Dear Dr Pinna,

Thank you very much for your response to our article on Acanthamoeba and contact lens disinfecting solutions. You are absolutely right, indeed, standard methods for testing of anti-Acanthamoeba effectivity of biocides are urgently needed. However, as such standards are not yet available, we used three different concentrations of amoebae, namely 103, 104, and 105 per ml (as stated in the Materials and Methods section of our article) intending to cover the whole range of concentrations we found in the literature.

You are right in what you stated on the various contact-lens solutions, however, it was our aim to compare all three different systems available on the market in spite of knowing that they would not all be optimal in their effectivity.

We fully agree to the measures you recommend, however, they depend on the compliance of the contact lens users. It is a problem of costs on one hand: the user wants to safe money by overwearing the lenses, not replacing the lens boxes and economizing with the lens solutions. One the other hand it is a problem of comfort, as using multipurpose or one step solutions is more comfortable. So we tried to give realistic recommendations, which help to minimize the risk, but are still user- friendly.

Horst Aspöck and Julia Walochnik

Dear Editor

I read with interest the article by Hiti et al.[1] 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 to 15 µg/ml) used in commercial contact lens solutions, PHMB is almost ineffective against amoeba cysts.[3-6] PHMB concentrations ranging from 45 to 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 al.[4] 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 al.[6] 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:
(a)Use a two-step 3% H2O2 system and neutralize after 9 hours' exposure (overnight).
(b)Replace the contact lens case regularly (preferably fortnightly).

References

(1) Hiti K, Walochnik J, Haller-Schober EM, et al. Viability of Acanthamoeba after exposure to a multipurpose disinfecting contact lens solution and two hydrogen peroxide systems. Br J Ophthalmol 2002;86:144-6.

(2) Meisler MD, Rutherford I. Acanthamoeba and disinfection of soft contact lenses. Rev Infect Dis 1991;13:S410-2.

(3) Hugo ER, McLaughlin WR, Oh K, et al. Quantitative enumeration of Acanthamoeba for evaluation of cyst inactivation in contact lens care solutions. Invest Ophthalmol Vis Sci 1991;32:655-7.

(4) Silvany RE, Dougherty JM, McCulley JP. Effect of currently available contact lens disinfection systems on Acanthamoeba castellanii and Acanthamoeba polyphaga. Ophthalmology 1990;97:286-90.

(5) Silvany RE, Dougherty JM, McCulley JP. Effect of contact lens preservatives on Acanthamoeba. Ophthalmology 1991;98:854-7.

(6) Zanetti S, Fiori PL, Pinna A, et al. Susceptibility of Acanthamoeba castellanii to contact lens disinfecting solutions. Antimicrob Agents Chemother 1995;39:1596-8.

(7) Burger RM, Franco RJ, Drlica K. Killing Acanthamoebae with polyaminopropyl biguanide:quantitation and kinetics. Antimicrob Agents Chemother 1994;38:886-8.

(8) Grey TB, Cursons RTM, Sherwan JF, et al. Acanthamoeba, bacterial, and fungal contamination of contact lens storage cases. Br J Ophthalmol 1995;79:601-5.

(9) Pinna A, Sechi LA, Zanetti S, et al.Bacillus cereus keratitis associated with contact lens wear. Ophthalmology 2001;108:1830-4.