Free-Living Amoebae
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Direct answer
Free-living amoebae are environmental protozoa that only occasionally infect humans, and the three important genera are Naegleria, Acanthamoeba and Balamuthia. Naegleria fowleri causes rapidly fatal primary amoebic meningoencephalitis after water enters the nose in warm fresh water. Acanthamoeba causes keratitis in contact-lens wearers and granulomatous encephalitis in the immunocompromised, while Balamuthia causes granulomatous encephalitis with possible skin lesions.
What you must remember
- Naegleria fowleri: enters through the nasal mucosa during swimming or nasal irrigation in warm fresh water, then travels along the olfactory nerve to the brain.
- Primary amoebic meningoencephalitis: an acute, fulminant meningoencephalitis in previously healthy swimmers, with motile trophozoites seen on a wet mount of CSF.
- Naegleria culture: grows on non-nutrient agar seeded with a lawn of bacteria such as Escherichia coli or Klebsiella.
- Acanthamoeba keratitis: a painful corneal ulcer, often with a ring-shaped infiltrate, in contact-lens wearers, confirmed by corneal scraping and culture.
- Acanthamoeba encephalitis: granulomatous amoebic encephalitis in immunocompromised people, with a subacute course.
- Balamuthia mandrillaris: granulomatous amoebic encephalitis in immunocompromised and occasionally healthy people, sometimes with skin lesions, diagnosed by PCR or immunofluorescence.
- Treatment: amphotericin B combined with miltefosine for Naegleria, and prolonged topical biguanide or diamidine therapy for Acanthamoeba keratitis.
A swimmer with fever and neck stiffness
A teenager develops high fever, headache and neck stiffness four days after swimming in a warm river, and the CSF shows a predominance of neutrophils.
- Suspect Naegleria in the right setting: warm fresh water exposure, recent nasal entry and a rapid meningitis picture point to primary amoebic meningoencephalitis.
- Examine fresh CSF promptly: a wet mount is examined for motile trophozoites, since the forms become difficult to see once the sample cools or degrades.
- Confirm by PCR and culture: PCR confirms the organism, and culture on non-nutrient agar seeded with bacteria grows the amoeba.
- Start empirical therapy early: amphotericin B combined with miltefosine is started without waiting for the final result, because survival depends on early treatment.
- Consider Acanthamoeba for a contact-lens ulcer: a painful ulcer with a ring infiltrate in a lens wearer is confirmed by corneal scraping, and treatment needs prolonged topical biguanide or diamidine therapy.
Each step moves from exposure history to laboratory proof, and treatment starts before the result when the picture is typical.
Where the exam tries to catch you
Free-living amoebae questions test the exposure story, the organ involved and the drug logic.
- Trap: treating an Acanthamoeba ulcer like bacterial keratitis Correct: antibacterial drops fail, and prolonged topical antiamoebic therapy with biguanides or diamidines is needed.
- Trap: dismissing a fulminant meningitis in a swimmer as bacterial Correct: Naegleria primary amoebic meningoencephalitis is rapidly fatal, and a CSF wet mount with early antiamoebic therapy is the key step.
- Trap: assuming these amoebae threaten only the immunocompromised Correct: Naegleria infects previously healthy swimmers, while Acanthamoeba keratitis mainly affects contact-lens users.
Frequently asked questions
Why is Naegleria fowleri infection almost always fatal?
Naegleria fowleri causes a rapid, destructive meningoencephalitis within days of exposure, and the brain is heavily involved by the time symptoms become obvious. Treatment is rarely effective. Early suspicion and combined antiamoebic therapy offer the only realistic hope.
How does Acanthamoeba keratitis differ from bacterial keratitis?
Acanthamoeba keratitis causes severe pain that is out of proportion to the findings, often with a ring-shaped corneal infiltrate in contact-lens wearers. Bacterial keratitis responds to standard antibiotic drops, which fail in Acanthamoeba keratitis. Corneal scraping and culture confirm the diagnosis.
What does Balamuthia mandrillaris cause?
Balamuthia mandrillaris causes granulomatous amoebic encephalitis in immunocompromised and occasionally healthy people, and it may produce skin lesions. It is diagnosed by PCR or immunofluorescence on tissue. Its clinical course is typically subacute, unlike the fulminant illness caused by Naegleria fowleri.
Where does Naegleria fowleri live?
Naegleria fowleri lives in warm fresh water, soil and sediment, in lakes, rivers and poorly maintained pools. Infection follows water entering the nose during swimming or nasal rinsing. Its presence in a water source is the key exposure history in exam questions.