What is acanthamoeba keratitis? | Symptoms, causes & treatment

Acanthamoeba keratitis is a vision-threatening corneal infection caused by free-living amoebae of the genus Acanthamoeba. It occurs most often in contact lens wearers with exposure to water or lapses in lens hygiene. Cardinal features are severe ocular pain (often out of proportion to clinical signs), photophobia, redness, and blurred vision. Diagnosis relies on a high index of suspicion plus corneal scraping for culture and/or polymerase chain reaction (PCR), supported by in vivo confocal microscopy. Treatment is prolonged topical antiamoebic therapy with biguanides, often combined with adjunctive agents; advanced cases may require keratoplasty.

Key Points

• Consider acanthamoeba keratitis in any contact lens wearer with severe pain, photophobia, and keratitis that is refractory to antibiotics or antivirals.
• Early disease may mimic viral or bacterial keratitis; classic ring infiltrate is a late finding.
• First-line therapy is intensive topical biguanides (polyhexamethylene biguanide 0.02% and/or chlorhexidine 0.02%), often for weeks to months.
• Topical corticosteroids are avoided initially; they may be added cautiously after antiamoebic coverage is established if inflammation is vision-threatening.
• Outcomes depend on time to diagnosis and depth of stromal involvement; delayed treatment increases the risk of scarring and keratoplasty.

Anatomy and Physiology

The cornea is an avascular, highly innervated tissue with five main layers: epithelium, Bowman layer, stroma, Descemet membrane, and endothelium. Dense subbasal corneal nerves account for pronounced pain with epithelial and anterior stromal disease. The tear film, epithelium, and intact contact lens hygiene form critical barriers to microbial adherence and invasion.

Etiology

• Acanthamoeba species are ubiquitous in soil, freshwater, tap water, hot tubs, and biofilms (including contact lens cases).
• Major risk factors:
  • Soft contact lens wear, especially overnight wear, poor case hygiene, infrequent case replacement, “topping off” solutions, homemade saline, and exposure to water (showering, swimming, hot tubs) while wearing lenses [1–4].
  • Corneal epithelial microtrauma (contact lens–related hypoxia or abrasion).
  • Non–contact lens corneal trauma with contaminated water or soil.

Pathophysiology

Trophozoites adhere to compromised epithelium and feed on corneal cells and bacteria. They secrete proteases that degrade the epithelial barrier and stromal collagen. Under stress (e.g., therapy), trophozoites encyst; cysts have a double wall that confers marked resistance to many antimicrobials and disinfectants. Perineural spread along corneal nerves produces radial keratoneuritis and disproportionate pain. In advanced disease, stromal inflammation leads to a ring-shaped infiltrate, thinning, and possible perforation. Acanthamoeba-associated scleritis may occur and can be severe.

Epidemiology

• Most U.S. cases occur in contact lens wearers. Incidence estimates vary by region and over time; outbreaks have been linked to lens solution failures and water exposure [2,3]. Precise current U.S. incidence is uncertain and varies across studies.
• Non–contact lens cases comprise a minority and are typically associated with corneal trauma and environmental exposure [1,4].

Classification

• By disease stage and depth:
  • Epithelial punctate keratopathy and pseudodendrites (early).
  • Anterior stromal perineural infiltrates (radial keratoneuritis).
  • Stromal ring infiltrate (intermediate to late).
  • Sclerokeratitis and corneal thinning/perforation (advanced).
• By risk context: contact lens–related vs non–contact lens–related (traumatic/environmental).
• By laterality: typically unilateral; bilateral disease is uncommon.

