Protozoa

Protozoa

Class Lobosea: Considered Organisms

  • Entamoeba coli
  • Entamoeba polecki: Uncommon, typically found in pigs and monkeys, occasional human infection.
  • Entamoeba gingivalis: Found in the mouth, associated with poor oral hygiene, no cyst stage, transmitted directly via oral contact.
  • Endolimax nana
  • Iodamoeba bütschlii: Non-pathogenic, intestinal amoeba, characterized by a large glycogen vacuole and a single nucleus (often described as "basket nucleus").
  • Blastocystis hominis: A common intestinal parasite, its pathogenicity is debated, can cause diarrhea, abdominal pain, and nausea, especially in immunocompromised individuals.
  • Naegleria fowleri
  • Acanthamoeba spp.
  • Balamuthia mandrillaris: Causes Granulomatous Amebic Encephalitis (GAE), similar to Acanthamoeba but can affect immunocompetent individuals, often found in soil, can cause skin lesions.

General Characteristics of Protozoa

  • Morphologic differentiation exists to help in identification.
  • Most reproduce by binary fission (asexual reproduction); some utilize other specialized modes of reproduction, such as schizogony (multiple fission) seen in Plasmodium species.
  • Two major methods of transmission for protozoal infections:
    • Ingestion of infective stage (e.g., cysts for Entamoeba histolytica).
    • Transmission through an arthropod vector (e.g., Anopheles mosquitoes for Plasmodium).
  • Modes of mobility among protozoa:
    • Pseudopodia: Used by amoebae (e.g., Entamoeba, Naegleria).
    • Flagella: Some protozoa possess one or several whip-like structures (e.g., Giardia, biflagellate form of Naegleria).
    • Cilia: Hair-like structures used for movement by ciliates (e.g., Balantidium coli).

Naegleria fowleri

  • Pathogenicity: Causes Primary Amebic Meningoencephalitis (PAM).
Method of Infection:
  • Trophozoite enters nasal cavity from infected water while swimming. The ameba thrives in warm freshwater environments (25^ ext{o} ext{C} to 45^ ext{o} ext{C} or 77^ ext{o} ext{F} to 113^ ext{o} ext{F}).
  • Free-living forms:
    • Trophozoite: The active, feeding, and reproductive stage responsible for infection. Motile by pseudopods.
    • Cyst: A dormant, resistant stage found in unfavorable environmental conditions, not typically found in human infections.
    • Biflagellate form: A temporary, non-replicating, motile form induced by environmental changes, such as nutrient depletion.
  • Migration to central nervous system (CNS) occurs via olfactory lobes, typically taking 7-10 days.
Diagnostic Stage:
  • Active trophozoites present in spinal fluid. PCR methods can also be used for diagnosis.
  • Trophozoite found in brain at autopsy.
  • PAM usually results in death (3-6 days after symptoms start).
  • Invades frontal cortex within 1-2 days.
Case Report Summary:
  • A 12-year-old girl in Arkansas was infected after swimming in Willow Springs Water Park, leading to a confirmed case of PAM.
  • Symptoms included fever, vomiting, and severe headache.
  • Immediate medical attention was sought, and the water park was subsequently shut down due to the outbreak.
Characteristics:
  • Naegleria fowleri is a free-living ameba found in fresh or saltwater, moist soil, and decaying vegetation, primarily during summer months.
  • Parasite can tolerate chlorinated water and survive temperatures up to 113^ ext{o} ext{F}.
  • Symptoms include headaches, fever, nausea, vomiting, coma, and death.
  • Purulent spinal fluid with neutrophil counts of 200-20,000 per ext{µL} is observed.

Acanthamoeba spp.

  • Details of morphological features: large potential nuclear karyosome and granular, vacuolated cytoplasm. Characterized by characteristic spine-like pseudopods (acanthopodia).
Method of Infection:
  • Trophozoite enters body through the respiratory tract (leading to GAE), broken skin, or through mucous membranes, with direct invasion of the eye possible (leading to Acanthamoeba keratitis).
Free-Living Forms:
  • Trophozoite
  • Cyst
Diagnostic Stage:
  • Trophozoites and cysts can be recovered from brain, skin biopsy, or corneal scraping; rarely observed in cerebrospinal fluid (CSF). Stains like Giemsa or calcofluor white can be used.
  • Trophozoite can reach CNS and/or other tissues, including bone via bloodstream over weeks to months.
Important Points:
  • Infection is more common in immunocompromised individuals. Granulomatous Amebic Encephalitis (GAE) is a chronic form of meningoencephalitis, developing over weeks to months, typically contrasting with the acute onset of PAM.
  • Onset of infection may take weeks to months with common infection routes being inhalation of contaminated dust or invasion of broken skin.
  • Cysts are resistant to chlorination and desiccation.
  • Acanthamoeba keratitis is commonly associated with improper contact lens care (e.g., swimming with contacts, using tap water to clean lenses). Symptoms include eye pain, redness, blurred vision, and light sensitivity.

