Keratitis by Acanthamoeba (AK) is a serious eye infection that is extremely difficult to treat and can lead to blindness. L'Acanthamoeba is a genus of free-living amoeba found in soil and water. It is also frequently found in tap water, and transmission of the parasite through the incorrect use of contact lenses is one of the main causes of AK infection in Western countries, so much so that it has prompted the organisation of many educational campaigns to inform contact lens wearers about the proper hygiene of these devices.
AK can enter the eye either as a result of direct contact with contaminated tap water, but also through wet hands, or due to the use of an ineffective multipurpose solution to disinfect against the parasite. Data indicate that AK cases are on the rise and some outbreaks have been recorded in recent years, often related to low disinfectant power of products used for contact lens care.
Engraving as a form of resistance to disinfection
Acanthamoeba has a life cycle comprising a phase in a mobile and infectious form, the trophozoite, and a phase in the form of cysts, which are more difficult to eradicate than the trophozoite form and have been shown to be impermeable to most disinfection methods that do not involve hydrogen peroxide or povidone iodine. It has also been observed that, as in other amoeba species, AK is able to form agglomerates of cysts, called spheroids, which are arranged to protect the trophozoites, so that the viable form can also better resist external aggression. In particular, encystment occurs when Acanthamoeba identifies the environment as unfavourable, e.g. temperature conditions, osmolarity or nutrient availability.
The risk of infection related to the materials from which contact lenses are made
A recent studio showed that the possibility of AK infecting the eye following contact lens wear may not only depend on the poor disinfectant power of the solutions, but also on the materials of which the lenses themselves are made. Indeed, both trophozoites and cysts of Acanthamoeba are able to bond to a wide variety of polymer surfaces.
Previous research on the interaction of Acanthamoeba with contact lenses have focused on its adhesion rate and the number of amoebae that can form bonds with the lens surface. Meanwhile, however, the contact lens industry continues to expand with new materials and surface chemicals.
A recent study therefore observed and quantified the behaviour of six different strains of Acanthamoeba (ATCC 30010, ATCC 30461, ATCC 50370, ATCC 50702, ATCC 50703 and ATCCPRA-115) on 7 different silicone hydrogels with which contact lenses are commonly made today, to determine whether the response of AK to different materials could play a role in the transmission of infection. In addition, the disinfection efficacy of three common multipurpose solutions was evaluated to describe the disinfection resistance of trophozoites, single cysts or spheroids.
The study actually showed that the trophozoites of Acanthamoeba Of all the strains tested, there was significantly more aggregation on specific contact lens materials than on others. This is an important result as such aggregation in response to a specific material is a potential risk factor for engraftment in the wearer of those lens types. In addition, etching occurred in less than four hours on contact lens materials, faster than in nature or in the laboratory. Finally, high resistance to disinfection with multipurpose solutions was observed.
New therapeutic possibilities
It is important to emphasise that, at present, there are no drugs authorised in any country for the treatment of keratitis by Acanthamoeba. The most widely used 'off-label' therapy to date is based on a combination of diamidine and biguanide, or other non-specific drugs such as antibiotics, steroids and antifungals are used. However, current recommendations from the Centres for Disease Control and Prevention (CDC) in the United States and the Royal College of Ophthalmologists in the United Kingdom recommend treatment with polyhexanide (0.02%) or chlorhexidine (0.02%) eye drops, either as monotherapy or with the addition of diamidine.
As far as the near future is concerned, it is expected that the first specific and effective drug for the pharmacological treatment keratitis by Acanthamoeba. After more than 15 years of research, the pharmaceutical company SIFI has, in fact, submitted for approval by the EMA (European Medical Agency), an application for marketing authorisation of a drug based on polyhexanide 0.08%, which is a candidate to become the first drug authorised for the monotherapy treatment of this serious eye disease. It is expected to be launched on the European market in early 2024.