Symptoms and Signs

• Common across stages

  • Severe ocular pain out of proportion to clinical signs
  • Photophobia, tearing, redness, blurred vision
  • Foreign body sensation, blepharospasm

• Early epithelial disease

  • Punctate epithelial erosions, epithelial irregularity
  • Pseudodendrites that may mimic herpes simplex but stain poorly with fluorescein/rose bengal
  • Small epithelial defects; minimal discharge

• Perineural/anterior stromal disease

  • Radial perineural infiltrates (radial keratoneuritis)
  • Focal anterior stromal haze or infiltrates
  • Reduced corneal sensitivity may be less marked than in herpes simplex

• Stromal/ring stage

  • Ring-shaped stromal infiltrate (classic but not pathognomonic)
  • Stromal edema, Descemet folds, endothelial dysfunction
  • Hypopyon in severe cases

• Complicated disease

  • Scleritis with severe, boring pain
  • Corneal thinning and perforation
  • Secondary glaucoma from inflammation

Complications

• Permanent corneal scarring and irregular astigmatism
• Corneal thinning and perforation
• Secondary bacterial or fungal superinfection
• Scleritis, occasionally necrotizing
• Need for therapeutic or optical keratoplasty
• Chronic ocular pain and photophobia
• Recurrent disease due to cyst survival

Diagnosis

Clinical evaluation

• Maintain high suspicion in contact lens wearers with severe pain and keratitis unresponsive to antibacterial/antiviral therapy.
• Elicit lens hygiene and water exposure history (shower, swim, hot tub) and case replacement practices.
• Slit-lamp findings: pseudodendrites, perineural infiltrates, or stromal ring; epithelium may be relatively intact early.

Imaging and laboratory testing

• Corneal scraping for:
  • Non-nutrient agar culture with Escherichia coli overlay (cyst/trophozoite growth) [1,5].
  • Stains (calcofluor white, Giemsa) for cysts/trophozoites.
  • PCR for Acanthamoeba DNA (high sensitivity/specificity; useful when culture is negative or slow) [1,5].
• In vivo confocal microscopy (IVCM): visualizes highly reflective double-walled cysts and trophozoites in vivo; supportive and expedites diagnosis, especially in deep stromal disease [1,5].
• Consider bacterial and fungal cultures concurrently due to overlapping presentations and potential coinfection.

Differential diagnosis

Treatment

Early initiation and sustained therapy improve outcomes. Management is typically off-label and requires access to compounding pharmacies in the U.S.

Medical management

• Topical biguanides (first-line; cysticidal)
  • Polyhexamethylene biguanide (PHMB) 0.02%: 1 drop hourly around the clock for 48–72 hours, then taper to q1–3h while awake; continue for weeks to months guided by response [1,5,6].
  • Chlorhexidine 0.02%: similar dosing; may be used in combination with PHMB [1,5,6].
  • Both agents are not commercially bottled for ophthalmic use in the U.S.; require extemporaneous compounding (off-label).
• Diamidines (adjunct; cysticidal)
  • Propamidine isethionate 0.1% or hexamidine 0.1%: add q1–3h if available; U.S. availability is limited and typically via compounding; off-label [1,5].
• Debridement
  • Gentle epithelial debridement at initiation can reduce organism load and improve drug penetration (repeat cautiously to limit toxicity) [1,5].
• Cycloplegics/analgesia
  • Atropine 1% bid–tid or cyclopentolate 1% tid for photophobia/ciliary spasm.
  • Systemic analgesics as needed.
• Topical corticosteroids (controversial)
  • Avoid during the first 2–3 weeks of antiamoebic therapy.
  • If inflammation is vision-threatening (e.g., scleritis, dense stromal inflammation), introduce a low-dose topical corticosteroid (e.g., prednisolone acetate 1% qd–bid) only after clear clinical response to antiamoebics, with close monitoring for recrudescence [1,5,6].
• Antifungals/azoles
  • Limited evidence for adjunctive azoles (e.g., voriconazole) against Acanthamoeba; use is off-label and specialist-directed [5].
• Systemic agents (refractory disease)
  • Oral miltefosine (FDA-approved for leishmaniasis) has been used off-label for refractory acanthamoeba keratitis and sclerokeratitis; consult cornea and infectious disease specialists due to variable dosing regimens and adverse effect profile [7].

