Protozoa are single celled organisms. Some of these microscopic organisms can cause intestinal infections. The infection causing protozoans found in humans are Entameba histolytica, Giardia lamblia, Cryptosporidium parvum, Isospora belli and Balantidium coli.
This type of ameba causes amebic dysentery and may spread to other organs of the body such as the liver or brain to cause amebic abscesses. Intestinal disease may be asymptomatic, mild or severe. Both acute and chronic cases of amebic dysentery occur.
Humans are the primary host for this parasite although occasional infections occur in animals such as dogs and cats. Within a host the ameba lives as a motile form but can form resistant cysts, which allow it to survive in the environment for an extended period.
Infection occurs by the feco-oral route. This means food or water contaminated by fecal matter from an infected host is normally the source of new infections. Poor sewage provision in developing countries means that this type of protozoa is more common in such countries. However, infections occur worldwide, spread by poor hygiene procedures. Ensuring clean water supplies and clean hygiene procedures for food handlers prevents its transmission.
This flagellated protozoan is a common cause of diarrhea in humans. It if frequently implicated in traveler’s diarrhea when tourists particularly to places with poor water treatment facilities develop the infection.
As with Entameba histolytica, Giardia lamblia forms a motile form within its host and a resistant cyst in the environment. These cysts may contaminate poorly maintained swimming pools so in addition to eating or drinking contaminated material, recreational swimming in contaminated water is a potential source of infection.
Prevention of giardiasis relies on careful hygiene procedures in the preparation of foodstuffs and drinks. Persons with diarrhea should not use swimming pools so they will not contaminate the water for other people.
Giardia lamblia is thought to be only a parasite of humans so there is no animal reservoir for this protozoan.
This protozoan infects many animal hosts and through them natural water sources. Cryptosporidiosis normally appears as a mild gastrointestinal upset but in immunocompromised subjects such as AIDS patients, it can cause a chronic debilitating condition.
The hardy cysts of Cryptosporidium parvum are resistant to some drinking water processing procedures. Therefore, most cases are water borne. In areas where water processing is deemed insufficient to eradicate the protozoa, people especially those at high risk may prefer to boil water prior to drinking or fit a fine pored filter designed to remove such cysts from their drinking water supply.
As with Giardia lamblia, Cryptosporidium parvum may survive in swimming pools allowing some cases to derive from this source. In addition, as natural watercourses may be contaminated by wild or domesticated animals, care should be taken when swimming or indulging in water-sports in lakes or rivers to avoid swallowing water, which may contain Cryptosporidium parvum cysts.
This is rarely seen in people with a normal immune system but AIDS patients occasionally develop gastrointestinal infections with this protozoan. It is transmitted by the feco-oral route so food hygiene is essential in its prevention.
This ciliated protozoan normally infects animals such as cows, pigs and horses. Human infection occur from the ingestion of food contaminated with farmyard manure. The thorough washing or peeling of vegetables to be eaten raw or the cooking of vegetables prevents balantidiasis.
While hygiene in all food preparation area is important in prevention of transmission of intestinal protozoa, some institutions have a high risk of transmission. These include mental institutions and childcare facilities. In such places, attention to personal hygiene of both the service users and staff is essential in preventing protozoal transmission.
The sexual practice of anal-oral stimulation also spreads these types of protozoa. Avoidance of such practices prevents transmission of protozoa and other gastrointestinal infections.
While this article has concentrated on protozoa that cause gastrointestinal symptoms, many other are found living in the human intestinal tract. These include Entameba dispar, Entameba coli, Entameba hartmanni, Endolimax nana, Iodameba bütschlii, Dientameba fragilis and Chilomastix mesnili. Like there more harmful relatives these are spread by feco-oral route and prevented by good hygiene.
Microbiology and Immunology on line University of California