Acanthamoeba
Acanthamoeba is a genus of free-living amoebae commonly found in various environmental sources, including soil, fresh water, and air. These protozoa are opportunistic pathogens capable of causing serious human infections such as Acanthamoeba keratitis and granulomatous amoebic encephalitis (GAE). Due to their ability to survive in extreme conditions and resist common disinfectants, Acanthamoeba pose a significant public health concern.
Acanthamoeba species are ubiquitous and distributed worldwide. Infections are relatively rare but can be severe, particularly in immunocompromised individuals. Acanthamoeba keratitis is more prevalent among contact lens wearers, with an estimated US annual incidence of 1 to 2 cases per million contact lens users annually [6]. GAE primarily affects patients with compromised immune systems, such as those with HIV/AIDS, cancer, or undergoing immunosuppressive therapy.
Acanthamoeba exists in two forms:
- Trophozoite: The active, feeding, and dividing stage. It is irregular in shape, typically 15–45 µm in diameter, with characteristic spine-like projections called acanthopodia.
- Cyst: The dormant, double-walled form that allows the organism to survive harsh environmental conditions. Cysts are smaller, around 10–25 µm in diameter, and are highly resistant to temperature changes, desiccation, and chemical agents.
The life cycle of Acanthamoeba includes:
- Trophozoite stage: Actively feeds on bacteria and organic matter in the environment or host tissues.
- Encystment: Under adverse conditions, trophozoites transform into cysts, which are resilient and can remain viable for years.
- Excystment: When favorable conditions return, cysts revert to trophozoites.
In humans, infection can occur when trophozoites or cysts come into contact with the eye, nasal passages, or skin wounds, or through inhalation of airborne cysts.
Acanthamoeba causes disease by:
- Adhering to host cells using mannose-binding proteins.
- Producing cytopathic enzymes such as proteases that degrade tissue.
- Inducing inflammation and immune responses.
The organism can invade the cornea, skin, lungs, or central nervous system, leading to tissue necrosis and severe inflammation.
Acanthamoeba infections present in several clinical forms:
- Acanthamoeba keratitis: Painful eye infection marked by redness, photophobia, blurred vision, tearing, and a ring-shaped corneal infiltrate. Often associated with improper contact lens hygiene.
- Granulomatous Amoebic Encephalitis (GAE): A rare but fatal disease with subacute onset. Symptoms include headache, altered mental status, seizures, and focal neurological deficits.
- Cutaneous acanthamoebiasis: Chronic, ulcerative skin lesions primarily in immunocompromised individuals.
- Microscopy: Identification of trophozoites or cysts in stained corneal scrapings, cerebrospinal fluid (CSF), or biopsy samples.
- Culture: Non-nutrient agar with a lawn of Escherichia coli is used to support Acanthamoeba growth.
- Molecular methods: PCR-based assays are highly sensitive and specific for detecting Acanthamoeba DNA.
- Immunofluorescence and ELISA: Useful in research and for confirmation in clinical labs.
Treatment is often challenging due to the organism's resistance and the difficulty in achieving early diagnosis.
- Acanthamoeba keratitis: Combination therapy with topical agents such as chlorhexidine, polyhexamethylene biguanide (PHMB), and propamidine isethionate. Oral antifungals or antibiotics may be added.
- GAE: No standard treatment. Experimental regimens may include miltefosine, pentamidine, sulfadiazine, fluconazole, and azithromycin.
- Cutaneous infections: Require a combination of systemic and topical antimicrobials.
Early detection and aggressive therapy significantly improve outcomes.
For contact lens users:
- Proper hand hygiene before handling lenses.
- Avoid using tap water to rinse lenses or lens cases.
- Use recommended cleaning and disinfecting solutions.
- Avoid swimming or showering with contact lenses.
Environmental measures:
- Regular maintenance of water systems.
- Disinfection protocols in healthcare and dental units.
Public awareness is crucial for early diagnosis and treatment.
- Marciano-Cabral F, Cabral G. Acanthamoeba spp. as agents of disease in humans. Clin Microbiol Rev. 2003;16(2):273-307.
- Visvesvara GS, Moura H, Schuster FL. Pathogenic and opportunistic free-living amoebae: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, and Sappinia diploidea. FEMS Immunol Med Microbiol. 2007;50(1):1-26.
- Centers for Disease Control and Prevention (CDC). Acanthamoeba. https://www.cdc.gov/parasites/acanthamoeba/
- Dart JK, Saw VP, Kilvington S. Acanthamoeba keratitis: diagnosis and treatment update 2009. Am J Ophthalmol. 2009;148(4):487-499.e2.
- Schaumberg DA, Snow KK, Dana MR. The epidemic of Acanthamoeba keratitis: where do we stand? Cornea. 1998;17:3–10. 10.1097/00003226-199801000-00001
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