handle occasional oocysts ingested in unfiltered municipal water, however the immune systems of many AIDS sufferers cannot handle even a few oocysts. The end result can be complete intestinal colonization and eventually death.

Some estimates suggest that Cryptosporidium infections contracted from municipal drinking water cause up to seven percent of all deaths among AIDS patients.