SN3 SHORT NOTE 5M ★ High-yield

Describe the free-living amoebae and the diseases they cause.

Microbiology · 2nd Professional — Para-Clinical

Naegleria Fowleri

A free-living amoeboflagellate found in warm fresh water — ponds, lakes, poorly chlorinated swimming pools, hot springs and warm water pipes. It thrives at temperatures up to 45°C, so infection peaks in summer
Three forms — the amoeboid trophozoite (the invasive form), a transient flagellate form, and a cyst
Entry is through the nose during swimming, diving or nasal irrigation with contaminated water, including ritual ablution and neti pot use with tap water. The amoeba penetrates the cribriform plate and travels along the olfactory nerves directly to the brain
Primary amoebic meningoencephalitis (PAM) — a fulminant, rapidly progressive purulent meningoencephalitis in a previously healthy young person, beginning with severe headache, fever and disturbance of smell or taste (reflecting the olfactory route), and progressing to coma and death within 3 to 7 days
Mortality exceeds 95%, with only a handful of documented survivors worldwide
Diagnosis — cerebrospinal fluid resembles bacterial meningitis (neutrophils, high protein, low glucose) but with NO organisms on Gram stain and a sterile culture. The diagnosis is made by seeing motile amoebae in a wet mount of fresh, uncentrifuged, unrefrigerated cerebrospinal fluid — which requires that someone thinks to look
Treatment — amphotericin B with miltefosine, rifampicin and azoles; and targeted temperature management. Outcomes remain poor

CLINICAL PEARL

Neutrophilic meningitis with no organisms and a sterile culture in a healthy young swimmer should prompt a wet mount of fresh cerebrospinal fluid. The picture is indistinguishable from partially treated bacterial meningitis, which is what it is nearly always called. Naegleria is only found by looking for movement in an unspun, unchilled specimen — centrifugation and refrigeration both destroy the amoebae, so the routine handling of the sample is what conceals the diagnosis.

Acanthamoeba and Balamuthia

Feature	Acanthamoeba	Balamuthia mandrillaris
Habitat	Ubiquitous — soil, dust, fresh and salt water, air conditioning units, contact lens solutions and tap water	Soil and dust
Forms	Trophozoite with spiny acanthopodia; a double-walled polygonal cyst	Trophozoite and cyst
Route	Through the cornea, or by inhalation and haematogenous spread — not through the cribriform plate	Haematogenous from skin or lung
Host	Immunocompetent for keratitis; immunosuppressed for encephalitis	May affect the immunocompetent
Disease	Acanthamoeba keratitis — a severe, exquisitely painful, sight-threatening corneal infection with a ring infiltrate, seen almost entirely in contact lens wearers who use tap water or homemade saline. Also granulomatous amoebic encephalitis — a subacute illness over weeks to months	Granulomatous amoebic encephalitis, often with a preceding skin lesion
Diagnosis	Corneal scraping with culture on non-nutrient agar seeded with E. Coli, on which the amoebae leave visible tracks as they feed; confocal microscopy; PCR	Biopsy and PCR
Treatment	Prolonged topical biguanides (polyhexamethylene biguanide) and diamidines; keratoplasty if it fails	Multidrug regimens; poor outcome

Comparison and Applied Aspects

Feature	Naegleria (PAM)	Acanthamoeba (GAE)
Host	Healthy young person	Immunosuppressed, chronically ill, or diabetic
Route of entry	Nose, through the cribriform plate along olfactory nerves	Lung or skin, then haematogenous
Onset and course	Fulminant — death in 3 to 7 days	Subacute or chronic — weeks to months
CSF picture	Neutrophilic, resembling bacterial meningitis	Lymphocytic, resembling tuberculous meningitis
Cysts in tissue	Absent — trophozoites only	Present — both trophozoites and cysts
Exposure	Swimming or nasal irrigation in warm fresh water	Contact lenses (keratitis); often none identified

Never rinse contact lenses or their cases in tap water, and never use homemade saline; this single instruction prevents most Acanthamoeba keratitis, and it must be given explicitly to every new lens wearer
Suspect Acanthamoeba in a contact lens wearer with keratitis and pain out OF proportion to the signs, particularly one not responding to antibacterial or antifungal drops; delay leads to corneal destruction
Use only boiled, distilled or sterile water for nasal irrigation and for ritual ablution; deaths have followed the use of tap water in a neti pot
Ask about swimming in warm fresh water in a young patient with fulminant meningitis, and request a wet mount of fresh unspun cerebrospinal fluid
These are free-living organisms that infect man incidentally, so there is no person-to-person transmission and no need for isolation — but the environmental source may affect others and should be identified
Chlorination at swimming pool levels kills Naegleria, so adequately maintained pools are safe; the risk lies in natural warm water and in poorly maintained facilities

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