Acanthamoeba Keratitis in China: Genotypic and Clinical Correlations

Purpose

To investigate the relationship between Acanthamoeba genotypes, clinical manifestations, and outcomes in Acanthamoeba keratitis (AK) patients.

Methods

This retrospective study included 159 culture-confirmed AK patients. Patients' data were collected, including demographics, initial diagnosis, treatments, and clinical features. The genotype of Acanthamoeba was identified through sequencing the Diagnostic Fragment 3 (DF3) region in the small ribosomal subunit RNA genes. The phylogenetic tree was constructed using the ClustalW model and maximum likelihood method. Cases with "poor outcome" were defined based on specific clinical criteria, including corneal perforation, keratoplasty, other eye surgery, duration of anti-amoebic therapy ≥8.0 months, and final visual acuity ≤20/80. "Better outcome" cases were the remainder. The correlation between T4 subtypes, clinical phenotypes, and clinical prognosis were further analyzed.

Results

In this study, AK was primarily attributed to the T4A genotype, with a positive correlation between geographical and genetic distances. The primary clinical associated with T4 subtypes was deep stromal infiltration. Results was also showed a significant association between T4 subtypes and clinical outcomes (P = 0.021). Further analysis revealed that T4C was closely associated with a better prognosis (P = 0.040) and T4D with worse outcomes (P = 0.013).

Conclusions

In China, AK was predominantly caused by the T4A subtype. Geographical distance positively correlated with genetic distance. Clinical prognosis varied among different subtypes, notably in T4C and T4D.

Translational Relevance

This study demonstrated the association between T4 subtypes and clinical phenotypes, as well as the effects of T4 subtypes on clinical prognosis.

Distribution and Current State of Molecular Genetic Characterization in Pathogenic Free-Living Amoebae

Free-living amoebae (FLA) are protozoa widely distributed in the environment, found in a great diversity of terrestrial biomes. Some genera of FLA are linked to human infections. The genus Acanthamoeba is currently classified into 23 genotypes (T1-T23), and of these some (T1, T2, T4, T5, T10, T12, and T18) are known to be capable of causing granulomatous amoebic encephalitis (GAE) mainly in immunocompromised patients while other genotypes (T2, T3, T4, T5, T6, T10, T11, T12, and T15) cause Acanthamoeba keratitis mainly in otherwise healthy patients. Meanwhile, Naegleria fowleri is the causative agent of an acute infection called primary amoebic meningoencephalitis (PAM), while Balamuthia mandrillaris, like some Acanthamoeba genotypes, causes GAE, differing from the latter in the description of numerous cases in patients immunocompetent. Finally, other FLA related to the pathologies mentioned above have been reported; Sappinia sp. is responsible for one case of amoebic encephalitis; Vermamoeba vermiformis has been found in cases of ocular damage, and its extraordinary capacity as endocytobiont for microorganisms of public health importance such as Legionella pneumophila, Bacillus anthracis, and Pseudomonas aeruginosa, among others. This review addressed issues related to epidemiology, updating their geographic distribution and cases reported in recent years for pathogenic FLA.

On the diversity and clinical importance of Acanthamoeba spp. from Group 1

Group 1 acanthamoebae are morphologically and phylogenetically distinct from all other Acanthamoeba species. They include five species, each labelled by its genotype: A. astronyxis (T7), A. tubiashi (T8), A. comandoni (T9), unnamed Acanthamoeba sp. (T17), and A. byersi (T18). Thought only environmental, they have recently attracted attention due to their recovery in cases of human keratitis and encephalitis, the main diseases caused by Acanthamoeba, where the usual causative agents are mainly species of Groups 2 and 3. Analysis of the available data confirms the pathogenic importance of these species, although it is probably minor compared to that of the species in Groups 2 and 3. In addition, it should be noted that there are difficulties in identifying genotypes by widely used molecular methods, and some misidentifications are revealed.

Discovery of repurposing drug candidates for the treatment of diseases caused by pathogenic free-living amoebae

Diseases caused by pathogenic free-living amoebae include primary amoebic meningoencephalitis (Naegleria fowleri), granulomatous amoebic encephalitis (Acanthamoeba spp.), Acanthamoeba keratitis, and Balamuthia amoebic encephalitis (Balamuthia mandrillaris). Each of these are difficult to treat and have high morbidity and mortality rates due to lack of effective therapeutics. Since repurposing drugs is an ideal strategy for orphan diseases, we conducted a high throughput phenotypic screen of 12,000 compounds from the Calibr ReFRAME library. We discovered a total of 58 potent inhibitors (IC50 <1 μM) against N. fowleri (n = 19), A. castellanii (n = 12), and B. mandrillaris (n = 27) plus an additional 90 micromolar inhibitors. Of these, 113 inhibitors have never been reported to have activity against Naegleria, Acanthamoeba or Balamuthia. Rapid onset of action is important for new anti-amoeba drugs and we identified 19 compounds that inhibit N. fowleri in vitro within 24 hours (halofuginone, NVP-HSP990, fumagillin, bardoxolone, belaronib, and BPH-942, solithromycin, nitracrine, quisinostat, pabinostat, pracinostat, dacinostat, fimepinostat, sanguinarium, radicicol, acriflavine, REP3132, BC-3205 and PF-4287881). These compounds inhibit N. fowleri in vitro faster than any of the drugs currently used for chemotherapy. The results of these studies demonstrate the utility of phenotypic screens for discovery of new drugs for pathogenic free-living amoebae, including Acanthamoeba for the first time. Given that many of the repurposed drugs have known mechanisms of action, these compounds can be used to validate new targets for structure-based drug design.

