FREE AMOEBA

Free-living amoebae

Amoebae that can exist independently in the environment or act as opportunistic parasites in humans

Opportunistic parasites

Microorganisms that cause disease mainly when host immunity is compromised

Common free-living amoebae

Naegleria fowleri, Acanthamoeba culbertsoni, Balamuthia mandrillaris

Natural habitat of free-living amoebae

Soil, freshwater, warm aquatic environments

Naegleria fowleri forms

Amoeboid (trophozoite), flagellate, and cyst

Infective stage of Naegleria fowleri

Trophozoite

Amoeboid (trophozoite) form

Feeding and dividing stage that moves using eruptive pseudopods

Eruptive pseudopods

Broad blunt cytoplasmic extensions that burst out rapidly for movement

Nuclear feature of Naegleria trophozoite

Single nucleus with large karyosome and no peripheral chromatin

Flagellate form of Naegleria

Temporary non-feeding biflagellate stage used for rapid movement

Function of flagellate stage

Helps amoeba move toward nutrient-rich environments

Cyst form of Naegleria

Spherical, resistant stage found in soil but never in human tissue

Thermophilic organism

Organism that thrives in warm or hot environments

Naegleria fowleri habitat

Warm freshwater such as lakes, ponds, hot springs, and swimming pools

Disease caused by Naegleria fowleri

Primary Amoebic Meningoencephalitis (PAM)

Primary Amoebic Meningoencephalitis

Acute, rapidly fatal infection of the brain and meninges

Mode of entry of Naegleria fowleri

Through the nasal cavity during swimming or diving

Route to the brain

Olfactory nerve through the cribriform plate to the olfactory bulb

Mechanism of tissue invasion

Production of lytic enzymes such as proteases, phospholipases, and hyaluronidase

Incubation period of PAM

1–14 days (average 5 days)

Early symptoms of PAM

Headache and fever

Late symptoms of PAM

Nausea, vomiting, stiff neck, confusion, coma

Cause of death in PAM

Increased intracranial pressure leading to brain herniation

Brain herniation

Displacement of brain tissue through rigid structures like the foramen magnum

Reason PAM is fatal

Compression of the medulla oblongata affecting respiration and circulation

CSF findings in PAM

Bloody CSF, increased protein, increased neutrophils, trophozoites present

Why neutrophils dominate in PAM

Infection mimics bacterial infection and IL-8 attracts neutrophils

Drug of choice for PAM

Amphotericin B

Adjunct drug for PAM

Miltefosine

Acanthamoeba culbertsoni

Ubiquitous free-living amoeba affecting CNS, eyes, skin, and lungs

Meaning of acantho

Spine or thorn

Life stages of Acanthamoeba

Trophozoite and cyst only

Acanthopodia

Spine-like projections used for movement and attachment

Cyst structure of Acanthamoeba

Double-walled with outer ectocyst and inner endocyst

Infective stages of Acanthamoeba

Both trophozoite and cyst

Routes of entry of Acanthamoeba

Corneal trauma, nasal passages, lungs, broken skin

Mode of CNS spread

Hematogenous spread

Reason Acanthamoeba reaches CNS from cornea

Spread through bloodstream despite cornea being avascular

Virulence factor of Acanthamoeba

Mannose-binding protein for attachment

Enzymes aiding tissue invasion

Cysteine protease, serine protease, metalloproteases

Amoebic keratitis

Corneal infection caused by Acanthamoeba

Major risk factor for amoebic keratitis

Contact lens use, especially sleeping with lenses

Symptoms of amoebic keratitis

Severe eye pain, blurred vision, corneal inflammation

Characteristic ring infiltrate

Ring-shaped corneal opacity due to peripheral inflammation

Cause of ring infiltrate

Central tissue destruction with fewer inflammatory cells

Diagnosis of amoebic keratitis

Corneal scraping showing trophozoites

Drug for amoebic keratitis

Polyhexamethylene biguanide with propamidine isethionate

Granulomatous Amoebic Encephalitis (GAE)

Chronic CNS infection caused by Acanthamoeba or Balamuthia

Granuloma

Collection of macrophages and lymphocytes due to persistent infection

GAE patient population

Immunocompromised or chronically ill individuals

CSF findings in GAE

Increased protein with lymphocytes and monocytes

Balamuthia mandrillaris

Free-living amoeba causing GAE and cutaneous infections

Route of entry of Balamuthia

Skin breaks or inhalation of cysts from dust

Trophozoite of Balamuthia

Amoeboid with long pseudopods and central karyosome

Cyst of Balamuthia

Double-walled with spherical inner wall

Treatment of Balamuthia infection

Same regimen as Acanthamoeba GAE