Entamoeba histolytica (Amoeba)

  • Diseases caused:
    • Amebiasis: Infection of the intestinal tract by E. histolytica, ranging from asymptomatic colonization to invasive disease.
    • Amebic dysentery: Severe form of intestinal amebiasis characterized by frequent, bloody, and mucus-filled stools.
    • Amebic hepatitis (or liver abscess): If liver is involved, can lead to painful enlargement of the liver due to trophozoite dissemination from the gut.
Distribution:
  • Worldwide.
Method of Infection:
  • Infection from ingestion of resistant, infective quadrinucleate cysts passed in feces, which can be transmitted through feces, fingers, food, fomites, and flies. Upon ingestion, excystation occurs in the small intestine, releasing eight metacystic trophozoites from each cyst, leading to infection.
Diagnostic and Infective Stage:
  • Trophozoites found mainly in soft or fluid feces, while cysts show up in feces more frequently. Trophozoites are motile and show progressive, directional movement.
Major Pathology and Symptoms:
  • Asymptomatic presentation is possible, but may also include vague abdominal discomfort, malaise, diarrhea, and bloody dysentery.
  • Amebic hepatitis: presence of enlarged liver, fever, chills, leukocytosis (increased white blood cells).
  • Ingested red blood cells may be used to identify E. histolytica due to its ability to engulf them (phagocytosis of RBCs is a key diagnostic feature).
  • Symptoms can include up to 10 stools per day with blood-tinged mucus. Invasive amebiasis often characterized by flask-shaped ulcers developing in the colon as trophozoites invade the intestinal wall.
Diagnostic Methods:
  • Antigen-based immunoassays on fresh or frozen fecal samples; serologic methods useful for extraintestinal diseases. Microscopic examination of stool for trophozoites and cysts.
Cyst Characteristics:
  • Cysts typically contain 1-4 nuclei (mature cysts are quadrinucleate), with one nucleus being visible. Presence of cigar-shaped chromatoid bodies and a well-defined cyst wall.
Connection:
  • E. histolytica is now classified as a sexually transmitted disease (STD).
Treatment Options:
  • Medications include Metronidazole, Iodoquinol, and Paromomycin.

Other Entamoeba Species

Entamoeba hartmanni

  • Non-pathogenic, commensal organism. Often referred to as the 'small race' E. histolytica due to morphological similarities but smaller size. Cyst morphology: Cyst is cigar-shaped containing 1-4 nuclei, with an even chromatin pattern in nuclei.

Entamoeba coli

  • Generally a non-pathogenic amoeba but can be misidentified as E. histolytica. Cysts: Can contain up to 8 nuclei, with irregularly shaped karyosomes and splintered chromatoid bodies (if present). Trophozoites are larger and have a more eccentric karyosome.

Endolimax nana

  • Trophozoites: note the large, usually irregular karyosome in the nucleus, often described as a "keyhole" or "blot-like" shape.
  • Cysts: ovoid shape, usually containing 1-4 nuclei.

Balantidium coli

  • Ciliate that can cause balantidiasis (balantidial dysentery). It is the largest protozoan pathogen for humans. Common reservoir host is pigs.
  • Cysts can be recovered from feces, characterized by having a large, kidney-shaped macronucleus and a small, spherical micronucleus, essential for genetic recombination. Trophozoites are large and ovoid, covered in cilia.
  • Treated typically with Tetracycline.

Plasmodium species (Malaria)

  • Species include:
    • Plasmodium vivax (most prevalent)
    • Plasmodium ovale
    • Plasmodium malariae
    • Plasmodium falciparum (most deadly)
Life Cycle and Method of Infection:
  • Transmitted by Anopheles mosquitoes that inject sporozoites during a blood meal. Sporozoites infect liver cells (exo-erythrocytic cycle), maturing into schizonts that release merozoites. Merozoites then infect red blood cells (erythrocytic cycle), causing characteristic clinical symptoms.
  • Diagnostic staging includes identification of trophozoites, schizonts, or gametocytes in blood, often characterized by specific morphology in Giemsa-stained blood smears.
  • Seasonal and environmental conditions influence transmission rates and geographical distribution.
Symptoms and Effects:
  • Symptoms of malaria typically include cyclical prolonged fevers, chills, sweats (known as malarial paroxysms), and can lead to severe anemia and death if untreated. Severe complications include cerebral malaria, severe anemia, and multi-organ failure. P. falciparum is associated with the most severe forms and highest mortality.