What is Acanthamoeba keratitis?
Acanthamoeba keratitis is a rare but serious eye infection of the cornea, the clear surface (or window) at the front of the eye. The infection is caused by a microscopic amoeba (a single-celled organism) called Acanthamoeba. These tiny organisms are commonly found in the environment – for example in rivers, lakes, seawater, soil and air. While these microbes do not usually cause any harm to humans, they can cause severe eye disease if they infect the cornea. Most infections come from exposure to fresh water – such as tap water, swimming pools, hot tubs and showers.
Acanthamoeba keratitis can be very painful – and, if left untreated, it may cause permanent sight loss or blindness due to corneal damage from the parasite. It can affect one or both eyes and is not contagious (so it can’t be passed from person to person). While anyone can develop Acanthamoeba keratitis, contact lens wearers are at particularly high risk of this infection.
What are the causes of acanthamoeba keratitis?
Acanthamoeba causes this eye infection. These amoebae are commonly found in water sources – typically in domestic hot and cold tap water, swimming pools, hot tubs and showers. While exposure to Acanthamoeba in contaminated water rarely causes any problems, the organism can occasionally infect cells on the outer surface of the cornea. The organism needs to make direct contact with the eye to cause Acanthamoeba keratitis – so it can’t be contracted from drinking water that contains the amoeba.
In the UK, most people who get acanthamoeba keratitis wear contact lenses. About 1 in 30,000 contact lens wearers become infected. But anyone with damage to the cornea (such as a corneal abrasion caused by contact lens wear or other minor eye injuries) is also at risk of developing the infection after exposure to Acanthamoeba. This is because the parasite can enter the eye more easily at the site of the injury where it can begin the process of infection.
Using tap water to clean or store contact lenses, or having poor contact lens hygiene, increases the risk of Acanthamoeba keratitis. Examples of poor lens hygiene are improper use of disinfection solutions (not following manufacturer’s instructions), reusing the solution or topping up the solution in the contact lens case, failing to empty and dry the contact lens case after use, and storing lenses in tap water overnight; the correct use of daily disposable lenses has been shown to reduce the risk of being infected by Acanthamoeba about three-fold compared to reusable lenses, probably because daily disposable use eliminates these lens hygiene issues and the need for a lens storage case. Wearing contact lenses when swimming or taking a shower also increases the risk of an infection. So does putting lenses in with wet hands from tap water.
Common risk factors for Acanthamoeba keratitis include:
- Use of reusable contact lenses
- Disinfecting contact lenses incorrectly – such as by using tap water or non-sterile solutions.
- Reusing solutions in the lens storage case.
- Failing to empty and dry the lens storage case properly after use.
- Poor lens handling hygiene – such as putting lenses in with wet hands after washing them in tap water.
- Swimming, using a hot tub or taking a shower while wearing lenses.
- The eyes coming into contact with contaminated water.
- An injury to the cornea (such as a scratch or abrasion).
What are the symptoms of Acanthamoeba keratitis?
The main symptom of Acanthamoeba keratitis is pain (which can be very variable from severe to absent) and inflammation. It can be difficult for health care workers to know whether someone has the infection as the appearance of the infected eye is often quite similar to that of other corneal infections.
The symptoms of Acanthamoeba keratitis include eye redness, light sensitivity, blurred vision and variable pain. These symptoms can take up to a few days to show up after the Acanthamoeba has entered the eye – and they can also be intermittent, meaning they can come and go before the infection becomes more serious.
The symptoms of acanthamoeba keratitis may include:
- Eye pain (which can be very severe - but may be painless in the early stages)
- Red eyes
- Sensation of something in the eye
- Blurred or poor vision
- Sensitivity to light
- Excessive tear production
Anyone experiencing any of these symptoms, particularly if they’re a contact lens wearer, should seek medical attention as soon as possible.
How is Acanthamoeba keratitis diagnosed?
Early diagnosis is essential for the effective treatment of Acanthamoeba keratitis. However, the infection is easy to misdiagnose in the early stages because the symptoms are so similar to other corneal infections. Sometimes a diagnosis can change when the doctor gets more information from laboratory tests – or how the eye(s) responds to treatment.
An eye doctor will usually review symptoms and perform an eye examination to look for signs of inflammation in the cornea. They may also carry out further tests including:
- Corneal scraping: a procedure to collect a sample of cells from the surface of the cornea, which they will then send for laboratory tests to check for infection.
- Confocal microscopy: this powerful microscope uses laser light to create highly detailed images of the cornea to see whether any Acanthamoeba cells are present.
How is Acanthamoeba keratitis treated?
Acanthamoeba keratitis needs immediate medical attention. Intensive and early treatment – usually with eye drops – is required before the infection results in permanent sight loss.
