Encystment of Acanthamoeba during incubation in multipurpose contact lens disinfectant solutions and experimental formulations.

OBJECTIVES: Acanthamoeba keratitis (AK) is most commonly found in contact lens wearers with poor lens care hygiene. Recent reports have indicated an increase in cases of AK in the United States associated with the use of Complete MoisturePlus multipurpose solution. This association may not appear explainable by differences in Acanthamoeba kill rates among multipurpose solutions. In this study the physiologic response of Acanthamoeba trophozoites to incubation in various contact lens solutions was investigated.
METHODS: Acanthamoeba trophozoites were incubated in commercial and experimental solutions and the percentage encystment and viability determined in conjunction with microscopic analysis of cyst formation.
RESULTS: Among commercial products, MoisturePlus solution induced encystment within 6 to 24 hr of incubation. Only immature cysts formed which failed to mature even after 5 days incubation in the solution although they remained viable. Experimental formulations of MoisturePlus solution, modified by omitting key ingredients, identified propylene glycol as the agent responsible for inducing encystment. However, tests of other commercial solutions containing propylene glycol produced reduced or no encystment.
CONCLUSIONS: The presence of propylene glycol was shown to be the key factor in inducing Acanthamoeba encystment. However, the reduced or absence of encystment in other commercial solutions containing propylene glycol suggests additional factors, such the buffering system, may be involved in the phenomenon. How these observations relate to the solution's role in the reported association with AK is unclear. Nevertheless, it is recommended that all lens care products be tested for propensity to induce Acanthamoeba encystment and formulations containing propylene glycol may warrant special scrutiny.