Amoeba Classification and Biology

Amebae

• Amebae are single-celled eukaryotic organisms.
• They are classified within the subphylum Sarcodina, class Lobosea.
• Intestinal species and extraintestinal species are further subdivisions.

Classification

• Subphylum: Sarcodina
• Class: Lobosea
• Intestinal Species: Entamoeba histolytica, Entamoeba hartmanni, Entamoeba coli, Entamoeba polecki, Endolimax nana, Iodamoeba bütschlii
• Extraintestinal Species: Entamoeba gingivalis, Naegleria fowleri, Acanthamoeba species

Intestinal/Lumen-Dwelling Protozoa

• Numerous protozoa inhabit the human gastrointestinal tract.
• Most protozoa are non-pathogenic, causing mild disease in commensals
• Pathogenic protozoa cause severe disease.
• Transmission often occurs via the fecal-oral route, ingesting contaminated food or water containing cysts.

Intestinal Protozoa: Life Cycle

• Mature cysts are ingested.
• Excystation occurs in the lower ileum.
• Trophozoites multiply via binary fission.
• Encystation occurs under unfavorable conditions.
• Cysts are excreted in feces.

Encystation

• Occurs when the environment is unfavorable for trophozoite reproduction (overpopulation, pH change, lack of nutrients, lack of oxygen).
• Trophozoites become spherical, 12-15 μm in diameter.
• Ribosomes aggregate to form elongated chromatoid bodies.
• Maturation involves two rounds of nuclear division; chromatoid bodies disappear.
• Excreted cysts are infective and viable for weeks to months depending on conditions.

Trophozoite Stage

• Trophozoites are actively growing, reproducing forms of the parasite.
• They are motile and fragile.
• Multiplication occurs via asexual binary fission.
• Trophozoites can be differentiated from cysts by microscopic analysis.

Intestinal Protozoa: Symptoms/Diagnosis

• Most ameba infections are asymptomatic.
• Common symptom is unexplained diarrhea.
• Diagnosis involves finding trophozoites and/or cysts in stool samples.
• Identifying characteristics include organism size, number and location of nuclei, cytoplasmic inclusions, and motility.
• Permanent stains facilitate visualization of microscopic structures.

Entamoeba histolytica

• A species which can become pathogenic leading to illness
• Transmission via ingestion of contaminated cysts in food or water.
• It leads to lethal systemic diseases.
• Trophozoites exhibit active metabolism.
• Reproduction and nutrient uptake occur in the trophozoite stage.
• Humans are the only definitive host.
• Characterized by amorphous shape.
• Trophozoite size ranges from 8-65 μm (average 12-25 μm).
• Cysts range from 12-18 μm.
• They move rapidly via pseudopodia.

Entamoeba histolytica: Modes of Transmission

• Hand-to-mouth infection (ingestion of cyst stage)
• Food/water contamination
• Unprotected sex
• Vectors (flies and cockroaches)

Entamoeba histolytica: General Characteristics (Cont'd)

• Nucleus has a small, central karyosome.
• Peripheral chromatin is fine and evenly distributed around nucleus.
• Nucleus is invisible to the naked eye until stained.
• The presence of red blood cells in the cytoplasm is a distinguishing feature.

Entamoeba histolytica Pathology:

• Infection can result in intestinal inflammation, ulcers, or abscesses.
• Extraintestinal lesions can occur in the liver (hepatic amebiasis), lungs (pulmonary amebiasis), or brain (cerebral amebiasis).

Amebiasis

• An estimated 50 million cases worldwide per year.
• Up to 100,000 deaths yearly.
• Third leading cause of parasitic deaths.
• Associated with long-term (> 1 month) residence in endemic regions.

Amebiasis (Cont'd)

• On average, one in 10 people infected with Entamoeba histolytica becomes ill (causes intestinal amebiasis)
• Asymptomatic cases arise from low parasite virulence, low host inoculation.
• Symptoms such as diarrhea, stomach pain, cramping, and colitis are common.
• Severe cases can involve amebic dysentery (bloody stools, stomach pain, fever)
• In rare cases, invasion of the bloodstream can result in liver abscesses (extraintestinal amebiasis).
• Diagnosis is typically serological unless trophozoites re-enter bloodstream.

Amebiasis (Cont'd)

• Infection can spread to other body parts (lungs or brain) in less frequent cases.

Amebiasis: Presentation and Diagnosis

• Symptoms typically appear 1-4 weeks post infection.
• Diagnosis is challenging due to similar morphology of other cells/parasites.
• Diagnosis is often misdiagnosed.
• Analysis of stool samples (and possible blood if intestinal penetration), staining, and wet mounts can assist in diagnosis..

Entamoeba coli

• A commensal, non-pathogenic ameba.
• Larger than Entamoeba histolytica (20-30 µm diameter); larger cysts (10-33µm).
• Primarily feeds on bacteria and other available cellular material.
• Does not invade tissues.
• Distinguished by granular instead of fine endoplasm.

Naegleria fowleri

• Found in warm freshwater environments.
• The life cycle includes cysts, trophozoites and flagellated forms.
• People can be infected through water-related activities and the parasite enters through the nose.
• The parasite migrates to the brain along the olfactory nerves, resulting in primary amebic meningoencephalitis.

Acanthamoeba spp.

• At least 5 species identified, are free-living in soil and freshwater.
• Trophozoites are ameboid forms.
• Known to cause keratitis in contact lens users.

Balantidium coli

• The largest protozoan; 28–152 µm, but average 35-50
• Exists in trophozoite and cyst forms.
• Trophozoite has cilia, and is ovoid to sac-shaped.
• Tapers at the anterior end and contains two nuclei.
• Cysts are 52-55 µm, spherical or oval in shape.
• Contain 1-2 vacuoles; mature cysts lose cilia.
• Transmission through contaminated food, water, or via fecal-oral/person-person routes
• Common symptoms are mild colitis, diarrhea (possible pus, mucus, blood), and abscesses. Can invade other organs including the liver, lungs or urogenital tract.

Pneumocystis carinii

• Cysts range from 3.5 to 7 µm with 4 to 8 nuclei (1-2 µm in size); varies on size.

• Trophozoites are 1-5 µm in size.

• Life cycle not fully understood.

• Believed to infect alveolar spaces in the lungs.

• Mature cyst ruptures releasing trophozoites.

• May infect other locations such as spleen, lymph nodes, and bone marrow.

• Commonly transmitted through person-to-person contact via exhaled droplets; can also be transmitted through the placenta resulting in still births.

• Most at risk are immunocompromised persons (e.g., AIDS) or young children.

• Respiratory symptoms (pneumonia) such as a non-productive cough, fever, rapid breathing, and cyanosis (bluish tint) are potential indicators.