Some Acanthamoeba species are distributed in natural and man-made environments, in a wide range of soil and aquatic habitats, also in clinical settings. The amphizoic organisms can exist as facultative parasites - causative agents of serious human disease, Acanthamoeba keratitis. The vision-threatening eye disease occurring particularly in contact lens wearers is reported with increasing prevalence in different regions of the world. The amoebic keratitis is difficult to diagnose as clinical symptoms are similar to those observed in other eye diseases. Moreover, bacterial, viral, fungal, and amoebic co-infections frequently occur; also amoebae act as carriers for ~ 20 species pathogenic for humans, e.g. from Pseudomonas, Legionella, Mycobacterium and Escherichia genera; thus the corneal disease is frequently misdiagnosed. Complex etiology, late proper recognition of amoebic infections, and the exceptional resistance of Acanthamoeba cysts to chemicals are important factors influencing diagnostic and therapeutic difficulties. Surgical interventions are needed as an alternative treatment in refractory Acanthamoeba keratitis. It should be taken into consideration that the knowledge and awareness of increasing threat generated by the amphizoic amoebae are still insufficient. This compilation presents selected aspects of eye disease that is becoming the increasingly significant for human health worldwide.
- Acanthamoeba keratitis
- risk factors
Eye diseases affecting the cornea are a major cause of blindness worldwide. Among different infectious agents, bacteria, fungi, viruses and protozoans may be causes of keratitis in contact lens users. At present, the epidemiology of microbial keratitis is complicated, diverse, and even controversial; the use of contact lenses is considered as the most important risk factor of corneal infections in humans [4,8]. It was emphasized in several reviews [8- 10] that the incidence rates of particular organisms causing keratitis associated with contact lens wear differ between parts of the world, from country to country and even from one population to another. Economic factors, various frequencies of contact lens wear in particular geographical locations, different availabilities and standards of eye care, and different methods of etiological agent isolation and culture in particular surveys were mentioned as factors influencing the differences and criteria taken into consideration in the studies.
The frequency of microbial keratitis cases caused by Gram-negative bacteria from genus
The highest proportion of fungal corneal infections (
Correlations between the contact lens wear and percentages of bacterial and fungal keratitis were statistically significant.
The viruses from
Protozoan eye disease,
Previous and recent studies continued in many centers with a participation of practicing researchers and other scientists are crucial for a better understanding of
1.1. Possible environmental sources of
Free-living amoebae belonging to
1.2. The developmental forms and classification of
The amoeboid, mitochondria-bearing protist is known as free-living organism that exists in two morphologic forms: trophozoite and cyst [4,12,18]. The life cycle of
Following the recognition of the amoebae and increasing number of isolates belonging to the genus
Acanthamoebaspp. as potential agents of human diseases
The protozoans belonging to
For this reason, these amoebae are called amphizoic amoebae, because they are able to exist in two different modes: as free-living-exozoic organisms and as endozoic parasitic organisms, within host tissues; thus, the free-living protozoans are also believed as facultative parasites.
Trophozoites and cysts of
Developmental stages of the amphizoic species may be causative agents of an systemic opportunistic disease developing in immunocompromised individuals. This is rare but almost always fatal granulomatous
3. Pathogenesis in AK
In the early phase of AK, there are nonspecific symptoms variable in their intensity starting with redness, photophobia, and excessive tearing. Most commonly, only one eye is involved. Active epithelial inflammations usually progress from the outermost layer of the cornea deeper, to the stroma. Symptoms of the devastating eye disease include loss of the visual acuity. In many cases of AK, excruciating eye pain occurs, in which the intensity is often incommensurate with relatively small degree of corneal deterioration. There are clinical and histopathological evidences that the severe pain is associated with
A pathogenesis of
Among the direct contributing factors, the following are listed: adhesion of
Among the indirectly contributing factors, there are amoebic and host determinants. Morpho-physiological features: the amoeboid motion and spine-like acanthopodia allow protozoans to modulate binding to biological and inert surfaces. Also, changes in the overall numbers of the amoebae as well as in the proportion of trophozoites and cysts dependent on the environmental conditions are also listed as the amoebic pathogenesis determinants. The temperature tolerance, osmotolerance and growth at different pH allow the amphizoic
4. Predisposing/ risk factors sufficient to contract AK
In several studies it is underlined that the initiating of AK is a multi-factorial process, in which both host and environmental determinants are likely involved, apart connected with
It is considered that
The estimation of AK findings in several countries showed various, generally relatively low, but constantly increasing number of the cornel disease incidents during the last few decades i.e. 1.36 cases per million contact lens wears in the United States, 17 to 21 cases per million in England, 1 per 30,000 contact lens wears in France, 0.05 per 10,000 in Holland [4,12,13]. However, “it is noteworthy”, as Khan  concluded “that variations in the incidence rate of
Some micro-traumas occurring earlier or appearing in connection with the use of the lenses predispose to contract AK; a human organism’s susceptibility, tissue specificity, tear factors, and secretory immunoglobulin A (sIgA), important in the specific immune defense mechanism, are among other host factors influencing development of this corneal disease. Environmental conditions such as temperature, osmolarity, and pH may be important in initiating AK.
