Acanthamoeba keratitis: the danger of water with contact lenses

Acanthamoeba keratitis is a rare but serious infection of the cornea, caused by a parasite found in water, that affects almost exclusively contact lens wearers. It most often occurs after lenses come into contact with water: shower, swimming pool, sea, spa or simply rinsing with tap water. Difficult to diagnose and slow to treat, it can threaten vision when it is managed too late. Understanding where this danger comes from, knowing how to recognise its earliest signs and, above all, preventing it through simple habits is essential for every lens wearer.

THE ESSENTIALS

Sources: Zimmerman 2017; Posarelli 2024 (see references at the bottom of the page).

What is acanthamoeba keratitis?

Acanthamoeba keratitis is an infection of the cornea caused by acanthamoeba, a microscopic parasite found in water and damp soil. This amoeba is common and harmless to most people: we come across it regularly without ever falling ill. The problem arises when it manages to settle on a weakened cornea, the transparent window at the front of the eye Maycock 2016.

In lens wearers, several conditions come together to favour this infection. By rubbing slightly against the surface of the eye, the lens can create tiny disruptions through which the parasite enters. It also provides a surface to which the amoebae adhere, especially once a biofilm has formed. Finally, water that comes into contact with the lenses or the case brings the parasite directly into contact with the eye. It is this combination that explains why the vast majority of cases occur in contact lens wearers.

Acanthamoeba keratitis remains a rare infection, but its frequency is closely linked to lens wear and the associated risk behaviours. It differs from more common keratitis, which is often bacterial, by how difficult it is to diagnose and how demanding its treatment is. For an overview of the various infections of the cornea, our article on keratitis details the other forms and their causes.

WATER AND LENSES

Why is water the main danger?

Water, even the water that looks cleanest, is never sterile. Tap water, pool water, spa water or water from lakes and rivers contains micro-organisms, including acanthamoeba. In everyday use, this presence has no consequence. But as soon as a contact lens comes into play, water becomes a vector that brings the parasite into prolonged direct contact with the cornea. That is why contact between water and lenses is the central risk behaviour for acanthamoeba keratitis Zimmerman 2017.

These behaviours are extremely common, often without wearers being aware of it. A large survey of lens wearers measured how frequent they are:

• 86% of soft contact lens wearers shower while keeping their lenses in;
• 62% swim while keeping their lenses in;
• 31% rinse their lenses with water, and 15% store them in water.

Yet the study points out that most of these wearers mistakenly believe these habits have only a minimal effect on their risk of infection Zimmerman 2017. Wearing lenses while sleeping further weakens the cornea and often adds to water exposure, as we explain in our article on the dangers of sleeping in your contact lenses.

WARNING SIGNS

The signs that should raise the alarm

The most telling sign is intense pain, often out of proportion to the apparent appearance of the eye. Acanthamoeba keratitis can at first resemble a simple irritation or conjunctivitis: redness, watering, discomfort in light and a feeling of a foreign body. This marked and persistent pain, in a contact lens wearer, should raise suspicion of the diagnosis and prompt a consultation without delay.

These signs are not specific: they can correspond to other, more common infections of the cornea. This is precisely why you should not attempt to self-diagnose. Faced with a painful eye in a contact lens wearer, only an ophthalmological examination can identify the cause. If in doubt, contact your ophthalmologist or go to an ophthalmology emergency service, and always mention that you wear lenses and whether your eyes have been in contact with water.

DIAGNOSIS AND TREATMENT

A difficult diagnosis, a long treatment

Acanthamoeba keratitis is known to be difficult to diagnose, particularly at its onset. Its early signs resemble those of other infections, especially viral or bacterial ones, which can delay identification of the parasite. Yet the earlier the diagnosis is made, the more likely the treatment is to be effective. This frequent delay in diagnosis is one of the factors that explain the potential seriousness of this infection Buchele 2023.

To confirm the diagnosis, the ophthalmologist has several tools available: examination with the slit lamp, corneal samples analysed in the laboratory, and specialised imaging techniques such as confocal microscopy, which makes it possible to visualise the parasite. These examinations are carried out in specialised facilities, and prompt referral to them improves care.

