Review Article

REVIEW ARTICLE

Acanthamoeba Keratitis : A Review
Pawan Prasher, Parineeta Sachdeva, Ravinder Nath Bhatnagar, Sachin Walia

Abstract:  Acanthamoeba is a ubiquitous, free-living protozoa that causes a serious and troublesome keratitis.  Acanthamoeba Keratitis continues to be a burgeoning and unsolved problem.1 Although soft contact lens wear is reported as the major risk factor in other parts of the world, reports from India suggest that Acanthamoeba Keratitis is more common among non-contact lens wearers.  Because it can lead to loss of useful vision, an increased awareness of this disease entity is essential.  Early suspicion and diagnosis may improve the medical and surgical outcome of this devastating disease.

Key words: Acanthamoeba, Keratitis, epidemic, contact lens, non-healing.

INTRODUCTION

The name Acanthamoeba comes from the Greek where acantho means curled referring to the spear shaped pseudopodia of the trophozoite.  Acanthamoeba are free-living, harmless organisms, however given the opportunity and the appropriate conditions, they can cause painful, sight-threatening as well as fatal infections and thus are considered opportunistic pathogens.2 At this moment there are more than 35 species known (based on cyst morphology, immunofluorescence with antibodies and on isoenzyme structure), among which possible causative agents for Acanthamoeba Keratitis are: A.castellani, A. polyphaga, A. hatchetii, A. culbertsoni, A.rhysodes and A. griffini.3 Acanthamoeba infections have become increasingly important in the past few years due to increasing populations of contact lens users and AIDS patients.2  

From a historical perspective Acanthamoeba Keratitis has been described as a recent epidemic.  It was extremely rare before the widespread use of contact lenses.  The first case of Acanthamoeba Keratitis, that involved Acanthamoeba polyphaga, was reported in 1974 when a Texas rancher splashed tap water from a contaminated river source into his eye.4 Very little is known about incidence of Acanthamoeba Keratitis before 1970s. The number of cases started to increase dramatically beginning in 1984, and by 1985, an association with the use of contact lenses was established, especially among individuals5 who used to swim while wearing their contact lenses and those who used home made saline. It is interesting to note that thiomersol, a mercury-based preservative used in contact lens solutions, was increasingly withdrawn from use at the same time owing to reports of thimerosal-related superior limbic keratoconjunctivitis and other allergic reactions.5

Acanthamoeba species have been isolated from many different sources, such as freshwater, seawater, chlorinated water from swimming pools, dental treatment units, and contact lens cases. Most of the strains found are not pathogenic.  Some pathogenic forms are known to survive for extended periods in fresh water.  Protozoa, in general, become airborne when encysted. The presence of pathogenic Acanthamoeba organisms in the atmosphere is an important factor in the prevalence of Acanthamoeba Keratitis, although this is not its main cause. The reported incidence of Acanthamoeba Keratitis in India varies from 1-3% of all keratitis in various published and unpublished series. Interestingly, unlike other parts of the world where use of the soft contact lenses is reported as major risk factor, reports from India suggest that of all the patients affected by Acanthamoeba Keratitis, majority are non contact lens users.6

RISK FECTORS

The main risk factors for Acanthamoeba Keratitis are wearing contact lenses (daily, extended-wear, rigid gas permeable [RGP], and PMMA), history of corneal trauma, non-sterile contact lens rinsing, omitted or chlorine based disinfection that has little protective action against the organism and swimming while wearing contact lenses.  Previous corneal oedema and exposure to contaminated substances increase the risk further. Over 6 80% of Acanthamoeba Keratitis could be avoided by use of contact lens disinfection systems that are effective against the organism.7

PATHOGENESIS

The life cycle of Acanthamoeba consists of two forms: trophozoites and cysts.  The trophozoites are the active, proliferating forms, which under adverse circumstances (dehydration, lack of food and contact with toxic substances)turn into cysts, which are the resistant, resting forms of the parasite.  the cysts reverse to trophozoite form under favourable circumstances.

