Microbial keratitis (Acanthamoeba sp.)


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Acanthamoebae are ubiquitous free-living protozoans, present in:

  • well water, drains, soil, dust
  • often present in domestic tap water (especially from storage tanks)

Can exist in two forms

  • motile, feeding and replicating form: trophozoite (most common form found in water and easily destroyed)
  • dormant form: cyst (highly resistant to disinfection, can survive for long periods in hostile environments)

Acanthamoeba keratitis, which may be caused by at least eight different species, is rare in the general population (estimated annual incidence: 1.4 per million per annum), even though most people have Acanthamoeba antibodies, suggesting that exposure is commonplace. It is much more common in contact lens wearers.
In the UK, higher incidence is found in hard water districts and where bathrooms are supplied by tank-stored water.
In 10% of cases there is associated scleritis. Acanthamoeba sclerokeratitis is associated with a poor clinical outcome.

Predisposing factors

Contact lens wear is associated with the majority of cases of Acanthamoeba keratitis (71-91%)

  • majority are soft lenses (particularly reusable or extended wear). Recently published case series suggest a potential increased risk of AK in orthokeratology users compared with other lens modalities
  • inadequate disinfection
  • use of non-sterile solutions
  • tap water rinsing of lenses and/or storage cases
  • contamination of storage case with bacteria and fungi (± biofilm) which provide substrate for Acanthamoebae
  • exposure to shower, pool, or hot tub water

Corneal trauma with exposure to soil or contaminated water.

Symptoms of microbial keratitis

  • pain (may be severe and out of proportion to degree of ocular inflammation; may also be painless in the early stages)
  • visual disturbance/loss
  • redness
  • epiphora
  • photophobia

Can be bilateral
May be a long history and condition may be misdiagnosed as herpetic, bacterial or fungal keratitis
NB in earliest stages, pain may be minimal

Signs of microbial keratitis

Early signs

  • epithelial or subepithelial infiltrates
  • pseudodendrites
  • radial keratoneuritis (infiltrates along corneal nerves)
  • recurrent breakdown of the corneal epithelium

Later signs

  • deep inflammation of the cornea consisting of a central or paracentral ring-shaped or disciform infiltrate or abscess
  • stromal thinning
  • extension of inflammation into sclera
  • anterior chamber cells and flare
  • hypopyon

Differential diagnosis

Signs may masquerade as herpes simplex with temporary improvement on anti-herpetic treatment, further delaying diagnosis

Suspect any painful epitheliopathy that:

  • does not respond to normal treatment
  • has known risk factor (e.g. contact lens wear or corneal trauma associated with soil or contaminated water)

(NB: Dendritic keratitis in a contact lens wearer should raise the index of suspicion of an Acanthamoeba infection)

Bacterial or fungal keratitis 
Concurrent bacterial or fungal infection can occur

Management by optometrist

Practitioners should recognise their limitations and where necessary seek further advice or refer the patient elsewhere

GRADE* Level of evidence and strength of recommendation always relates to the statement(s) immediately above

Non pharmacological

Cease contact lens wear immediately (both eyes)
(GRADE*: Level of evidence=low, Strength of recommendation=strong)



Management category

A1: emergency (same day) referral to ophthalmologist without intervention. Acanthamoeba keratitis can be difficult to treat; therefore prompt, aggressive therapy is vital. Delay is associated with a poorer prognosis. Emphasise to the patient the urgency of the condition and instruct them to attend the local hospital eye department or hospital A&E the same day, explaining that you will leave a message so that they are expected. Telephone the department to explain what you have done,
preferably leaving your message with a doctor or other health care professional. Advise patient to take lenses and lens case for possible culture.

Possible management by ophthalmologist

Diagnosis will usually be confirmed by histology (corneal scrape) and culture or PCR. Cystic form can also be imaged by confocal microscopy.
Intensive (day and night) topical medical treatment with either a biguanide or a diamidine or a combination of the two:
Biguanides: polyhexamethylene biguanide (PHMB), chlorhexidine.
Diamines: propamidine, dibromopropamidine, hexamidine (not available as commercial licensed ophthalmic products).
PHMB is sometimes prescribed as topical monotherapy.
The anti-fungal drug voriconazole is sometimes prescribed as topical monotherapy.
Collagen cross-linking therapy has been used as therapy in Acanthamoeba keratitis.
Continuous treatment may be necessary for weeks or months
(A 2015 Cochrane systematic review identified a paucity of evidence to inform robust conclusions for treating AK in practice).
Systemic analgesia as necessary.
Topical steroid to limit inflammation.
Topical antibiotics as necessary for secondary bacterial infection.
Penetrating keratoplasty if corneal irregularity, thinning and/or scarring is severe following complete control of infection.