Simultaneously, the amoebae were found in contact lens and in storage cases that may be potential sources and reservoirs of the facultative parasites [12,18,47-53]. In spite of this, the incidence rate of AK in wearers of contact lenses is remarkably low in comparison with the contact lens storage cases contaminated with
Additionally, it has been confirmed for various
In persons not using contact lenses, other circumstances influence as important for contract AK [4,47]. The different
There have been no reports of
5. When the clinician should suspect
It is known, that the emerging vision-threatening AK is difficult to diagnose because clinical manifestations are similar to those observed in the course of other infectious eye diseases.
In anamnesis, in the early stage of this eye disease - patients are complaining of photophobia, excessive tearing, and reduced visual acuity; the clinical manifestations of this keratitis may also include redness and eyelid edema [13,17,50,54].
Particularly, if a presence of any foreign particles will be excluded, appearing of excruciating eye pain with intensity incommensurate with degree of corneal deterioration may suggest that
6. Differential diagnosis of AK
6.1. A tentative diagnosis of AK
The non-invasive methods are useful for the tentative diagnosis of AK, in which the slit-lamp that provides magnification from 10 to 25 times and
The use of a slit lamp is indicated in any acute situation that requires magnification to inspect the anterior segment of the eye. Active epithelial inflammations and hyper reflective tissues in the affected eye may be visualized by the slit lamp; a corneal ulcer and characteristic, ring-like corneal infiltration may occur in some patients. However it should be underlined that the characteristic ring infiltrate is seen in approximately 50% of AK cases.
Different clinical presentations may occur in various causes of keratitis. However, in clinical AK practice there are nonspecific signs variable in their intensity starting with photophobia, redness, and excessive tearing that present with similar symptoms as viral, bacterial and fungal keratitis (4,7, 9-12].
In the slit lamp, corneal epithelial disease caused by
Bacterial corneal infection appears typically as a one gray-white stromal infiltrate with well-demarked borders. Critical sings for fungal keratitis, e.g.
In the early epithelial stage of
Active epithelial inflammations usually progresses from the outermost layer of the cornea -a superficial keratitis -to deeper stroma -the stromal or interstitial keratitis.
In the initial epithelial phase, typical signs of AK include epithelial or sub-epithelial infiltrates, pseudodendrites resembling these observed in
Later signs of AK develop in 3-8 weeks and include a deep inflammation of the cornea consisting of a central stromal thinning and melting, anterior chamber cells and flare, hypopyon and extension of inflammation into sclera. The latter is generally reactive reaction rather than extension of infection; later in the disease course, the slowly progressive stromal opacifications and neovascularization may occur.
Common findings in viral keratitis are: highly reflective, desquamating epithelial cells in superficial epithelial layer, multiple dendritic cells in basal epithelial layer, the absence of subepithelial nerve plexus, and hyperreflective keratocytes in the anterior stroma.
In bacterial keratitis, confocal micrographs typically reveal leucocytes infiltrating the corneal stroma and adherent to vessel walls. In some cases, the dendritic cells are present intrastromally; the bacteria themselves cannot be detected with the confocal microscopy.
Filamentous fungi and bacteria (e.g.
The examination of affected eyes by
It was evident also in our studies on monitoring of
6.2. Why clinical manifestations are not sufficient to indicate a causative agent of keratitis
Knowledge and awareness of threat are necessary as the most important step in proper AK diagnosis as it is underlined by J. Lorenzo-Morales et al. .
The careful anamnesis is very important and helpful. Most of the clinical symptoms of
Undoubtedly, the non-invasive
Also, in our several studies we analyzed serious keratitis cases regarding patients who were under suspicion of
It has been reported that
6.3. Laboratory evaluation
Literature data as well as results of our studies indicated that microscopic visualization of amoebae in unstained or stained slides prepared directly from corneal scraping is usefulness for AK diagnostics. Also, laboratory examinations of specimens from
Moreover, culture methods are considered as the gold standard of diagnosis, which needs, however, collaboration between clinicians and laboratory staff and, also, the familiarization with a morphological characteristic of
Simultaneously, molecular methods of classification of
7. Treatment options in AK
7.1. Factors complicating effective pharmacotherapy in AK
AK treatment is difficult and often unsuccessful despite of advances in pharmacotherapy. There are several factors that are listed as influencing difficulties and still not fully effective of applied therapy [12,18].
AK is often incorrect diagnosed due to nonspecific clinical symptoms; similar clinical pictures to this observed in the amoebic keratitis may give a wide range of agents e.g. viral
Additionally, mixed amoebic, fungal viral and bacterial keratitis may occur that complicate therapeutic management.
It is also underlined that extremely high resistance of
Among the abovementioned facts, diagnostic mistakes that cause delayed in a beginning of an efficient treatment may result in a prolonged, severe course of AK and vision deterioration.