Experimental keratitis in rats caused by Acanthamoeba griffini: A kinetic histopathological study

The aim of this study was to evaluate the inflammation process that resulted from the inoculation of Wistar Rats with Acanthamoeba griffini, a virulent T3 Acanthamoeba genotype that produces keratitis. Haematoxylin and eosin, periodic acid stain, immunohistochemistry and morphometry were used to analyse tissues from rats of an Acanthamoeba keratitis (AK) model. Two weeks after inoculating the rats with A griffini trophozoites, the thickness of the stroma had diminished, followed by an increase in thickness at 4 weeks. At the latter time, an abundance of inflammatory infiltrate cells was observed, some found to express IL-1β, IL-10 and/or caspase 3. Intercellular adhesion molecule-1 was expressed in corneal blood vessels amid the abundant vascularization characteristic of the development of AK. Through an immunohistochemical technique, trophozoites were detected at 2 and 4 weeks post-inoculation. By 8 weeks, there were a low number of trophozoites and cysts and the corneas of infected rats were similar in thickness to those of the controls. Thus, the rats were capable of healing experimental AK in the present rat model. Diverse immunological mechanisms regulated the inflammatory process in acute AK induced by A griffini in a murine model.

Continuous positive airway pressure-associated cutaneous amoebiasis in an immunosuppressed patient

Organisms of the genus Acanthamoeba are environmentally ubiquitous and colonizers of the oral mucosa in humans. While largely asymptomatic in healthy persons, Acanthamoeba infection can cause disseminated disease with poor prognosis in immunosuppressed populations. Here we report a unique case of cutaneous amoebiasis associated with continuous positive airway pressure use in an immunosuppressed patient.

Fulminant and fatal encephalitis caused by Acanthamoeba in a kidney transplant recipient: case report and literature review

Acanthamoeba is the most common cause of granulomatous amebic encephalitis, a typically fatal condition that is classically described as indolent and slowly progressive. We report a case of Acanthamoeba encephalitis in a kidney transplant recipient that progressed to death within 3 days of symptom onset and was diagnosed at autopsy. We also review clinical characteristics, treatments, and outcomes of all published cases of Acanthamoeba encephalitis in solid organ transplant (SOT) recipients. Ten cases were identified, and the infection was fatal in 9 of these cases. In 6 patients, Acanthamoeba presented in a fulminant manner and death occurred within 2 weeks after the onset of neurologic symptoms. These acute presentations are likely related to immunodeficiencies associated with solid organ transplantation that result in an inability to control Acanthamoeba proliferation. Skin lesions may predate neurologic involvement and provide an opportunity for early diagnosis and treatment. Acanthamoeba is an under-recognized cause of encephalitis in SOT recipients and often presents in a fulminant manner in this population. Increased awareness of this disease and its clinical manifestations is essential to attain an early diagnosis and provide the best chance of cure.

Characterization of a new pathogenic Acanthamoeba Species, A. byersi n. sp., isolated from a human with fatal amoebic encephalitis

Acanthamoeba spp. are free‐living amoebae that are ubiquitous in natural environments. They can cause cutaneous, nasopharyngeal, and disseminated infection, leading to granulomatous amebic encephalitis (GAE) in immunocompromised individuals. In addition, they can cause amoebic keratitis in contact lens wearers. Acanthamoeba GAE is almost always fatal because of difficulty and delay in diagnosis and lack of optimal antimicrobial therapy. Here, we report the description of an unusual strain isolated from skin and brain of a GAE patient. The amoebae displayed large trophozoites and star‐shaped cysts, characteristics for acanthamoebas belonging to morphology Group 1. However, its unique morphology and growth characteristics differentiated this new strain from other Group 1 species. DNA sequence analysis, secondary structure prediction, and phylogenetic analysis of the 18S rRNA gene confirmed that this new strain belonged to Group 1, but that it was distinct from the other sequence types within that group. Thus, we hereby propose the establishment of a new species, Acanthamoeba byersi n. sp. as well as a new sequence type, T18, for this new strain. To our knowledge, this is the first report of a Group 1 Acanthamoeba that is indisputably pathogenic in humans.