Although Acanthamoeba are ubiquitous in nature, yet the incidence of keratitis is rather low.  The explanation proposed is that either the Acanthamoeba are weak pathogens or the cornea under normal circumstances forms an adequate barrier.2

Men and women are equally affected and the majority of cases are unilateral. Of individuals iwth Acanthamoeba Keratitis, 85% wear contact lenses; abrasion of the cornea is implicated.  The contact lens, when placed on the eye, can introduce the pathogen through an abrasion previously caused by the contact lens.1 It has been found out that more cysts and trophozoites adhere to the unwashed lenses than to the washed lenses. Trophozoites thatadhere to the lenses have surface projections (acanthamoeba, filopodia, and lobopodia) where as adherent cysts have been found to have wrinkled ectocysts.  These rough surfaces provide the means by which cysts adhere to the lens surface.8 

The first step in the infection involves adhesion of the trophozoite to the corneal epithelium.  Pathogenic strains of Acanthamoebai have been found to produce a variety of proteases, which facilitate corneal invasion, resulting in parasite-mediated cytolysis of the 7 cornea. The stromal disease occurs later.  The infection causes destruction of the corneal epithelium and stroma, followed by an infiltration of inflammatory cells and eventually formation of descemetocoele and perforation.  Limbitis and scleritis can also occur, either by an immunological reaction secondary to primary corneal infection or by direct spread of infection from the cornea.2

Interaction of Acanthamoeba with biofilms and hydrogel lenses: Free-living amobae can grow successfully as commensals and parasites, particularly with gram-negative bacteria (e.g. Escherichia coli) that naturally exist as part of the external eye flora. Acanthamoeba organisms are also known to be part of the natural eye flora in non-contact lens wearers. Contact lenses act to increase the number of amebic trophozoites and cysts in the eye. It could be that other naturally occurring bacteria produce a symbiotic relationship that could favour amoebic infection.

It has been found that hydrogel contact lenses are particularly suitable for supporting the growth of biofilms of Pseudomonas aeruginosa. This biofilm, in turn, increases the adsorption of Acanthamoeba organisms to the lens, which suggest that contact lenses that are already contaminated with a bacterial biofilm provide an increased chance of development of Acanthamoeba Keratitis.1

CLINICAL FEATURES 

The most striking feature of Acanthamoeba Keratitis is the variability of the presentation. It can be a devastating infection if recognition is delayed.  The course of the disease is protracted, with remission and exacerbations.9 It usually starts as a unilaterally red eye with epiphora, foreign body sensation, pain and photophobia.  The early signs can be non-specific and present as epithelial irregularities and opacities.  However, in some cases the epithelium can be completely intact.  One of the first signs of Acanthamoeba Keratitis is a pseudo-dendrtitc epithelial lesion.  At this stage the lesion can strongly resemble viral keratitis (herps simplex and zoster).  The corneal sensitivity can be decreased, which obscures the differential diagnosis from herpes simplex even more.  In a further stage of the disease (or sometimes simultaneously); there are a number of stromal abnormalities like nummular infiltrates (as seen in adenoviral infections) and radial keratoneuritis.  The keratoneuritis is characterized by liner, radial, branching infiltrates of the parasite along the corneal nerves into the anterior stroma.  There is associated anterior chamber reaction leading to hypopyon in 39% of the cases.10 A ring shaped stromal infiltrate is characteristic of advanced infection and is nearly pathognomonic for Acanthamoeba Keratitis.  Eventually the keratitis can lead to necrotic zones in the stroma, with the formation of descemetocoele and corneal perforation.  In the majority of cases the infection is mainly limited to the cornea, but sometimes there is scleral involvement presenting as scleral nodules and inflammation.  The two most striking condition of the Acanthamoeba Keratitis are-an excruciating pain which is not always in relation to the clinical findings and a remarkable lack of corneal neovascularisation in spite of the chronic course and severity of inflammation.  The exact reason for the lack of neovascularisation is not clear and it is thought to be due to insufficient immunogenecity of the Acanthamoeba leading to failure to generate full inflammatory cascade necessary for vascular growth.11

The ocular history may be one of the most important tools in the clinical assessment of patients with Acanthamoeba Keratitis. It is critical to take a complete history when working up a patient with a nonspecific but nonhealing keratitis.  For example, the presentation of a herpetic lesion in a contact lens wearer should raise a strong suspicion for Acanthamoeba.  Suspicion should be increased with a history of ocular exposure to soil or water, or a history of trauma.  It should be remembered that the disease could also occur in the absence of contact lens wear.6 Depending upon the time of presentation of Acanthamoeba keratitis can be divided into three stages: early (less than 1 month), middle (1-2 months), and late (more than 2 months).12 The early stages of Acanthamoeba keratitis may mimic herpes simplex keratitis in several ways.  The initial features include fluctuating epithelial defects, epithelial haze, pseudodendrites, ocular hyperemia, and severe ocular pain due to a keratoneuritis.