It is also emphasized that some chemicals can induce amoebic encystment that subsequently, by excystment, may lead to repeated development of trophozoites, thus an activation of the dormant cysts can lead to recurrence of the disease. It is why not only amoebicidal effects but also the cysticidal efficacy of applied therapeutics is very important [43-46].
In some research works, also in our experimental studies it has been reported that higher concentrations of drugs and some new-synthesized imidazole derivatives may be
7.2. Treatment recommended in management in AK
Currently, there are not known single-treatment methods effective against both trophozoites and cysts of
The mainstay agents that are used as a first-line treatment for
However, particularly after earlier improper treatment in other centers, the combination drug therapy with the antimicrobial agents is used more or less successfully. Additionally, such factors as human organism status, a virulence of amoeba strains, phase of infection, and kind and concentration of the chemicals applied may determine variability in effects of drugs on trophozoites and cysts of several
The most frequently used agents that achieve sufficient high concentrations at the site of infection and are effective against trophozoites and cysts of
Although the low doses of topical steroids can be useful to diminish inflammation in cases of controlled infection but the use of topical corticosteroids is controversial.
Systemic corticosteroids are preferred over topical ones in cases of severe inflammation. This route of administration provides better ocular safety profile (less concentration in the cornea) but less body safety profile. However there are some suggestions that steroid use may result in increased pathogenicity of the amoebae .
The corneal transplantation can be performed for therapeutic or optical indications. Therapeutic, usually penetrating, keratoplasty is applied when the infectious process spreads to the corneal stroma, causing corneal melting and thinning despite of aggressive prolonged anti-amoebic therapy [4,12,18]. In a case of threatening or completed perforation of the cornea, the surgery must be performed urgently. Some authors recommend systemic steroids prior to surgery if concomitant limbitis or scleritis is present .
Sacher et al. show that pretreatment of
The size of corneal graft should be minimum to excise an inflamed and necrotic tissue. Although remaining clinically healthy cornea is frequently also infected, this tissue should be saved because of the higher risk of rejection with large/decentrated grafts and because the possibility of repeat grafting should be kept in mind in the event of recurrence; a further graft represents a new food source for the organism and can be used to attract residual amoebae .
In a case of therapeutic keratoplasty for AK, the topical steroids in combination with anti-amoebic drugs are applied for 6-12 months following keratoplasty, to relieve pain, lessen the inflammation, and prevent graft rejection and recurrence of infection. Corneal grafts performed in the eyes with active inflammation are the high-risk transplants and they required systemic immunosuppression similar to this given in organ transplants (cyclosporine and/or mycophenolate mofetil). Apart from a poor graft survival, the postoperative glaucoma is a frequent complication.
In optical keratoplasty performed after resolution of active keratitis there is an excellent prognosis for both graft survival and visual outcome .
Promising clinical results were reported from amoebicidal effect of combined riboflavin and UV-A (ultraviolet light A, 365nm wavelength) exposure -corneal cross-linking (CXL) that was used for stabilization of corneal melting which can delay surgical treatment [12,72,73]. CXL has also an antimicrobial effect that is due to the effect of UV light interacting with riboflavin as the chromophore. It damages both the DNA and RNA of pathogens. Photoactivated chromophore for infectious keratitis (PACK)-CXL is an alternative to standard antibiotic therapy in treating infectious corneal disorders, and may help reduce the microbial resistance to antibiotics and avoid therapeutic keratoplasty in some cases .
Many chemicals and antimicrobials were examined and are still tested
8. Prevention and prognosis of AK
The contact lens wearers must be well educated as for the proper use and care of their lenses; do not use saline solution for lens storage, and do not to swim wearing contact lenses or use the swimming goggles. It is also very important to educate the ophthalmologist to be aware of signs and symptoms of AK and be able to early diagnose and initiate suitable treatment.
The prognosis for visual recovery with only mild residual stromal involvement is very good; in other cases, the visual prognosis is poor. Generally, a prediction depends on inflammation status at the time of diagnosis and the prompt initiation of proper treatment.
A retrospective review indicates that early diagnosis (less than 18 days) results in better final visual acuity and less likely needs keratoplasty . In the early stage of infection, trophozoite forms are predominated, and the infection is confined to the superficial corneal layers. With time as the process progresses, the microorganisms enter to the deeper corneal stroma and encyst. Cysts are much more resistant to anti-protozoan drugs compared to trophozoites. Severe inflammation, scleral involvement, late diagnosis, and retardation of the therapy initialization are associated with poor clinical outcomes. In 10% of cases, there is associated scleritis.
Complex infective etiology and late recognition of amoebic infections were the important factors influencing diagnostic and therapeutic difficulties in AK. Laboratory examinations including
In some severe cases, keratoplasty and prolonged application of a mixture of drugs may be an appropriate option for visual rehabilitation.
Moreover, as our studies and experience show,
Human infections with facultative parasitic
Therefore, further educational efforts directed first of all to contact lens users are desirable for the prevention of this vision-threatening corneal disease.
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