Anti-Acanthamoeba efficacy in contact lens disinfecting systems

Hiti et al¹ produced an interesting study investigating the susceptibility of three strains of Acanthamoeba to a PHMB (polyhexamethylene biguanide) based multipurpose solution (MPS), a one step peroxide system (3% H₂O₂), and a two step peroxide system (0.6% H₂O₂). However, no comment was made regarding the choice of strains tested, only one of which (A castellanii 4CL) was typical of those that cause keratitis in humans. This isolate belongs to genotype T4, the predominant genotype in Acanthamoeba keratitis.^2,3 Their second strain (A hatchetti 11DS) is of T6 genotype, which, with the exception of one incident,⁴ has not previously been reported as causing keratitis. The third strain (A lenticulata 72/2) belongs to genotype T5,^2,3 and as the authors themselves state “has not been reported to cause Acanthamoeba keratitis.” Although testing of all three strains was of interest to the microbiologist, the only result that can be extrapolated to clinical practice and be of clinical relevance to the contact lens wearer is that for A castellanii (4CL).

The majority of two step peroxide systems currently available contain 3% hydrogen peroxide. The authors tested a weak peroxide solution (0.6% or 6000 μg/ml), but gave no comment as to why they chose a system with such a low active concentration. The two step system was, however, found to be effective neat after an 8 hour exposure, but not as a 1:2 dilution (3000 μg/ml). This result was to be expected as Zanetti et al, in 1995,⁵ demonstrated that a 1:2 dilution of a 3% hydrogel peroxide solution killed cysts of A castellanii after a 9 hour exposure, but that a 1:10 dilution (0.3% or 3000 μg/ml) had lost its cysticidal effect.

The MPS tested was designed for use with rigid gas permeable lenses and contained PHMB at a concentration of 0.0005% (5 ppm or μg/ml). This is higher than that found in the majority of soft lens MPS, which contain 0.0001% PHMB. As the minimum trophozoite amoebicidal concentration (MTAC) of PHMB is 1 μg/ml and the minimum cysticidal concentration (MCC) is 3 μg/ml,⁶ the solution has performed as expected against the clinically relevant strain. Also, a recent study by our group⁷ testing the efficacy of a variety of MPS for soft lenses, including one containing 0.0005% PHMB, found this concentration to be cysticidal after the manufacturer’s recommended disinfection time of 4 hours. A reduced effect occurs with 0.0001% PHMB.

One step peroxide systems have been shown to be less effective than two step systems, as the peroxide in the one step solutions is rapidly neutralised.⁸ Therefore, again, the results for the one step peroxide system were as expected. In addition, a recent cohort study of CL wearers in Hong Kong has shown that lens storage cases were more likely to yield “no bacterial growth” and less likely to yield “heavy bacterial growth” if MPS were used in preference to one step peroxide solutions.⁹

The results for all three solutions when tested against clinically relevant strains of Acanthamoeba were as expected. The authors concluded by recommending the two step peroxide system (0.6% H₂O₂) for overnight soaking of contact lenses as an effective disinfectant against Acanthamoeba. However, antimicrobial efficacy is not the only factor that should be considered when advising a CL wearer about a disinfecting solution. Thought must be given to ease of use and personal compliance. It has been suggested that the introduction of MPS has led to a reduced incidence of the infection since 1995.¹⁰ Although MPS may not be as cysticidal as a two step peroxide system, compliance by the CL wearer is higher. In addition, the MPS kills bacteria and fungi within lens cases which reduces growth factors for Acanthamoeba.

CL wearers should practise good hygiene, cases should be changed monthly and only filled with sterile solutions such as MPS—tap water should never come into contact with a lens storage case. Scrubbing of cases, as recommended by the authors, is not required with frequent (monthly) replacement. If such measures are followed then the lens case is kept free of Acanthamoeba.¹¹ The use of MPS with PHMB at 0.0001% is trophozoiticidal at 24 hours with partial effect against cysts.⁷ If two step hydrogen peroxide solutions are used instead, then the peroxide should be at a concentration of 3% and not 0.6% as recommended by the authors as co-contaminating bacteria, which may be present in the lens case, can produce catalase which aids neutralisation of the active peroxide.