Middle stages of the disease are often characterized by recurrent or persistent epithelial defects overlying nummular stromal infiltrates. This presentation often mimics herpetic disease. Frank stromal ulceration and lysis can ocur during this stage.     

In the late or advanced stages of the disease, a ring infiltrate can appear, along with satellite lesions and stromal abscesses with a suppurative appearance.  The presence of satellite lesions can mimic a fungal infection, but the presence of an annular keratitis and severe pain can aid in making the correct diagnosis.  These features can also help to distinguish an atypical mycobacterial infection from Acanthamoeba.  Progressive stromal loss can eventually lead to descemetocele formation and perforation.

The classical symptoms may not always be there, as reported by Sharma et al.6 who in their study on 39 patients with non contact lens related Acanthamoeba keratits reported that ocular pain disproportionate to the degree of keratitis was not noted for any of the patients in their study and radial keratoneuritis was seen only in one patient.

DIFFERENTIAL DIAGNOSIS

Herpes zoster virus: It is associated with painful skin vesicles along a dermatomal distribution of face, not crossing the midline.  The pseudodendritis in this condition are raised mucous plaques, do not have terminal bulbs, and do not stain well with fluorescein.

Herpes simplex virus: The patients are often young.  The rash does not follow a dermatome nor obey the midline.  Corneal dendrites have true terminal bulbs and stain well with fluorescein.

Recurrent corneal erosion syndrome: The healing erosion often has a dendritiform appearance.  There is often history of recurrent attacks of acute ocular pain, photophobia, and tearing, often at the time of awakening or during sleep when the eyelids are rubbed or opened.  There may be history of prior corneal abrasion of the involved eye.

Contact lens related pseudodendritis: The epithelial abnormalities do not typically branch, do not have terminal bulbs and stain minimally. There is no skin involvement.

Other possible differential diagnoses are: a fungal keratitis or keratitis caused by Mycobacteria, a toxic keratopathy caused by abuse of local anaesthetics or other eye drops and an infectious crystalline keratopathy.2

LABORATORY DIAGNOSIS

There are a number of laboratory techniques to confirm the diagnosis: bacteriological (smears and cultures) and eventually histopathological.  Diagnostic scraping for cultures and stains be a routine treatment for any suspicious nonhealing ulceration or keratitis.

Smears- Although several stains like Giemsa, Gram and PAS may highlight Acanthamoeba trophozoites and cysts obtained from smears, Calcofluor White and Masson trichrome stains are often preferred over Gram and Giemsa-Wright stains.2 KOH wet mount is also effective in identifying cysts and it has several advantages in the India scenario 13 as:

  • It can be used to identify and differentiate the presence of fungi (a common cause of keratitis in Indian population) while simultaneously looking for Acanthamoeba.

  • The procedure is inexpensive and not time consuming and does not require much infrastructure development to set up.

  • It can be used to determine the need to include non-nutrient agar among culture media.      

Culture- It is suggested that culture material should be taken not only from infected cornea, but also from contact lenses, the preservation liquid and the contact lens holder on 1.5% non-nutrient agar covered with E.coli (E.coli are a food source for Acanthamoeba trophozoites). Cultures are considered to be positive when amoebic migration tracks called "mow tracks" through the "lawn" of bacteria are seen (sometimes as early as 2-3 days) or when trophozoites are seen under the microscope. Culture results may take 7 to 10 days if trophozoites are harvested, but may be delayed for longer periods if only cysts have been obtained, to allow time for the cysts to transform to the trophozoite stage. Both smears and culture have a sensitivity of 65%. Even in early cases, a sample of epithelial cells can improve diagnostic yield. Later cases may require a biopsy of stroma.  The stains mentioned above can be used for such biopsies.

If available, tandem confocal microscopy can be a helpful and noninvasive method for examining corneas for the presence of both trophozoites and cysts. Various specific characteristic features of both cysts and trophozoites have been described to aid in diagnosis.14 Recently PCR has been found to be more sensitive diagnostic test than culture. So it could be particularly useful in confirming the clinical diagnosis in culture negative patients.

