A parasite is an organism that obtains food and shelter from another organism and derives all benefits from this association. The parasite is termed obligate when it can live only in a host; it is classified as facultative when it can live both in a host as well as in free form. Parasites that live inside the body are termed endoparasites whereas those that exist on the body surface are called ecto-parasites. Parasites that cause harm to the host are pathogenic parasites while those that benefit from the host without causing it any harm are known as commensals.
The organism that harbors the parasite and suffers a loss caused by the parasite is a host. The host in which the parasite lives its adult and sexual stage is the definitive host whereas the host in which a parasite lives as the larval and asexual stage is the intermediate host. Other hosts that harbor the parasite and thus ensure continuity of the parasite's life cycle and act as additional sources of human infection are known as reservoir hosts. An organism (usually an insect) that is responsible for transmitting the parasitic infection is known as the vector.
INTESTINAL AND UROGENITAL PROTOZOA
Intestinal and luminal protozoa significant to human health include
Entamoeba histolytica (Amebae)
Balantidium coli (Ciliates)
Giardia lamblia and Trichomonas vaginalis (Flagellates)
Cryptosporidium parvum and Isospora belli (Sporozoa)
AMEBIASIS (amebic dysentery, amebic hepatitis)
E. histolytica is the major cause of amebic dysentery.
0.5 to 50% of the population world wide harbors E. histolytica parasites with the higher rates of infection being in underdeveloped countries. 1 to 3% of the population of the USA are infected. Infection is associated with poor hygiene. Humans are the principal host, although dogs, cats and rodents may be infected.
Trophozoite: This form has an ameboid appearance and is usually 15-30 micrometers in diameter, although more invasive strains tend to be larger. The organism has a single nucleus with a distinctive small central karyosome (Figure 1A,B). The fine granular endoplasm may contain ingested erythrocytes (Figure 1C). The nuclear chromatin is evenly distributed along the periphery of the nucleus.
Cyst: Entameba histolytica cysts are spherical, with a refractile wall; the cytoplasm contains dark staining chromatoidal bodies and 1 to 4 nuclei with a central karyosome and evenly distributed peripheral chromatin (Figure 2).
Infection occurs by ingestion of cysts on fecally contaminated food or hands. The cyst is resistant to the gastric environment and passes into small intestine where it decysts. The metacyst divides into four and then eight amoebae which move to the large intestine. The majority of the organisms are passed out of the body with the feces but, with larger bolus of infection, some amebae attach to and invade the mucosal tissue forming "flask-shaped" lesions (bomb craters). The organisms encyst for mitosis and are passed through with feces (Figure 3). There are no intermediate or reservoir hosts.
from : http://pathmicro.med.sc.edu