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G. C. Booton, C. E. Joslin, M. Shoff, E. Y. Tu, D. J. Kelly, P. A. Fuerst; Genotypic Identification of Acanthamoeba sp. Isolates Associated With an Outbreak of Acanthamoeba Keratitis (AK). Invest. Ophthalmol. Vis. Sci. 2007;48(13):3234.
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An increase in Acanthamoeba keratitis (AK) cases has been documented in the Chicago, Ill. USA area from 1 June 2003 through 30 November 2005. Epidemiological analysis indicates a significant increase in AK cases during this period when compared to historical numbers. It was hypothesized that the increased infection rate and skewed geographical distribution of cases may be due to changes in the type and amounts of water treatment chemicals. Alternatively, a more pathogenic strain of Acanthamoeba could be responsible for the increase in AK cases. In this study we use genotypic data to test the hypothesis that a new, or a known more pathogenic genotype of Acanthamoeba, is the cause of the recent surge in AK.
Previous sequence analysis of the 18S ribosomal RNA gene (18S rDNA) of Acanthamoeba isolates has resulted in the placement of Acanthamoeba strains into 15 different genotypic classes. Most cases (~97%) of AK are associated with a single genotype (designated T4) of Acanthamoeba. Rarely, AK cases are associated with other genotypes. Other more serious infections caused by Acanthamoeba, including granulomatous amebic encephalitis (GAE), are associated with genotype T4 and also with additional rare genotypes not observed in AK infections. In this study we have determined the genotypes of 11 Acanthamoeba sp. isolates from the Chicago AK outbreak by sequencing a highly informative region of the 18S rDNA.
DNA sequencing shows these isolates are predominantly genotype T4 (82%), whereas the remaining isolates were genotype T3 (18%). Both genotypes have previously been observed in AK cases. In addition, DNA sequences are overwhelmingly similar to previously sequenced isolates.
There is no support for the hypothesis that cases of AK in the Chicago outbreak are the result of infection by a new Acanthamoeba genotype. The high sequence similarity between these isolates and previously sequenced T3 and T4 genotype isolates from AK cases does not support the hypothesis that they represent more pathogenic Acanthamoeba of known genotypes. Lastly, the results lend support to the hypothesis that increased AK cases in the Chicago area may be due to changes in water treatment, permitting increased bacterial colonization of the water, increased Acanthamoeba colonization, and ultimately an increase in AK cases due to an increased abundance of Acanthamoeba in the water supply.
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