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Abstract
This thesis was divided into two sections. The first section of this thesis was aimed at evaluating the risk factors associated with contact lens-associated bacterial keratitis in South India (Chapter 2). The second section of the thesis was aimed at evaluated on genetic risk factors associated with contact lens and non-contact lens-associated bacterial and sterile keratitis (Chapter 3).
The aim of this thesis was to identify trends and risk factors of contact lens-associated microbial keratitis and to determine the genetic susceptibility to microbial keratitis, sterile keratitis and bacterial induced contact lens keratitis in a South Indian population. This was achieved by retrieving retrospective data over a period of one decade to identify the common risk factors associated with contact lens-associated microbial keratitis. In addition, genes associated with inflammatory diseases were identified from the literature and screened for genetic variants called single nucleotide polymorphisms and analysed for their potential role in susceptibility to microbial keratitis, sterile keratitis, or bacterially-induced contact lens keratitis.
Among 20,903 lenses wearers who attended L. V. Prasad Eye Institute (LVPEI) during the time period, 125 cases of contact lens-associated microbial keratitis (CL-MK) were identified. Pre-existing ocular conditions such as meibomian gland dysfunction was noted to be protective in some cases. Yearly replacement contact lenses or wearing contact lenses on an overnight wear schedule were identified as independent risk factors for developing CL-MK. Variables such as central location of the infiltrate (OR, 4.33; CI, 1.12 - 16.77; p = 0.03) and severity score (OR, 1.91; CI, 1.4 - 2.61; p< 0.001) were associated with vision loss. Cases that healed within one week were less likely to have significant vision impairment compared to cases that healed over more than two weeks. Using data obtained from culture reports, 83 (66.4%) cases were bacterial, 3 (2.4%) cases were fungal, 1 (0.8%) case was caused by Acanthamoeba, and 9 (7.2%) cases yielded mixed micro-organisms. In 21 (16.8%) cases there was no growth of microbes and the remaining 8 (6.4%) cases were treated empirically with antibiotics without culture.
Even though these risk factors were identified, they cannot explain all of the risks for developing microbial keratitis. This might be due to differences in genes that encode aspects of the innate and adaptive immunity that may predispose the individual to these adverse events. Patients with contact lens and non-contact lens associated microbial keratitis and contact lens-associated inflammatory events (n=145), and unaffected controls from the same ethnicity (n=160) who presented to the cornea and contact lens clinic at LVPEI Institute in Southern India between May 2010 and February 2012 were recruited. DNA was extracted from their peripheral blood and candidate genes associated with inflammatory diseases, IL (Interleukin) 1ï ¢, IL6, IL8, IL10, IL12B, TLR (Toll-like receptor) 2, TLR4, TLR5, TLR9, Cathelicidin antimicrobial Peptide (CAMP), and Beta defensin (DEFB) 1 were selected for study. A possible ninety-six single nucleotide polymorphisms (SNPs) from these genes were identified from dbSNP, Hap map, and Indian genome variation databases.
The SNPs rs4986791 in TLR4, and rs10499563 in IL6 were associated with susceptibility to keratitis in general. The SNPs rs1800795 in IL6, rs4073 in IL8, rs2569254 and rs730691 in IL12Bwere associated with reduced susceptibility to keratitis in general. For MK cases, the TLR5 SNP rs764535, or the DEFB1 SNP rs2980928 led to increased susceptibility, whereas, SNPs in TLR2 (rs5743706), TLR4 (rs10983755), TLR5 (rs2241096) or TLR9(rs187084) led to reduced susceptibility. For sterile keratitis (i.e. the combination of CLARE, CLPU and IK), the TLR5 SNP rs764535 or the DEFB1 SNP rs2980928 led to increased susceptibility but the TLR2 SNP rs5743706 or the IL6 SNP rs2069827 led to decreased susceptibility. For bacterially driven contact lens keratitis (CL-MK, CLARE, CLPU, IK), the TLR5 SNP rs764535 increased susceptibility whereas the TLR2 SNP rs5743706, the TLR4 SNP rs10983755, or the TLR9 SNP rs187084 reduced susceptibility.
In conclusion, the most significant risk factor for CL-MK was sleeping in lenses, which is similar to finding from studies in other countries. A central location of infiltrates was associated with vision loss and a lower severity was associated with significant visual recovery. Several SNPs were associated with increased susceptibility to MK and other adverse events with contact lens wear, yet others were protective. In future studies the functional analysis of these genes and production of their proteins should be undertaken to confirm these SNP associations. Together, this information might help in identifying patients at risk of developing keratitis with contact lens wear, and in the development of novel prophylactic and/or therapeutic strategies.