The treatment is long and demanding: it relies on specific eye drops, often given very frequently at the start, for several weeks or even several months, with close monitoring. A review of 158 studies made it possible to clarify the prognosis of complicated forms: severe complications remain rare — corneal perforation in 2.2% of cases, enucleation in about 1% — but a corneal transplant is needed in 16.7% of cases. These figures underline the importance of prevention and of early treatment Posarelli 2024.

RISK FACTORS

Who is affected and why?

The vast majority of acanthamoeba keratitis cases occur in contact lens wearers, whether they use soft or rigid lenses. The risk increases when wear is combined with water-related behaviours or insufficient care: wearing lenses in the shower, at the pool, during water sports, rinsing equipment with tap water or overnight wear without a prescription Zimmerman 2017.

The point is not to give up lenses, which remain a comfortable solution for many people, but to correct how they are used. Most cases could be avoided by following simple hygiene rules and by ensuring no contact whatsoever between the lenses and water. This is reassuring: the main risk factor is a modifiable behaviour, unlike a predisposition over which one would have no control.

Care mistakes also play a role: reusing the solution in the case, not renewing the case regularly, or topping up the fluid instead of changing it all favour the formation of a biofilm in which the parasite thrives. These mistakes, which are very common, are detailed in our article on common contact lens care mistakes. Correcting them is an integral part of prevention.

PREVENTION

How can you protect yourself from acanthamoeba keratitis?

The prevention rule fits into one sentence: never let water touch the lenses. In practice, this comes down to a few systematic habits:

• remove your lenses before showering, bathing or swimming;
• never rinse or store your lenses in tap water;
• for swimming, if wearing lenses is essential: watertight goggles and daily disposable lenses discarded straight away;
• wash and carefully dry your hands before any handling.

Care of the equipment completes this protection. You should use only a suitable solution, renew it completely with each use, rinse the case with the solution and not with water, leave it to air-dry face down, and replace it regularly. Respecting the wearing and replacement schedule prescribed by the ophthalmologist is part of the same set of good practices.

Finally, regular ophthalmological follow-up makes it possible to check the tolerance of the lenses and to remind you of the safety instructions. Faced with any unusual symptom, in particular intense pain after contact with water, you should remove your lenses and consult quickly. Prevention and responsiveness are the two keys that allow you to continue wearing lenses while keeping this rare but serious risk at bay Maycock 2016.

FREQUENTLY ASKED QUESTIONS

Frequently asked questions

Scientific sources

1. Posarelli M, Passani A, Nardi M, et al. The incidence of severe complications in acanthamoeba keratitis. Surv Ophthalmol. 2024;69(5):769-778. PMID 38885760
2. Maycock NJ, Jayaswal R. Update on Acanthamoeba Keratitis: Diagnosis, Treatment, and Outcomes. Cornea. 2016;35(5):713-720. PMID 26989955
3. Buchele MLC, Nunes BF, Filippin-Monteiro FB, et al. Diagnosis and treatment of Acanthamoeba Keratitis: A scoping review demonstrating unfavorable outcomes. Cont Lens Anterior Eye. 2023;46(4):101844. PMID 37117130
4. Zimmerman AB, Emch AJ, Geldis J, et al. Water Exposure is a Common Risk Behavior Among Soft and Gas-Permeable Contact Lens Wearers. Cornea. 2017;36(8):995-1001. PMID 28410356

Further reading

• Sleeping in your contact lenses: what are the dangers for your eyes?
• Contact lens hygiene: the most common mistakes
• When should you seek emergency care in ophthalmology?

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Medical disclaimer

This article is intended for general information and does not replace a medical consultation. Wearing contact lenses should be the subject of a prescription and follow-up by a healthcare professional. In the event of a red eye, intense pain, a drop in vision or persistent discomfort, in particular after the lenses or eyes have come into contact with water, remove your lenses and consult an ophthalmologist or an ophthalmology emergency service quickly.