Acanthamoeba is a genus of microscopic, single-celled organisms found globally in various environments. These amoebae are free-living, meaning they can survive without a host. While most species are harmless, some are opportunistic pathogens capable of causing infections in humans and animals.
Where Acanthamoeba is Found
Acanthamoeba is exceptionally widespread in nature. It is commonly isolated from soil, dust, air, and a wide range of water sources, including lakes, rivers, oceans, tap water, swimming pools, and hot tubs. The organism’s prevalence also extends to sewage, air-conditioning units, and dental equipment.
The survival of Acanthamoeba across diverse habitats is due to its two-stage life cycle. The first stage is the active, feeding form known as the trophozoite, where the amoeba moves and feeds on bacteria, yeast, and other organic particles.
When faced with unfavorable conditions like temperature extremes or a lack of food, the trophozoite transforms into a dormant cyst. This cyst has a double-walled structure that makes it resistant to environmental stressors, including chlorine. This hardiness allows it to persist for long periods in treated water until conditions become favorable again.
Common Infections Caused by Acanthamoeba
The most frequent infection caused by this amoeba is Acanthamoeba keratitis (AK), a serious and painful infection of the cornea, the clear outer layer of the eye. Symptoms of AK often include severe eye pain, redness, blurred vision, a sensation of something in the eye, and intense sensitivity to light. As the infection progresses, a characteristic ring-shaped infiltrate may form on the cornea.
The vast majority of AK cases occur in individuals who wear contact lenses. The infection typically begins when the amoeba, present in contaminated water or lens solutions, becomes trapped between the contact lens and the eye. Minor corneal abrasions, sometimes caused by the lenses, can provide an entry point for the organism.
While AK is the most common, Acanthamoeba can cause other, rarer infections, primarily in individuals with weakened immune systems. One such condition is Granulomatous Amebic Encephalitis (GAE), a severe infection of the brain and spinal cord. Disseminated infections can also occur, leading to skin lesions, but these forms are not a significant risk for the general healthy population.
Diagnosis and Treatment
Diagnosing an Acanthamoeba infection requires laboratory tests, as its symptoms can mimic other common eye infections. For suspected Acanthamoeba keratitis, a physician will take a small sample of cells from the cornea (a corneal scraping). This sample is then examined under a microscope or cultured to confirm the amoeba’s presence.
For systemic infections like GAE, diagnosis is more complex and may involve brain imaging scans and biopsies of affected tissue. Early and accurate diagnosis is important for effective treatment but can be challenging because the initial symptoms are often nonspecific, which can delay the process.
Treatment for Acanthamoeba infections, particularly AK, can be a long process, often lasting for months. It involves a combination of prescription antiseptic eye drops that must be administered frequently, sometimes around the clock in the early stages. In severe cases of AK with significant scarring and vision loss, a corneal transplant may be required.
Prevention Strategies
Preventing Acanthamoeba infections centers on avoiding exposure to the organism, especially for those in high-risk groups. For contact lens wearers, meticulous hygiene is the most effective defense against Acanthamoeba keratitis.
A primary rule is to never use tap water, bottled water, or any non-sterile water to clean or store contact lenses. Always use fresh, commercially prepared sterile disinfecting solution for cleaning and storage, and never top off old solution. Contact lens cases should be cleaned, rinsed with sterile solution, air-dried after each use, and replaced every one to three months.
It is also strongly recommended to remove contact lenses before any activity involving water. This includes showering, swimming in pools, lakes, or oceans, and using hot tubs.