Acanthamoeba Keratitis: Symptoms, Causes, Diagnosis, and Treatment
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
Editorial note: This article is for educational purposes only and is not a substitute for professional medical advice. Severe eye pain that doesn't improve with standard treatment should be evaluated by an eye care specialist promptly. Information reflects current published clinical sources as of August 2026.
TL;DR
Acanthamoeba keratitis (AK) is a rare parasitic eye infection in which a microscopic amoeba invades the cornea — the clear front covering of your eye. Left untreated, it can damage the eye and cause loss of sight. Contact lens wearers account for at least 90% of cases, usually through habits like sleeping in lenses, storing them in tap water, or swimming in them. It is often misdiagnosed at first (75–90% of initial diagnoses are wrong) because it mimics common eye infections, and about 40% of cases need surgery. The good news: it is curable, mostly preventable, and outcomes are far better when treatment starts within about three weeks of symptoms.
Quick Answer
What is acanthamoeba keratitis?
A rare infection of the cornea caused by an amoeba — a single-celled organism that lives in freshwater, seawater, and soil.
Contact lens wearers make up at least 90% of cases; it is infectious but not contagious from person to person.
Symptoms include severe eye pain, a "something stuck in my eye" feeling, light sensitivity, watery eyes, and sometimes a ring-shaped mark on the cornea.
It is curable — especially if treatment starts within about three weeks of symptoms — and mostly preventable with proper contact lens hygiene.
What Is Acanthamoeba Keratitis?
Acanthamoeba keratitis (AK) is a rare parasitic eye infection from a certain type of amoeba. It affects the cornea, the clear, dome-shaped front covering of your eye. When not treated, it can damage your eyes and cause loss of sight.
This condition is sometimes known as "amoebic keratitis." AK usually affects one eye at a time, but it can affect both. It starts in the outermost layer of your cornea, the epithelium. As it gets worse, the infection extends deeper.
Understanding the Amoeba Behind It
An amoeba is a single-celled organism. It's similar to bacteria but a little more complex. It isn't a true parasite because it can go through its entire life cycle without needing to infect humans or any other animals — but amoebas act like parasites when they infect humans or animals.
Scientists know of at least 20 species of acanthamoeba worldwide. They can live almost anywhere humans do — freshwater, seawater, soil, and many other places. Researchers know of eight (possibly nine) acanthamoeba species that can cause AK, and two species cause most cases: A. castellani and A. polyphaga.
During their life cycles, acanthamoeba take two forms:
Form | Characteristics | What it means for infection |
|---|---|---|
Active form | Mobile, feeding stage | Enters the cornea and drives the infection |
Cyst form | Toughened outer shell | Resists treatments; can lie dormant in the cornea and return months later |
The cyst form is remarkably durable. In this state, acanthamoeba can survive:
Extreme temperatures — from -4°F to 132°F (-20°C to 56°C)
Lack of nutrients and water — up to 20 years (maybe more) at room temperature
Chemicals and toxins — including some medications that treat parasitic infections
Sunlight — cyst forms survive ultraviolet (UV) rays that kill many microbes
Knowing that is important because acanthamoeba can enter your body in both active and cyst forms. That's why you should take proper precautions.
How Common Is It?
AK is rare overall. Research indicates there could be up to 1,500 cases in the U.S. each year.
What Are the Symptoms?
The symptoms of AK happen when the active form of these microbes enters the corneas of your eyes. The symptoms may not remain constant — they cycle back and forth between better and worse.
A hallmark of AK is pain that feels disproportionate — often much more severe than the other symptoms or visible eye changes would suggest.
Symptom | What to notice |
|---|---|
Eye pain | Sometimes severe; often worse than visible changes suggest |
Foreign body sensation | Feeling like something is stuck in your eye; washing doesn't help; nothing visible |
Watery eye | Constant tearing (epiphora) |
Light sensitivity | Photophobia — discomfort in bright light |
Redness or irritation | Eye redness or general irritation |
Cornea appearance | Cloudy or dirty-looking cornea, or a ring-shaped area on its surface |
Vision changes | Blurred or clouded vision, usually in severe or advanced cases |
What Causes Acanthamoeba Keratitis?
AK is infectious — meaning you can catch it — but it isn't contagious: you can't directly catch it from someone else.
The most common ways for acanthamoeba to infect your eyes are contact lenses, contaminated water, and eye injuries.