Doctors will usually prescribe antiseptic drops – sometimes with additional antibiotic and antifungal eyedrops. Anti-inflammatory and pain relief medications are given by mouth as needed and anti-inflammatory eye drops may be added in some cases after two or more weeks. These treatments are needed for up to five months in half of patients and for longer in the remaining half; this prolonged course of treatment is effective in most patients. A corneal transplant may be recommended for patients with the most severe infections that have progressed to advanced stages or have not responded to the medical treatments.
Potential treatments for Acanthamoeba keratitis include:
Topical antiseptics (chlorhexidine, polyhexanide (also called PHMB), and propamidine (Brolene) are the most common treatments for Acanthamoeba keratitis. These are applied directly to the surface of the eye and may need to be continued for many months.
Antibiotics or antifungal medications
These may be recommended if the eye is also infected with bacteria or fungi as well as Acanthamoeba and one anti-fungal drug (voriconazole) has some effect on Acanthamoeba and may be used as an eye drop or taken by mouth.
Steroid eye drops or oral non-steroidal anti-inflammatory drugs (NSAIDs)
These may be prescribed to help reduce pain and inflammation.
Surgery may be needed to treat the most serious infections that don’t improve with topical therapy. This may include a corneal transplant where the damaged cornea is removed and replaced with a healthy one from a suitable donor; the failure and complication rate is high in this setting.
How can Acanthamoeba keratitis be prevented?
Preventing Acanthamoeba keratitis is crucial, particularly as treatment can be lengthy and is not always effective. Practicing good contact lens hygiene – for example, by cleaning and drying hands before touching lenses – will reduce the chance of developing Acanthamoeba keratitis. It’s also important to avoid any activities that may increase the risk of exposing the eyes to Acanthamoeba – such as swimming, using a hot tub or showering while wearing contact lenses.
Contact lens users should also follow the advice from their optician and the manufacturer’s instructions for cleaning and storing their lenses and storage case. They might also want to consider using daily disposable lenses rather than reusable lenses – use of daily disposable lenses (providing they are used according to manufacturer’s instructions) are three times less likely to be linked to Acanthamoeba keratitis compared to re-usable lenses – probably because the use of daily disposables eliminates the risks of lens case contamination by Acanthamoeba which is found in up to 5% of lens cases from unaffected individuals.
Top tips for preventing acanthamoeba keratitis:
- Use daily disposable lenses if possible; these are not available in some prescriptions.
- Remove contact lenses before sleeping.
- Replace contact lenses regularly, according to the manufacturer’s recommendations.
- Remove contact lenses before any activity involving contact with water including showering, swimming, or using a hot tub. If you have to use contact lenses when swimming use goggles and replace the lenses for new after the swim – these measures have not been shown to be as safe as not using them at all but are likely to reduce the risk.
- Visit the optician for regular eye examinations.
- Always wash and dry your hands well before touching the eyes or handling contact lenses.
- For reusable contact lens users:
- Rinse and store contact lenses only with the recommended disinfecting solution (never tap water or saline).
- Fill the contact lens storage case with fresh solution every time it is used – never mix fresh solution with old or used solution.
- Rinse the contact lens storage case with sterile contact lens solution (never tap water) and leave it open to dry after every use.
- Replace the lens case monthly.
Latest Research on Acanthamoeba keratitis
Current Acanthamoeba keratitis research is focussed on improving diagnosis and understanding why some people are more severely affected than others. We also need to find targets for drug treatment.
Here are some of our latest stories on AK research:
Acanthamoeba keratitis clinical trials
You could play an important part in eye research by being a participant in a clinical research study that may benefit many people. You could even help shape clinical research by becoming more actively involved and having a say. Patients, carers, or anyone with an interest can help.
What are clinical trials
Clinical trials are research studies that find out if a medical strategy, treatment, or device is safe and effective for humans. They are a key research tool for improving medical knowledge and patient care. The people who carry out research are mostly the same doctors and healthcare professionals who treat people. Their aim is to find better ways of treating patients and keeping people healthy.
Here are some ways to find out about research projects and clinical trials that you can get involved in.
UK Clinical Trials Gateway
The UK Clinical Trials Gateway run by the National Institute for Health Research (NIHR) provides easy to understand information about clinical research trials running in the UK, and provides a large range of information about these trials. It is designed to enable patients and clinicians to locate and contact trials of interest. Visit their website and select the eye condition that you are interested in.
NIHR Clinical Research Network Portfolio
The NIHR Clinical Research Network Portfolio is a database of high-quality clinical research studies in England, Northern Ireland, Scotland and Wales. Within this, the Ophthalmology Specialty Group supports a national portfolio of research studies in ophthalmology and the vision sciences. See their website for details.
If you wish to join a trial it is always best to discuss this with your doctor or clinical team first.
Last updated January 2023
Approved by Professor John Dart, Moorfields Eye Hospital and UCL Institute of Ophthalmology