Acanthamoeba Keratitis

 Etiology and PathogenesisTop

1. Etiologic agent: Free-living amoebas of the Acanthamoeba species; typically A castellanii or A polyphaga.

2. Reservoir and transmission: Amoebas are commonly found in nature and present in warm natural fresh water reservoirs (ponds, lakes, geothermal springs, irrigation channels, retention tanks), artificial reservoirs (fountains, waterspouts, swimming pools, water parks, garden ponds), and moist soil. Humans are accidental hosts. Acanthamoeba spp are also found in some domestic and wild animals (dogs, cattle, sheep, beavers). The site of entry of the protozoan is corneal microinjuries in persons who wear contact lenses; it often dwells in contact lens disinfectant solutions.

3. Risk factors: Improper storage, disinfection, and rinsing of contact lenses.

4. Incubation and contagious period: Several weeks to up to a few months following infection. Human-to-human transmission is not thought to occur.

EpidemiologyTop

The prevalence of keratitis (the most common type of Acanthamoeba spp infection) is 1 to 33 cases per 1 million of contact lens users.

CLINICAL FEATURES AND NATURAL HISTORYTop

Chronic painful keratitis associated with a significant risk of loss of vision develops in contact lens users. The signs and symptoms are usually unilateral and include severe pain, lacrimation, photophobia, and redness. Eyelid edema may be present.

DIAGNOSISTop

Diagnostic Tests

1. Identification of the etiologic agent:

1) Microscopic examination of Giemsa-stained or Wright-stained corneal ulcer swabs or contact lens containers and storage solutions; characteristic findings include trophozoites surrounded by inflammatory infiltrates with the predominance of lymphocytes, plasma cells, histiocytes, and giant-cell granulomas. Motile protozoans can be identified in contact lens solution samples.

2) In vitro culture of the amoeba.

3) Molecular assays (polymerase chain reaction [PCR]).

2. Other: Corneal confocal microscopy (CCM; sensitivity ~90%).

Diagnostic Criteria

Diagnosis is confirmed by microscopic or molecular identification of Acanthamoeba spp.

Differential Diagnosis

Suppurative bacterial keratitis, viral keratitis, onchocerciasis, leprosy with ocular complications.

TREATMENTTop

Topical polyhexamethylene biguanide, chlorhexidine digluconate, hexamidine, propamidine isethionate, or silver halides. The treatment should last 6 to 12 months. The use of glucocorticoid eye drops is controversial. Corneal transplant is sometimes needed.

PROGNOSISTop

In patients with stromal infiltrates the prognosis of cure is poor. Favorable prognostic factors in terms of vision acuity following treatment include infection without significant vision worsening prior to treatment, acquiring infection when practicing water sports (swimming), unaffected corneal epithelium integrity, treatment with chlorhexidine, nonuse of glucocorticoids (initiation of glucocorticoids prior to antiparasitic treatment is associated with worse prognosis).

PREVENTIONTop

Specific Prevention

None available.

Nonspecific Prevention

Persons who wear contact lenses and practice water sports should remember about regular ophthalmic follow-up and develop proper lens storage, wearing, and removing habits.