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I read with interest the article by Hiti et al1 on the susceptibility of Acanthamoeba to a multipurpose disinfecting contact lens solution and two hydrogen peroxide systems. The association between Acanthamoeba keratitis and contact lens wear is now firmly established. Thus, the use of contact lens disinfecting solutions effective at killing Acanthamoeba organisms is important in preventing corneal infections. I would like to make a few comments on this article.
The lack of standard methods for testing disinfecting solutions against Acanthamoeba represents a critical problem when sensitivity assays are set up.2 The authors say that 1 ml of disinfectant per well was applied in 24 well plates, but the amount of Acanthamoeba suspension distributed in each well is missing. PHMB (0.0005%) was found to be ineffective against A hatchetti and A lenticulta cysts after 8 hours’ exposure. This is not surprising. Indeed, at concentrations (0.5–15 μg/ml) used in commercial contact lens solutions, PHMB is almost ineffective against amoeba cysts.3–6 PHMB concentrations ranging from 45–90 μg/ml are needed to kill 99.9% of Acanthamoeba cysts in under 1 hour of exposure.7
The authors observed that a one step 3% H2O2 system with catalase was ineffective against Acanthamoeba cysts after 8 hours’ exposure. Silvany et al4 reported similar results. Because the catalyst is present from the very beginning of the disinfection step, the H2O2 is neutralised long before any disinfection can occur. Therefore, adequate exposure time before neutralisation is crucial.
The authors also found that a two step 0.6% H2O2 system was effective against A castellanii and A hatchetti cysts after 8 hours’ exposure. However, Zanetti et al6 observed that an equivalent dilution of 3% H2O2 was ineffective against the cysts of a corneal isolate of A castellanii after 9 hours’ exposure. These variations in susceptibility may depend on inherent strain differences.2 Therefore, unlike the authors, I would not recommend the two step 0.6% H2O2 system as a safe disinfectant against Acanthamoeba. From previous data,6,8,9 I suggest that the following measures should result in less contact lens case and contact lens contamination, thereby possibly reducing the risk of microbial keratitis.
Naturally: wash hands before handling contact lenses
Use “one day” disposable contact lenses
If other types are preferred:
Use a two step 3% H2O2 system and neutralise after 9 hours’ exposure (overnight)
Replace the contact lens case regularly (preferably every 2 weeks).
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