Contact Lenses
Contact lens wearers make up at least 90% of AK cases — usually because of a combination of factors:
Risky habit | Why it matters |
|---|---|
Wearing contacts for too long | Prolonged wear irritates the cornea and traps microbes |
Improper storage | Using tap water to clean or store lenses introduces amoebas |
Incorrect cleaning | Contacts and cases aren't properly sanitized |
Swimming or showering in lenses | Water exposure drives amoebas onto and under lenses |
Contaminated items | Using tainted storage cases or solution |
Contaminated Water
Acanthamoeba can survive in water easily, especially in cyst form. Even treated drinking water, bottled water, or swimming pool water may not have a high enough chlorine concentration or other disinfectants. That's why you should never use tap water with contacts or wear contacts while swimming.
Under ordinary circumstances, acanthamoeba from these sources can't infect your eyes. But they can when:
You have an existing eye infection
You have an eye injury, like a corneal scratch
You're immunocompromised
Eye Injuries
The cornea is like your eye's windshield, but injuries make it less effective. Injuries are like weak points or small gaps in the cornea surface where it's easier for microbes to get in. Injuries to your corneas usually involve contact lenses, your fingers or fingernails, plants or plant material, or dirt or soil.
Possible Complications
There are a few main complications possible with AK. It can damage your corneas and lead to loss of sight in the affected eye(s). Because acanthamoeba in cyst form can live in your corneas for extended periods and return to their active forms later, recurrence is possible. And because it's usually a painful and disruptive condition, it can interfere with work, hobbies, and time with loved ones.
How Is It Diagnosed?
AK is tricky to diagnose early on, and an initial misdiagnosis happens in about 75% to 90% of cases. That's because:
It has the same symptoms as more common viral or bacterial eye infections
AK is rare, so eye care specialists and healthcare providers usually don't suspect it at first
The tests involve a scraping or biopsy of the cornea (which is invasive) or equipment that isn't available outside major medical centers
Your eye care specialist will do an eye exam, including a slit lamp exam, which lets them look into your eyes for signs or clues. They'll also ask about your symptoms, recent activities, or whether there could be other contributing factors.
The standard practice is to treat any eye infection as if it were viral or bacterial first — and suspect it's AK if treatment doesn't work. If it doesn't, your eye care specialist will likely recommend starting treatment for AK.
Corneal Tissue Tests
Testing for AK may involve taking samples of corneal tissue:
Test | What it involves |
|---|---|
Corneal scraping | A sample of the cornea's outermost layers is taken for testing |
Corneal biopsy | A larger tissue sample; can detect deeper infections than a scraping |
Both tests are more invasive and can be painful. But your provider will use numbing drops or medications to help with that. Corneas also regenerate quickly, which reduces how long you feel pain or discomfort. The wait time for results from corneal tissue testing can take several days — your provider will likely begin treatment to help with your symptoms in the meantime.
How Is Acanthamoeba Keratitis Treated?
Treating AK has two main goals: getting rid of the infection and reducing pain and other disruptive symptoms. Medications are usually the first option, and surgery may be necessary in more severe cases.
Yes, AK is curable — and this is easier to do when you get treatment sooner rather than later.
Medications
Active forms of acanthamoeba are very sensitive to certain medications. In cyst form, acanthamoeba species can resist treatment, but some medications can still overcome that.
The main form of treatment is topical antiseptic drops, such as chlorhexidine and polihexanide (also known as polyhexamethylene biguanide, or PHMB).
Many of the more common AK medications aren't available in the United States anymore because the condition is so rare. But some pharmacies may be able to custom-make (compound) these medications for you. Your eye care specialist is the best person to tell you about medication options and where you might be able to get them.
Surgery
About 40% of AK cases don't respond well enough to medication alone. When that happens, surgery may be the next best option:
Surgery | How it works |
|---|---|
Epithelial tissue removal (debridement) | Removes the outermost corneal layer, which may remove active or cyst-form amoebas and helps medication reach deeper layers |
Tissue grafts or special bandages | Amniotic membrane transplants or a bandage contact lens help the eye heal |
Keratoplasty and corneal transplants | Removing the cornea may be the only treatment in some cases; it is then replaced with a transplant |
What Is the Outlook?
AK is a condition that can be very painful — often much more than your other symptoms or visible eye changes would suggest — and disruptive. It's important not to ignore it, because it gets harder to treat the longer you wait.
Positive outcomes are much more likely if you get medical care before the infection spreads past the second layer of your cornea — that usually means starting treatment within three weeks of when your symptoms start.
If you have AK symptoms lasting more than a few days, see an eye care specialist. Doing so gives them time to try bacterial infection treatments first, and if those don't work, you can begin AK-specific treatments right away.