Histopathology- Even though Acanthamoeba initially provoke a superficial involvement, ther will eventually be deeper stromal invasion and cyst formation.  This is probably the reason why corneal smears turn out negative if taken some time after the initial symptoms and thus the need for corneal biopsy from the deeper stroma. The specimen can be stained with HE, PAS, Grocott and calcofluor white.  The cyst morphology as such is insufficient to determine species identification for which immunofluorescence with antibodies is needed.  The cysts and trophozoites are found in the ulcerative zone and in the surrounding unaffected stroma or sometimes cysts can be found at the level of descemet's membrane.11

TREATMENT

Acanthamoeba keratitis is difficult to both diagnose and treat.  It is more useful to diagnose the keratitis at an earlier stage. Epithelial debridement combined with topical treatment is highly therapeutic in early cases. But, unfortunately, diagnoses are commonly delayed for weeks to month.15 Although both medical and surgical options exists, no consensus on the most appropriate methods of treatment has yet been reached, and the prognosis for infected eyes is generally guarded.  Prevention and early diagnosis are the best current means of dealing with Acanthamoeba keratitis.

ANTIMICROBIAL THERAPY

The main difficulty in treating Acanthamoeba keratitis is the resistance of Acanthamoeba cysts to anti-microbials.  Acanthamoeba may persist in the encysted form for months and may reactivate after therapy has been discotinued.  When the disease spreads, the Acanthamoeba invades the deeper layers of the stroma, which seriously limits the efficacy of topical treatment.  The topical antimicrobials may be toxic to the cornea, including to keratocytes, increasing the risk of long-term therapy. Ocular medications effective against Acanthamoeba in vivo include the following:

  • Biguanides

  • Polyhexamethylene biguanide (PHMB) (0.02% and 0.1%)

  • Chlorhexidine biguanide (0.02% and 0.1%)

  • Benzamidines

  • Propamidine isethionate (0.1%)

  • Pentamidine isethionate (0.05% to 0.1%)

  • Hexamidine diisethionate (0.1%)

  • Imidazole solutions: Miconazole (1%); Clotrimazole(1%)

  • Neomycin (e.g. Neosporin, or Neomycin-Polymyxin B-Gramicidin)

THERAPEUTIC SCHEME (ACCORDING TO LINDQUIST)16

  1. Loading dose (first 3 days): Chlorhexidine 0.02% or PHMB 0.02+Propamidine isethionate 0.1%+/- neomycin solution to be administered hourly, day and night, with each drug given at same interval separated by 5 minutes.

  2. Intensive treatment phase (4-7 days): Same combination is given every 2 hours while awake and every 4 hours at night.

  3. Maintenance phase (minimum 4 months): Chlorhexidine or PHMB alone or in conjunction with propamidine, 3-4 times daily.

Any drug causing toxicity may be discontinued as long as chlorhexidine or PHMB therapy is maintained. Immunologic methods are being investigated as a form of prevention, and oral immunization of animals recently has been successful in the prevention of Acanthamoeba keratitis by including immunity before infection occurs. Immunization via mucosal surfaces induces anti-Acanthamoeba lgA antibodies in the tears and provides solid protection against the development of Acanthamoeba keratitis.  Unlike other immune effector mechanisms that rely on cytolysis, inflammation, release of toxic molecules or the induction of host cell death, the adaptive immune apparatus prevents Acanthamoeba infections of the cornea by simply preventing the attachment of the parasite to the epithelial surface.  The beauty of this mechani efficacy. Immunization thus may eventually become the best approach for reduction of the incidence of amoebic infection in humans.17  

SURGICAL THERAPY

Penetrating Keratoplasty-In rare cases, keratoplasty is indicated for perforation or intractable keratitis unresponsive to medical therapy and/or debridement. In the majority of cases, however, penetrating keratoplasty is reserved for visual rehabilitation in eyes in which the infection has been completely cleared and in which all cyst forms are believed to have been eradicated.  For this reason, prolonged (e.g. 1 year or more) use of topical antiamoebic agents is indicated prior to considering penetrating keratoplasty.  Because recurrence of Acanthamoeba keratitis in a graft can be a devastating complication with a poor prognosis, use of postoperative prophylactic antiamoebics, up to 1 year following surgery should also be a strong consideration.  

Deep lamellar keratectomy with a conjunctival flap is a suitable approach to help control the infection and to help relieve pain in patients with advanced Acanthamoeba keratitis.18 

PREVENTION

Proper contact lens disinfection and care is the most important step in preventing Acanthamoeba keratitis. It has been observed that the risk of developing Acanthamoeba increases markedly among daily wearers of disposable contact lenses who are less prone to clean their lenses regularly. Those who use chlorine-based disinfectants, which are not effective against Acanthamoeba, are also at higher risk for developing disease.19 So contact lens wearers should be specifically warned about the ubiquitous presence of this hardy organism in soil and fresh water (including tap water).