Evidence base

*GRADE: Grading of Recommendations Assessment, Development and Evaluation (www.gradeworkinggroup.org)

Sources of evidence

Alkharashi M, Lindsley K, Law HA, Sikder S. Medical interventions for acanthamoeba keratitis. Cochrane Database Syst Rev. 2015;2:CD010792

Bagga B, Sharma S, Gour RPS, Mohamed A, Joseph J, M Rathi V, Garg P. A randomized masked pilot clinical trial to compare the efficacy of topical 1% voriconazole ophthalmic solution as monotherapy with combination therapy of topical 0.02% polyhexamethylene biguanide and 0.02% chlorhexidine in the treatment of Acanthamoeba keratitis. Eye (Lond). 2021;35(5):1326-1333

Carnt N, Stapleton F. Strategies for the prevention of contact lens-related Acanthamoeba keratitis: a review. Ophthalmic Physiol Opt. 2016;36(2):77-92

Carrijo-Carvalho LC, Sant'ana VP, Foronda AS, de Freitas D, de Souza Carvalho FR. Therapeutic agents and biocides for ocular infections by free-living amoebae of Acanthamoeba genus. Surv Ophthalmol. 2017;62(2):203-218

Cope JR, Collier SA, Schein OD, Brown AC, Verani JR, Gallen R, Beach MJ, Yoder JS. Acanthamoeba keratitis among rigid gas permeable contact lens wearers in the United States, 2005 through 2011. Ophthalmology. 2016;123(7):1435-41.

Iovieno A, Gore DM, Carnt N, Dart JK. Acanthamoeba Sclerokeratitis: epidemiology, clinical features, and treatment outcomes. Ophthalmology 2014;121(12):2340-7

Lorenzo-Morales J, Khan NA, Walochnic J. An update on Acanthamoeba keratitis: diagnosis, pathogenesis and treatment. Parasite. 2015;22:10

Niederkorn JY The biology of Acanthamoeba keratitis. Exp Eye Res. 2021;202:108365

Ross J, Roy SL, Mathers WD, Ritterband DC, Yoder JS, Ayers T, Shah RD, Samper ME, Shih CY, Schmitz A, Brown AC. Clinical characteristics of Acanthamoeba keratitis infections in 28 states, 2008 to 2011.  Cornea. 2014;33(2):161-8

Scanzera AC, Tu EY, Joslin CE Acanthamoeba keratitis in minors with orthokeratology (OK) lens use: A Case Series. Eye Contact Lens. 2021;47(2):71-73


What is Acanthamoeba?

Acanthamoeba is a protozoan (single-celled organism) that is very widespread throughout the environment, especially where there is standing water, and it may be present in inadequately treated tap water. It has two forms: the trophozoite, which is active, capable of feeding, moving and reproducing, and which is easily destroyed; and the cyst, which is dormant and difficult to destroy. Acanthamoeba can change between these two forms, depending on whether it is in a favourable or a hostile environment.

Acanthamoeba is normally harmless to humans, but if it is transferred to the eye on a contaminated contact lens it can infect the cornea (the clear window at the front of the eye). Such infections can be difficult to treat owing to the lack or non-availability of anti-amoebic drugs. It is far better to prevent the infection by the use of effective contact lens hygiene, in particular avoiding contact of the lens and lens case with tap water.

Patients with early Acanthamoeba keratitis usually complain of discomfort, redness and light sensitivity of the affected eye. In the later stages the eye can become very painful as the nerves and deeper parts of the cornea become affected.

How is Acanthamoeba Keratitis managed?

An optometrist who suspects such an infection is advised to refer the patient as an emergency (same day) to the ophthalmologist, who will try to confirm the diagnosis and then prescribe special eye drops given day and night. Often the patient will be admitted to hospital. If there is much scarring of the cornea following the eventual elimination of the infection, and vision is badly affected, a corneal transplant may be recommended.

Microbial keratitis (Acanthamoeba sp.)
Version 13
Date of search 15.12.21
Date of revision 24.03.22
Date of publication 06.07.22
Date for review 14.12.23
© College of Optometrists