It's also important to follow your provider's treatment instructions exactly. If you don't, you may be prone to reinfection — the cyst form can live dormant in your corneas for extended periods, which can cause AK symptoms to return months after treatment.
AK can cause severe and permanent damage if it goes untreated for too long. Possible complications include glaucoma, iris atrophy (shrinking or moving out of place), cataracts, and chronic defects in the cornea's outermost layer. In more severe cases, major vision loss is possible.
Can Acanthamoeba Keratitis Be Prevented?
AK is mostly preventable, though uncommon cases can happen for reasons you can't control. The table below summarizes the key habits:
Prevention step | Detail |
|---|---|
Wear contacts as instructed | Don't wear them too long, leave them in overnight, or wear them while swimming or showering |
Store contacts properly | Use nothing but contact lens solution — never tap water |
Clean lenses and case | Lens solution usually doesn't disinfect lenses; ask your specialist how and how often to sanitize |
Discard after infection | Throw contacts away and replace the case after any eye infection to prevent reinfection |
Keep water out of eyes | Use artificial tears for dryness; use lens solution, not tap water, to rinse eyes |
Extra caution if higher risk | If immunocompromised or with an eye injury, avoid swimming or showering in ways that let water reach your eyes |
Wear eye protection | Protecting your eyes prevents injuries — and injuries are how microbes get in |
Conclusion
Acanthamoeba keratitis is one of the most painful eye conditions that exists — and it disproportionately targets people who wear contact lenses, who account for at least 90% of cases. The amoeba behind it is tiny, but its cyst form is extraordinarily tough, surviving extreme temperatures, years without nutrients, and even some treatments.
The three things to remember: pain that seems worse than it looks is a signature warning sign; the condition is frequently misdiagnosed at first, so an eye infection that doesn't improve on standard treatment deserves a follow-up and a question about AK; and timing matters — starting treatment within about three weeks of symptoms, before the infection reaches the deeper corneal layers, dramatically improves the odds of a full recovery.
The prevention side is almost entirely in your hands: proper lens wear, lens solution instead of tap water, regular cleaning, and eye protection during risky activities.
Wear contact lenses and have eye pain that won't quit, a gritty "something's stuck in my eye" feeling, or light sensitivity that lasts more than a few days? Take your lenses out, stop using tap water around your eyes, and book an appointment with an eye care specialist — and mention the possibility of acanthamoeba if routine treatment isn't working.
Frequently Asked Questions
1. What is acanthamoeba keratitis? A rare parasitic eye infection in which an amoeba invades the cornea — the clear, dome-shaped front covering of the eye. Without treatment it can damage the eye and cause vision loss.
2. Who is most at risk? Contact lens wearers, who account for at least 90% of cases — especially those who wear lenses too long, store them in tap water, swim or shower in them, or use contaminated cases and solution. People who are immunocompromised or have eye injuries also have higher risk.
3. Is acanthamoeba keratitis contagious? No. It is infectious but not contagious — you can't catch it directly from another person.
4. Can you tell if you have it on your own? There's no way to tell if you have AK (or any amoeba-related infection) on your own. Only certain medical tests, such as corneal scraping or biopsy, can show it.
5. Why is it often misdiagnosed? An initial misdiagnosis happens in about 75% to 90% of cases because AK looks like common viral or bacterial eye infections, it's rare so specialists don't suspect it first, and confirmatory tests are invasive or only available at major medical centers.
6. How is it treated? Usually topical antiseptic drops such as chlorhexidine or PHMB. About 40% of cases need surgery — debridement, tissue grafts or bandage lenses, or in severe cases a corneal transplant.
7. Is it curable? Yes. It's curable, and it's much easier to cure when treatment starts sooner — ideally within about three weeks of symptom onset, before the infection spreads past the second corneal layer.
8. Is it an emergency? No — it isn't an emergency condition. But you should make an appointment and see an eye care specialist as soon as possible.
References
This article is based on medically reviewed clinical information, including Acanthamoeba Keratitis, supplemented by the peer-reviewed literature cited therein, including Acanthamoeba Keratitis (StatPearls, 2023), the American Academy of Ophthalmology EyeWiki entry on Acanthamoeba Keratitis, the CDC's Parasites — Acanthamoeba resource, Acanthamoeba keratitis: a review of biology, pathophysiology and epidemiology (Ophthalmic & Physiological Optics, 2021), Notes from the Field: Acanthamoeba Keratitis Cases — Iowa, 2002–2017 (MMWR, 2019), and Challenges in Acanthamoeba Keratitis: A Review (Journal of Clinical Medicine, 2021).
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