Acanthamoeba keratitis

What is Acanthamoeba keratitis?

Acanthamoeba keratitis (AK) 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 (single-celled organism) called Acanthamoeba which is commonly found in the environment – in rivers, lakes, seawater, soil and air. 

While these microbes do not usually cause any harm to humans, they can act as parasites and 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 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 causes Acanthamoeba keratitis?  

AK is caused by a microbe called Acanthamoeba commonly found in water sources, typically in domestic hot and cold tap water, swimming pools, hot tubs, and showers. While exposure to Acanthamoeba in water rarely causes problems, occasionally the organism can infect cells on the outer surface of the cornea. 

The organism needs to make direct contact with the eye to cause AK – so it can’t be contracted from drinking water that contains the amoeba.

In the UK, most people who get AK wear contact lenses. 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.

Common risk factors for Acanthamoeba keratitis include:

• Use of reusable contact lenses
• Using tap water to clean or store contact lenses
• Disinfecting contact lenses incorrectly – such as by not following the manufacturer’s instructions
• Reusing solution or topping up the solution 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
• An injury to the cornea (such as a scratch or abrasion)

What are the signs and symptoms of Acanthamoeba keratitis? 

The symptoms of Acanthamoeba keratitis include eye redness, light sensitivity, blurred vision, your eyes watering, and feeling like there is something in your eye. It can cause severe pain, although the pain varies and it may not be painful in the early stages.

If you have any of these symptoms, particularly if you wear contact lenses, seek medical attention as soon as possible.

Symptoms can take up to a few days to show up after the Acanthamoeba has entered the eye. They can also be intermittent, meaning they can come and go before the infection becomes more serious.

It can be difficult to know whether someone has the infection, as the appearance of the infected eye is often quite similar to that of other corneal infections.

Is Acanthamoeba keratitis serious?  

Acanthamoeba keratitis needs immediate medical attention and intensive treatment before it results in permanent sight loss or blindness due to corneal damage from the parasite. AK can be very painful, and treatment can be lengthy and is not always effective. 

How is Acanthamoeba keratitis diagnosed? 

An eye doctor will usually ask you about your symptoms and examine your eye to look for signs of inflammation in the cornea. They may carry out a test called corneal scraping, which involves collecting a sample of cells from the cornea's surface, which is sent to the lab to check for infection.

Another test they may carry out is confocal microscopy, which uses a powerful microscope with laser light to create highly detailed images of the cornea to see whether any Acanthamoeba cells are present.

Early diagnosis is essential for the effective treatment of AK. 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 lab tests, or from how the eye responds to treatment.

What are the treatments for Acanthamoeba keratitis?  

Doctors usually prescribe antiseptic eye drops, such as chlorhexidine, polyhexanide (also called PHMB), and propamidine (Brolene). You may also be prescribed additional antibiotic and antifungal drops or medication, as well as anti-inflammatory and pain relief tablets if needed – anti-inflammatory eye drops are sometimes added after two or more weeks. 

These treatments are needed for up to five months in half of people with AK and for longer in the remaining half. This prolonged course of treatment is effective for most people.

A corneal transplant may be recommended for people with the most severe infections that have progressed to advanced stages or have not responded to the medical treatments. This may include a corneal transplant where the damaged cornea is removed and replaced with a healthy one from a suitable donor, but when used for advanced AK this has a high failure and complication rate.

What can help Acanthamoeba keratitis?  

Preventing Acanthamoeba keratitis is crucial, particularly as treatment can be lengthy and is not always effective. Practising good contact lens hygiene, and avoiding the risk factors listed above, will reduce the chance of developing AK.

People who wear reusable contact lenses are nearly four times as likely as those wearing daily disposables to develop a rare sight-threatening eye infection. So switching to daily disposable lenses can reduce the risk, providing they are used according to manufacturer’s instructions.

This is probably because daily disposable use eliminates lens hygiene issues and the need for a lens storage case. Contamination by Acanthamoeba is found in up to 5% of lens cases from unaffected people.

These tips can help prevent AK:

• 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 with new lenses after the swim. These measures have not been shown to be as safe as not using contact lenses 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.
• Follow the advice from your optician and the manufacturer’s instructions for cleaning and storing your contact lenses and storage case.
• For reusable contact lens users:
  
  • Rinse and store reusable contact lenses only with the recommended disinfecting solution (never tap water or saline).
  • 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.

What research is there into Acanthamoeba keratitis?  

Current Acanthamoeba keratitis research is focused on improving diagnosis and understanding why some people are more severely affected than others. We also need to find targets for drug treatment.

For more than 70 years our research has been fuelling projects helping to unlock the secrets of dozens of different eye conditions. Right now, the brilliant minds we fund are working to find new treatments and cures for AK, and to understand the condition better.

A study led by Fight for Sight, the NIHR Moorfields Biomedical Research Centre and Moorfields Eye Charity, was able to show the increased risk of Acanthamoeba keratitis from wearing reusable contact lenses compared to daily disposables. The researchers estimated that 30-62% of cases in the UK, and potentially in many other countries, could be prevented if people switched from reusable to daily disposable lenses.

See our research projects