Protozoa, single celled animals have evolved to exploit every ecological niche. Some are free living while others are parasitic and capable of causing severe, sometimes life-threatening, infection.
A number of parasitic protozoa are intracellular parasites living within other cells. One of the major causes of illness and contributors to mortality in tropical and sub-tropical countries is malaria. The cause of malaria is an intracellular parasite Plasmodium species. Four different species of Plasmodium cause infection in humans, these are Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae and Plasmodium ovale. Plasmodium parasites multiply in the liver before infecting the red blood cells of their host. Outside of tropical areas, the morphological similar parasites Babesia species occasionally cause infection but they are far less common than malaria.
The cysts of Cryptosporidium parvum contaminate water supplies. When ingested this intracellular protozoon causes gastroenteritis. Immune suppressed individuals, such as AIDS patients, suffer particularly badly with this infection.
Another, very common, intracellular parasite is Toxoplasma gondii the cause of toxoplasmosis. This extremely common parasite occurs worldwide and is capable of causing infection in many different animals. Another morphologically similar parasite causing infection in dogs is Neospora caninum. Both of these parasites infect their hosts when they are ingested by the host.
Plasmodium and Babesia both infect their host via the bite of an insect. Another parasite gaining admission to its host by an insect bites is the Trypanosome. Trypanosoma brucei is a flagellated protozoan, which causes the potentially fatal disease sleeping sickness. This is another sickness to afflict people living in Sub Saharan Africa. Trypanosoma cruzi causes Chagas disease in parts of the Americas. These active protozoa propel themselves around by means of flagella.
A flagellate also causes trichomoniasis. The protozoon, Trichomonas vaginalis commonly infects a person as a sexually transmitted disease, with the symptoms more often apparent in women.
Another flagellate capable of causing infection is the intestinal parasite Giardia lamblia. Frequently associated with food and water contamination giardiasis, caused by this protozoon, occurs worldwide.
A number of amoebae live within the human gastrointestinal tract. Only one of these causes infection and that is Entamoeba histolytica. Amoebic dysentery caused by Entamoeba histolytica is a severe form of gastroenteritis but some infections are asymptomatic. Entamoeba histolytica infections occasionally spread to other organs particularly the liver.
Not all pathogenic protozoa depend on a host for their existence. Some such as Giardia species form resistant cysts allowing them to survive in the environment. Some naturally free-living amoebae occasionally cause infections. Acanthamoeba species, Naegleria species, Balamuthia mandrillaris, and Sappinia diploidea may all cause potentially fatal infections of the central nervous system, usually in immunocompromised patients. These amoebae normally reside in soil, vegetation or water sources. Acanthamoeba species also causes occasional eye infections most commonly in contact lens wearers.
As can be seen there are many different pathogenic protozoa. Treatment for man of these infections is available. The drug of choice for treating these infections depends on the causative protozoon.
CDC Fact sheet
Health Protection Agency
The American Social Health Association
World Health Organization