Chapter 7 - The Amebas

1. Chapter 7 - The Amebas Taxonomy P. Sarcodina S.P. Amoebozoa (Rhizopoda) C. Lobosea O. Amoebida O. Schizopyrenida

2. P. Sarcodina • Characterized by cytoplasmic extensions of the body called pseudopodia; flagella, when present, are restricted to specific developmental stages • Internal organs are not fixed in position within the cell, but rather they move freely within the cytoplasm • Most members of phylum are free living; a few are parasitic and commensalistic • O. Amoebida • Usually uninucleate; no flagellate stage

3. Family Endamoebidae • Parasites or commensals of the digestive systems of arthropods and vertebrates • Genera and species are primarily distinguished on the basis of their nuclear structure • Type example: Entamoeba

4. Genus Entamoeba • Morphology • Ectoplasm is usually distinguishable from the endoplasm • Nucleus is characterized by a a peripheral layer of chromatin forming a beaded ring • Cytoplasm of trophozoites contains numerous helical bodies with ribonuceoproteins - chromatoidal bodies • Chromatoidal bodies develop in the motile stages of the amebas and mature in the cyst, but disappear as the cyst ages • Golgi bodies and mitochondria are completely absent in these organisms

5. Entamoeba histolytica • Best known species of ameba that parasitizes humans • Causative agent of amoebiasis/amoebic dysentery • Global in its distribution; the incidence of infection exceeds 400 million cases! • First discovered in Russia in 1873 by Losch

6. Life Cycle: Overview

7. Life Cycle (detail) • The uninucleate trophozoite inhabits the colon, rectum and, at times, the lower end of the small intestine of humans and other primates • It is typically monopodial, producing one large fingerlike pseudopodium • Trophozoites proliferate mitotically (binary fission) in the host gut • Under certain adverse environmental and/or physiological circumstances the trophozoites assume precystic characteristics by becoming more spherical and as food vacuoles are extruded, shrinking in size

8. Life Cycle cont. • Encystation begins with the secretion by the precyst trophozoite of a thin, surrounding hyaline membrane to form a cyst • Nucleus of the cyst then undergoes 2 mitotic divisions to produce 4 vesicular nuclei in the mature cyst • Cysts represent the infective form and pass out of the host feces • When food or water contaminated with cysts is ingested by a host, cysts pass through the stomach to the ileum where excystment occurs • Upon excystation, the tetranucleate organism undergoes mitosis, giving rise to 8 small, uninuceate metacystic trophozoites, which pass down to the large intestine where they feed, grow and reproduce

9. Note: • Main source of infection is the cyst-passing, asymptomatic carrier or chronic patient - luminal amoebiasis • Acutely ill patients, those with invasive amoebiasis, are not significant transmitters since they pass the noninfective trophozoite • Flies and roaches have been implicated as vectors in the spread of this ameba, since cysts can survive for long periods in their digestive tracts, later to be regurgitated or passed out in feces on food

10. Pathology • In humans living in temperate zones, the organism often produces the nonpathogenic luminal form of the disease - luminal amoebiasis • In the tropics and subtropics, the invasive form of the disease is more common • In the infected individual who develops dysentery, the mucosal ulceration may penetrate deeper into the intestinal tissue, causing vast areas of tissue to be destroyed; the overlying mucosal epithelium then may be sloughed off, exposing these necrotic areas • The destructive process is usually followed by a regenerative period, resulting in thickening of the intestinal wall as a result of the deposition of fibrous connective tissue Human colon showing amoebic ulcer

11. Pathology cont. • Trophozoites may also be carried to the liver via the hepatic portal system causing hepatic amoebiasis • The first sign of hepatic involvement is the formation of an early hepatic abcess containing a matrix of necrosed hepatic cells, which eventually become liquified Liver abcess

12. Symptoms Invasive amoebiasis may manifest itself in a number of ways: 1. Acute amoebiasis - severe diarrhea (blood and mucus in liquid) develops and a fever 2. Chronic amoebiasis - continuous attacks of diarrhea with other mild intestinal problems 3. Hepatic amoebiasis - abscesses may rupture the abdominal wall or extend through the diaphragm into the lungs; any of these manifestations can be fatal

13. Prevention • Avoid food and water contaminated with feces containing the cysts are the most common vehicles for transmission • Boil or iodination of drinking water in endemic areas • Westerners travelling to developing countries should drink bottled water, avoid ice cubes, salads and those fruits not peeled by the person consuming them

14. Entamoeba coli • An intestinal ameba, generally considered to be nonpathogenic in humans • Does not ingest or invade host tissue; usually feeds on bacteria, yeast and fragments of intestinal debris • Life cycle is similar to the of E. histolytica, including precystic, metacystic and trophozoite stages, with infection of the host initiated by ingestion of cysts

15. Entomoeba gingivalis • Cosmopolitan in distribution • Commonly found in the tartar and debris associated with the gingivital tissues in the mouth • Food vacuoles may contain epithelial cells, WBCs and occasionally RBCs • No indication that it is pathogenic • No cyst is formed; it is transmitted either directly (kissing) or indirectly via trophozoite-contaminated food, chewing gum, etc.

16. Order Schizopyrenida • Members of this order possess both a flagellated and amoeboid form • During the amoeboid form that reproduction (asexual) occurs • The amebas occur in soil and water and mostly consume bacteria • Family Vahlkamphidae • Although members of this family typically live in the soil and water and feed on bacteria, they can become facultative parasites of animals, including humans • Example: Naegleria fowleri

17. Naegleria fowleri • Responsible for primary amebic meningoencephalitis (PAM) • Most victims have a history of recent exposure to warm, fresh or brackish water, such as swimming pools, ponds, lakes and streams • Life Cycle • Includes flagellated and amoebic trophozoites and cysts; rapid transformation from one form to the other • Flagellated trophozoites are capable of rapid movement through the water, and transmission to humans likely occurs when the nasopharyngeal mucosa is invaded by these forms • Amoebic trophozoite form migrates through the nervous system to the brain, where inflammation occurs followed by death • No cyst stage occurs in the human host

18. Family Hartmannellidae • Contains a single genus, Acanthamoeba • Worldwide in distribution; isolated from freshwater, brackish water, hot tubs, sewers, etc. • Like Naegleria, it’s a facultative parasite of humans • Other than the absence of the flagellated trophozoite stage, it has a life cycle similar to N. fowleri • Responsible for acute PAM, Acanthomoeba causes chronic PAM Acanthamoeba

19. Family Hartmannellidae cont. • Acanthamoeba have been implicated in skin infections • Recently gained public attention because of the devastating effects of its invasion to the human eye (= keratitis), corneal inflammation and corneal perforation often followed by blindness) Keratitis. Note the ring-shaped infiltrate of the cornea