Duration: Intensive therapy typically maintained for at least 2–4 weeks, followed by a slow taper over several months to reduce relapse risk. Recurrences may occur if cysts persist; re-escalation is common.

Procedural and surgical management

• Therapeutic deep anterior lamellar keratoplasty (DALK) or penetrating keratoplasty (PK) for:
  • Imminent/perforated cornea
  • Medically uncontrolled infection
  • Dense central scarring impairing vision after quiescence
• Continue antiamoebic therapy before and after keratoplasty to reduce recurrence from residual cysts [1,5].
• Amniotic membrane transplantation may be used as a surface adjunct in select cases; it does not treat infection.
• Glaucoma management if intraocular pressure is elevated.

Special populations

• Pediatric: Rare; similar management principles. Emphasize caregiver-supervised lens hygiene if contact lenses are used.
• Pregnancy/lactation: Topical biguanides have minimal systemic absorption; use only if potential benefit justifies potential risk; consult obstetrics for systemic agents.
• Immunocompromised: Keratitis usually occurs in immunocompetent hosts; however, scleritis and extraocular disease may be more severe. Involve infectious disease if dissemination is suspected.

Prognosis

Prognosis correlates with disease stage at diagnosis and timeliness of antiamoebic therapy. Epithelial-limited disease treated early often resolves with good vision. Stromal/ring-stage disease carries higher risks of scarring, protracted treatment, and need for keratoplasty. Recurrence can occur during taper. Scleritis portends a guarded visual prognosis [1,5,6].

Prevention and Patient Counseling

• Do not expose contact lenses or cases to water (no showering, swimming, or hot tubs with lenses in).
• Wash and dry hands before handling lenses.
• Rub and rinse lenses with appropriate solutions; avoid “topping off.”
• Replace lens cases at least every 3 months; air-dry daily.
• Adhere to wearing schedules; avoid overnight wear unless specifically prescribed.
• Consider daily disposable lenses to reduce biofilm risk.
• Seek prompt care for painful red eye that does not improve within 24 hours.

See general overview at keratitis for broader context: https://myvisioncare.org/disease/keratitis.

When to Seek Immediate Care

• Sudden or severe eye pain, especially with light sensitivity or blurred vision
• A red eye that worsens or does not improve within 24 hours of stopping contact lens wear
• White spot on the cornea or a ring-shaped corneal haze
• Decreased vision or new floaters
• Severe headache with nausea/vomiting and eye pain
• Recent eye injury with soil or water exposure

Call 911 for severe eye pain with sudden vision loss, penetrating eye injury, or if the eye appears ruptured.

References

1. EyeWiki. Acanthamoeba Keratitis. American Academy of Ophthalmology. https://eyewiki.aao.org/Acanthamoeba_Keratitis. Accessed July 24, 2026.
2. CDC. Acanthamoeba Keratitis Multiple States, 2005–2007. MMWR. 2007;56(21):532–534. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5621a1.htm.
3. Joslin CE, et al. Acanthamoeba keratitis: risk factors and clinical presentation..
4. Dart JKG, Saw V, Kilvington S. Acanthamoeba keratitis: diagnosis and treatment update. Am J Ophthalmol. 2009;148(4):487–499.e2. doi:10.1016/j.ajo.2009.06.009..
5. StatPearls. Acanthamoeba Keratitis. Treasure Island (FL): StatPearls Publishing; updated 2024. https://www.ncbi.nlm.nih.gov/books/NBK549903/.
6. American Academy of Ophthalmology Preferred Practice Pattern. Infectious Keratitis PPP (2023)..
7. FDA. Impavido (miltefosine) Prescribing Information. https://www.accessdata.fda.gov/drugsatfda_docs/label/2014/204684s000lbl.pdf.

Disclaimer: This article is for informational purposes and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified eye care professional about your specific condition. If you have sudden vision loss, severe eye pain, or an eye injury, seek emergency care immediately.