Acanthamoeba

Background

Microbiology

• One of the free-living amoebae
• Seventeen genotypes (T1-T17), of which 10 have caused disease in humans
• Most common is Acanthamoeba castellanii complex (T4)
• Species traditionally organized morphologically into 3 groups

Group	Species	Trophozoite Size	Cyst Size
I	A. astronyxis, A. comandoni, A. tubiashi, A. byersi	30-125 μm	16-35 μm
II	A. castellanii, A. polyphaga, A. rhysodes, A. hatchetti	15-45 μm	≤18 μm
III	A. culbertsoni, A. royreba, A. lenticulata	15-45 μm	<18 μm with different morphologies

Life Cycle

• Cysts and trophozoites can cause disease

Epidemiology

• More common in immunocompromised patients, with risk factors including AIDS, liver disease, diabetes, organ transplantation, corticosteroid use, chemotherapy, and rituximab

Clinical Manifestations

Granulomatous amebic encephalitis

• Subacute presentation of encephalitis (fever, headache, nuchal rigidity, behaviour changes, altered level of conciousness, focal neurological deficiets)
• Often preceded by cutaneous lesions by months
  • May include ulcers, nodules, and subcutaneous abscesses
  • Amoebic granulomas on biopsy

Non-neurological disseminated acanthamoebiasis

• Rarely, patients may have disseminated disease without evidence of CNS involvement
• Mostly seen in patients with AIDS, transplant, or prolonged corticosteroid use
• Most commonly involves skin, but can involve liver, lungs, and bones

Amoebic keratitis

• Sight- and eye-threatening infection
• Often secondary to corneal trauma or improper contact lens hygiene
• Present with severe pain, photophobia, tearing
• May see dendiform epitheliopathy or stromal ring

Diagnosis

• Ameba culture on nutrient agar with layer of gram-negative bacteria at 30ºC (cornea/skin) or 37–C (brain/lung)
• Species identification done with 18S rDNA sequencing

Management

Granulomatous amebic encephalitis

• Can consider a combinaction of pentamidine, azoles, flucytosine, sulfadiazine, miltefosine, amikacin, and voriconazole

Amoebic keratitis

• Can be treated with any of:
  • PHMB
  • Chlorhexadine
  • Topical and oral voriconazole
  • Propamidine
  • Hexamidine