Summary
Highlights
The pathogenesis of E. histolytica begins with the ingestion of cysts via the fecal-oral route. Cysts excyst in the small intestine, and trophozoites invade the large intestine, causing mucosal destruction, ulcers, and bleeding. Trophozoites can then enter the bloodstream via the mesenteric vein, travel to the liver through the portal vein, and cause liver abscesses, leading to liquefactive necrosis. Symptomatic carriers can spread the cyst form without being ill. Flies and cockroaches act as mechanical vectors, and cysts are resistant to routine chlorination.
Entamoeba coli is a non-pathogenic commensal amoeba, often confused with E. histolytica. Its trophozoite has a single nucleus, dirty-looking cytoplasm, blunt pseudopods, and an eccentric karyosome. The cyst form has eight nuclei and splinter-like chromatid bars. E. hartmanni is a smaller, non-pathogenic amoeba similar to E. histolytica, but it does not cause tissue invasion or ingest red blood cells. Both indicate poor sanitation practices if found.
Entamoeba polecki is a zoonotic amoeba found in pigs and monkeys, occasionally infecting humans, and is usually non-pathogenic. Its cyst has a single nucleus with angular chromatid bars. Endolimax nana is the smallest intestinal amoeba, commensal and harmless, characterized by a large, irregular karyosome and a "crosseyed" appearance in its four-nucleated cyst. Iodamoeba bütschlii, another commensal, is known for its large glycogen vacuole and a cyst with a unique 'basket of flowers' karyosome.
Entamoeba gingivalis exists only in the trophozoite stage (no cyst) and resides in the oral cavity, particularly around gum areas. It ingests white blood cells and is associated with poor oral hygiene but is non-pathogenic. Free-living amoebas, such as Naegleria fowleri and Acanthamoeba species, are then introduced. Naegleria fowleri, the causative agent of primary amoebic meningoencephalitis (PAM), is highly fatal. It has amoeboid, flagellated, and cyst forms, entering the brain via the nasal passage when swimming in contaminated warm fresh water, leading to rapid brain destruction.
Acanthamoeba species cause Acanthamoeba keratitis (eye infection) and granulomatous amoebic encephalitis (GAE), a chronic nervous system infection, particularly in immunocompromised patients and contact lens users who clean lenses with tap water. Transmission occurs via inhalation, eye ulcerations, or broken skin. The trophozoite has a single large nucleus and thorn-like appendages (acanthopodia) for movement and attachment. The cyst form has a characteristic double-walled structure with a wrinkled outer wall and a polygonal inner wall.
Flagellates possess whiplike flagella for locomotion. Most exist only as trophozoites and live in the intestine, except for Trichomonas tenax and Trichomonas vaginalis. Giardia lamblia (also known as Giardia intestinalis) is the only pathogenic intestinal flagellate, residing in the small intestine (duodenum). It causes giardiasis via contaminated water/food and has a low infective dose. Its trophozoite is pear-shaped with two nuclei, four pairs of flagella, and two ventral sucking discs, leading to malabsorption. The cyst is ovoid with four nuclei.
Dientamoeba fragilis, a flagellate formerly classified as an amoeba, exists only as a trophozoite. It typically has two nuclei with rose-like karyosomes and is fragile. It is associated with pinworm (Enterobius vermicularis) co-infection. Trichomonas species (vaginalis, hominis, tenax) only exist in the trophozoite stage (no cyst). T. vaginalis causes trichomoniasis, a sexually transmitted infection, characterized by a jerky, tumbling motility and a foul-smelling greenish-yellow vaginal discharge ('strawberry cervix'). T. hominis and T. tenax are commensals in the intestine and oral cavity, respectively.
Balantidium coli is the only human pathogenic ciliate, causing balantidiasis. It is the largest parasitic protozoan, moving by cilia. It possesses two nuclei: a large, kidney-shaped macronucleus (controls daily functions) and a small micronucleus (for reproduction). Pigs are the natural host, and transmission occurs through ingestion of infective cysts. Treatment often involves tetracycline, which indirectly reduces the parasite's food source by killing intestinal bacteria.
This section introduces protozoans as important eukaryotic microorganisms in clinical parasitology. It discusses their basic characteristics, such as possessing a nucleus and organelles, varying in shape and size, and reproducing both asexually (binary fission) and sexually (genetic exchange). A key distinguishing feature is the absence of cell walls, which allows for flexibility and shape changes. The internal structure, including the nucleus (control center with genetic material like DNA) and cytoplasm (composed of endoplasm for metabolism and ectoplasm for protection), is also covered.
Protozoans are classified into four main groups based on their methods of locomotion: Sarodina (Amoebas) use pseudopodia (false feet), with Entamoeba being an example. Mastigophora (Flagellates) use flagella (whiplike tails), exemplified by Giardia lamblia and Trichomonas vaginalis. Ciliophora (Ciliates) use cilia (tiny hair-like structures), with Balantidium coli as an example. Sporozoans or Apicomplexa, such as Plasmodium species (malaria), lack obvious locomotory organelles.
Intestinal amoebas are single-celled parasites primarily living in the human intestines, moving via pseudopods. All amoebas, except Entamoeba gingivalis, have a cystic stage. Most amoebas inhabit the intestines, again with the exception of E. gingivalis, which resides in the mouth. Generally, amoebas are commensal (harmless), but Entamoeba histolytica is pathogenic. Distinguishing features for Entamoeba genus include peripheral chromatin and chromatid bars in both trophozoite and cyst stages.
This part differentiates between the two life forms of protozoans: the trophozoite and the cyst. Trophozoites are motile, feeding, and vegetative (active growth) stages found in diarrheal or liquid stool, capable of causing symptoms if pathogenic. Cysts are non-motile, non-feeding, and the infective stage, found in formed stool, and are responsible for transmission through contaminated food or water, surviving outside the body.
Laboratory diagnosis involves wet preparation and permanent stains. Wet mounts show trophozoite motility and internal structures, while iodine wet mounts enhance visualization of cyst structures. Permanent stains confirm identification, and ocular micrometers measure organism size. PCR is highlighted as the method of choice to differentiate pathogenic from non-pathogenic amoebas.
Entamoeba histolytica is the only pathogenic amoeba, causing diseases like intestinal amoebiasis (amoebic colitis, amoebic dysentery) and extraintestinal amoebiasis (liver abscess). Transmission occurs via ingestion of infective cysts through contaminated food or water. Key diagnostic features of its trophozoite form include progressive, directional motility, centrally located karyosome, evenly distributed peripheral chromatin, and significantly, ingested red blood cells in the cytoplasm. The mature cyst has four nuclei and can survive for a month in the environment.