CONCLUSIONS

Acanthamoeba keratitis has been described as a recent epidemic with soft contact lens wear as greatest risk factor.  With most of the literature focusing on contact lens related Acanthamoeba keratitis, ophthalmologists may hesitate to diagnose this entity in patients without contact lenses, which may eventually lead to significantly delay in diagnosis and hence poor visual outcome in such patients.  Hence a high index of suspicion is needed for this disease entity.  Patients with therapy resistant keratits, even non - contact lens wearers should be examined for the presence of Acanthamoeba by means of specific cultures, histopathological staining and if necessary-corneal biopsy, and appropriate therapy should be instituted at the earliest to prevent the progression of the disease process and prevent visual loss.

REFERENCES

  1. Kumar R, Lloyd D. Recent Advances in the Treatment of Acanthamoeba Keratitis. Clinical Infectious Diseases 2002; 35: 434-441.

  2. Khan NA. Pathogenesis of Acanthamoeba infections. Microb Pathog. 2003 Jun;34 (6):277-85

  3. Claerhout and kestelyn PH. Acanthamoeba keratitis: A Review. Bull. Soc. Belge. Ophthalmol 1999; 274;71-82    

  4. Wilhelmus KR. Introduction: the increasing importance of Acanthamoeba. Rev Infect Dis 1991; 13:S367-8.

  5. Buck SL, Rosenthal RA, Schlech BA. Methods used to evaluate the effectiveness of contact lens care solutions and other compounds against Acanthamoeba: a review of the literature. CLAO J. 2000;26(2):72-84.

  6. Sharma S, Garg P, Rao G N. Patient characteristics, diagnosis, and treatment of non-contact lens related Acanthamoeba keratitis Br J Ophthalmol 2000; 84: 1103-1108 (October).

  7. IIIingworth CD, Cook SD, Karabatsas CH and Easty DL. Acanthamoeba keratitis: risk factors and outcome. British Journal of Ophthalmology, 1995, Vol 79, 1079-1082.

  8. John T. Desai D, Sahm D. Adherence of Acanthamoeba castellani cysts and trophozoites to extended wear soft contact lenses. Rev Infect Dis 1991; 13:S419-20.

  9. Cohen EJ, Buchanan HW, Laughrea PA etal. Diagnosis and management of Acanthamoeba keratitis.  Americal Journal of Ophthalmology. 1985;100:385-89

  10. Mannis MJ, Tamaru R, Roth AM etal. Acanthamoeba sclerokeratitis: Determining diagnostic criteria. Arch Ophthalmol. 1986; 104:1313-17

  11. Kremer I, Cohen EJ, Eagle RC et al. Histopathological evaluation of stromal inflammation in Acanthamoeba keratitis. CLAO J. 1994; 20:45-48.

  12. Bacon AS, Frazer DG, Dart JK, et al. A review of 72 consecutive cases of Acanthamoeba keratitis, 1984-1992. Eye 1993;7(Pt 6):719-725

  13. Srinivasan M, Burman S, George C, Nirmalan PK. Non con-contact lens related Acanthamoeba keratitis at a tertiary eye care center in south India: Implications for eye programmes in the region. Med Sci Monit; 2003; 9(4): CR177-81.

  14. Pfister DR, Cameron JD, Krachmer JH, Holland EJ. Confocal microscopy findings of Acanthamoeba keratitis. Am J Ophthalmol. 1996; 121(2):119-128.

  15. Alizadeh H, Niederkorn JY, McCulley JP. Acanthamoeba keratitis. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. st. Louis, Mo:Mosby; 1997: 1267-1273.

  16. Lindquist TD. Treatment of Acanthamoeba keratits. Cornea 1998; 17:11-16.

  17. Niederkorn JY The role of the innate and adaptive immune responses in Acanthamoeba keratitis. Arch Immunol Ther Exp (Warsz). 2002; 50(1) : 53-59.

  18. Cremona G, Carrasco MA, Tytiun A, Cosentino MJ. Treatment of advanced acanthamoeba keratitis with deep lamellar keratectomy and conjunctival flap.  Cornea. 2002 Oct; 21(7): 705-8.

  19. Redford CF, Bacon AS, Dart JK, Minassian DC. Risk factors for acanthamoeba keratitis in contact lens users: a case-control study. BMJ 1995; 310:1567-1570.

Address for Correspondence:

Dr. Pawan Prasher
17/Doctor's Hostel, Govt. Medical College, Patiala.
Email: drpawanprashar@yahoo.com