Acanthamoebic keratitis in a healthy Australian man

The Natural History of Acanthamoeba Keratitis: A Systematic Literature Review

INTRODUCTION

Acanthamoeba keratitis (AK) was first identified in 1972 and the first patient cured with propamidine was reported in 1985. Treatment outcomes, before the advent of the first effective anti-amoebic treatment, were known to be poor and often required therapeutic keratoplasty (TK) but have not been evaluated in detail. Analysis of these outcomes has value for several reasons: it gives an historical perspective, describes the natural history of AK when the disease was minimally modified by the early treatments and provides a benchmark against which current treatments can be compared and how these have changed the therapeutic results.

METHODS

We conducted a systematic literature review for the period 1970-1995 using PRISMA guidelines. The population of interest comprised patients with AK treated without products having established anti-amoebic activity against both trophozoites and cysts (biguanides or diamidines). The outcomes of interest were medical cure, TK and enucleation. Proportions and 95% confidence intervals were estimated.

RESULTS

Fifty-six case reports were eligible. Risk factors for AK were reported in 44/56 patients: contact lens wear in 30/44 (68.2%) and trauma in 14/44 (31.8%). The mean time from presentation to diagnosis was 7.3 weeks (standard deviation 9.3 weeks); 13/56 (23.2%) were diagnosed within 4 weeks. Topical treatments given to patients included corticosteroids (85.2%), antibiotics (85.2%), antivirals (72.2%) and antifungals (51.8%). Final visual acuity was ≥ 20/40 in 17/33 (51.5%) patients with no missing data. Medical cures were reported in 11/56 patients (19.6%), TK in 38/56 (67.9%), other surgery in 4/56 (7.1%) and enucleation in 3/56 (5.4%).

CONCLUSION

This study suggests that, before the availability of propamidine as the first effective treatment for AK, the clinical outcome of these patients was poor with only a few patients cured without surgery. These findings should be interpreted with caution because they rely on case reports and series that are subject to inherent bias.

Antigens of selected Acanthamoeba species detected with monoclonal antibodies

Acanthamoeba species are ubiquitous soil and freshwater protozoa that have been associated with infections of the human brain, skin, lungs and eyes. Our aim was to develop specific antibodies to aid in rapid and specific diagnosis of clinically important isolates. Mice were variously immunised with live mixtures of Acanthamoeba castellanii strain 112 (AC112) trophozoites and cysts, or with sonicated, formalin-fixed or heat-treated trophozoites, or with a trophozoite membrane preparation. Eight hybridoma cell lines secreting monoclonal antibodies reactive with A. castellanii epitopes were generated. Seven of the new antibodies (designated AMEC1-3 and MTAC1-4) were isotyped as IgMkappa and one (MTAC5) as IgG1kappa. All of the novel antibodies bound to AC112 cysts, and MTAC4 and MTAC5 also bound to trophozoites as measured by flow cytometry on unfixed cells. Single chain antibody fragments that retained parental antibody binding characteristics were engineered from three of the hybridomas (AMEC1, MTAC3 and MTAC4). Four monoclonal antibodies (AMEC1, AMEC3, MTAC1, MTAC3) bound reliably to unfixed cysts of clinical isolates of A. castellanii (two strains) and Acanthamoeba polyphaga (two strains), belonging to Pussard-Pons morphological group II, and to Acanthamoeba lenticulata and Acanthamoeba culbertsoni, belonging to Pussard-Pons morphological group III. None of the antibodies bound to cysts or trophozoites of the environmental group I species, Acanthamoeba tubiashi. Antibodies AMEC1, MTAC3, MTAC4 and MTAC5 reacted with buffered formalin-fixed AC112 by immunohistochemistry, and also stained Acanthamoeba in sections of infected rat cornea and buffered formalin-fixed, paraffin-embedded infected human cornea. These antibodies may be useful in diagnosing pathogenic Acanthamoeba species in clinical specimens, provided that cysts are present.

Antimicrobial susceptibility of 19 Australian corneal isolates of Acanthamoeba

PURPOSE

To determine the in vitro drug susceptibility of Australian corneal isolates of Acanthamoeba and to correlate the results with patient treatment and visual outcome.

METHODS

Acanthomoeba isolates were obtained from a reference laboratory Cyst suspensions were prepared from 19 strains and exposed to 10 antimicrobial agents for 7 days. The minimum drug concentrations required to inhibit excystation were determined. Inhibited cells were then plated out to determine minimum cysticidal concentrations.

RESULTS

Overall, propamidine proved to be the most active anti-Acanthamoeba agent tested. The disinfectant polyhexamethylene biguanide, either pure or in Baquacil, was also effective. Pentamidine, hexamidine, chlorhexidine and chloroxylenol had intermediate activity, while neomycin, amphotericin B and povidone-iodine had poor activity. There was no clear relationship between in vitro susceptibility and visual outcome.

CONCLUSIONS

Propamidine and polyhexamethylene biguanide drops are recommended as initial choices for the treatment of Acanthamoeba keratitis. The variability in the susceptibility to any one agent suggests that individual testing of isolates is necessary to identify the most appropriate treatment. A number of factors influence visual outcome in these cases; further studies are required to resolve the importance or otherwise of in vitro susceptibility.

Outbreak of keratitis presumed to be caused by Acanthamoeba

PURPOSE

A sharp increase of Acanthamoeba keratitis from two cases per year to 30 cases per year at our institution prompted this study to determine whether there was a change in the clinical characteristics, basic epidemiology, and outcome of this disease.

METHODS

We reviewed all cases of Acanthamoeba keratitis diagnosed at the University of Iowa Hospitals and Clinics from mid-1993 through 1994.

RESULTS

We screened 217 patients with keratitis by tandem scanning confocal microscopy and suspected Acanthamoeba in 51 patients. Diagnosis was confirmed by cytology in 43 patients (48 eyes). There were no positive cultures. Patients examined within four weeks of onset of symptoms were younger (mean age, 32.6 +/- 15.4 years) and wore contact lenses (11 of 18 patients), and infrequently herpes simplex keratitis (four of 18 patients) was diagnosed. Patients examined after four weeks were older (mean age, 54.0 +/- 19.5 years), infrequently wore contact lenses (six of 25 patients), and often had herpes simplex keratitis (12 of 25 patients).

CONCLUSIONS

Corneal examination with tandem scanning confocal microscopy was associated with a marked increase in the detection of Acanthamoeba, strongly suggesting that the disease is more prevalent than suspected. Acanthamoeba may account for many cases of clinically presumed herpes simplex keratitis, the leading cause of corneal blindness in the United States. Acanthamoeba should be considered in the differential diagnosis of any unexplained keratitis, even those of short duration.

Corneal virulence, cytopathic effect on human keratocytes and genetic characterization of Acanthamoeba

Acanthamoeba keratitis is a sight-threatening complication of corneal trauma or contact lens wear. Although the majority of corneal isolates of Acanthamoeba belong to Group II in the Pussard-Pons classification based on cyst morphology, they have been placed in at least six species and their genetic relatedness is uncertain. The aim of this study was to determine the virulence of, and the relationship among, strains derived from the cornea, the nasal mucosa, and other environmental sources. To assess virulence, 10(4) trophozoites of each strain were incubated with monolayers of human corneal fibroblasts. By day 7, 12 of 29 strains tested had induced significant cytopathic changes. In addition, inocula of 10(4) cysts or trophozoites with 10(6) Corynebacterium xerosis were injected into the corneas of Porton rats; 11 amoebic strains induced infection within 7 days. The correlation between the virulence of trophozoites in vitro and in vivo was 86%. Using allozyme electrophoresis, 23 Acanthamoeba strains clustered into 5 major phylogenic divisions. Three divisions contained one or more strains that were virulent in the rat cornea. Virulent Pussard-Pons Group II strains clustered tightly to a fixed allelic difference of 13.6%. The eight corneal isolates clustered to 33%, dividing into three lineages. Five avirulent nasal isolates were strongly differentiated from other Group II strains. The results were not in accord with species designations based primarily on morphological criteria. These data suggest that particular subsets of Acanthamoeba strains are virulent in the human cornea.

Free-living amoebae: pathogenicity and immunity

Free-living amoebae causes three well-defined disease entities: (i) primary amoebic meningoencephalitis, caused by Naegleria fowleri, (ii) granulomatous amoebic encephalitis and (iii) chronic amoebic keratitis, caused by species of Acanthamoeba. Both Naegleria infections and chronic amoebic keratitis occur in healthy individuals while granulomatous amoebic encephalitis is often associated with patients with acquired immunodeficiencies. The different pathogenic behaviour of these organisms is associated with differences in life cycle, amoeboidal locomotion, enzyme composition (such as phospholipase A), and cytotoxins, as well as natural host immunity. Immunity against these amoebae (whether acquired or natural) involves a combination of complement, antibody and cell-mediated immunity. Evidence suggests that the major mechanisms of immunity against these amoebae is activation of phagocytic cells, especially neutrophils, by lymphokines and opsonization of the amoebae by antibody which promote an antibody dependent cellular destruction of the organism.

Microbiological and histopathological confirmation of acanthamebic keratitis

A healthy 42 yr-old woman presented with a left keratitis which she had had for 3 months. No organisms could be grown by culture of corneal scrapings for bacteria and fungi, and the condition failed to respond to topical therapy. Amebic keratitis was diagnosed following corneal biopsy and cultures which grew Acanthamoeba of a species similar to, but not identical with, Acanthamoeba polyphaga. Medical treatment was continued for 6 mth. During this time the corneal infiltrate became less prominent but visual acuity remained impaired by both corneal opacity and cataract.

Acanthamoebic keratitis diagnosed by paracentesis and biopsy and treated with propamidine

A previously healthy 53-year-old man had keratitis of the right eye for six months, unresponsive to topical medical therapy. Acanthamoeba was grown from tissue obtained by corneal biopsy and from aqueous from an anterior chamber tap. The patient was treated with propamidine isethionate 0.1% drops and dibromopropamidine isethionate 0.15% ointment, and after two and a half months the ocular inflammation was continuing to resolve. This case supports a role for the diamidines in the treatment of acanthamoebic keratitis.