Acanthamoeba Facts, Eye Infection, and Keratitis
What Is Acanthamoeba?
Acanthamoeba is a microscopic, single-celled organism that is found in the environment and sometimes becomes a parasite in humans. It can cause a disease called Acanthamoeba keratitis in the eye. In this often painful condition, the patient's cornea becomes cloudy and the person finds it difficult to see. If the disorder isn't treated, permanent vision problems or even blindness can result. Contact lens wearers are especially susceptible to the disease. The disorder is rare, but recently there have been increased reports of people suffering from the condition.
Many species of Acanthamoeba exist and multiple forms infect humans. The different species are often hard to tell apart visually, but scientists can identify them genetically. They have a widespread distribution and some interesting features. The organisms sometimes live in our body without making us sick. Unfortunately, they can also cause disease.
The CDC (Centers for Disease Control and Prevention) lists nine species of Acanthamoeba that can infect humans. The species are often referred to as a single entity, but researchers have found differences between them.
Distribution in the Environment
Acanthamoeba species are very common in the environment. The list of places where they've been found is alarmingly long. The amoebas are found in soil, on plants, and in fresh water. They can be found in tap water, swimming pools—even chlorinated ones— lakes, rivers, and ponds. They have also been found in sea water and in the air. They've been discovered in and on items and equipment that we use, including bottles of mineral water and distilled water, vegetables, shower heads, heating and air-conditioning devices and ducts, hospital equipment, sewage, and contact Iens cases.
It's thought that most of us are exposed to the organism on a routine basis and that it frequently enters our body. In most cases, this doesn't make us sick. Another interesting observation is that although the incidence of Acanthamoeba keratitis is highest in contact lens users, many people who wear the lenses don't develop a corneal infection. In addition, when the cornea is infected, the infection is prevented from travelling further into the body.
The observations described above suggest that our bodies have very effective ways to protect us from the parasite, or at least from most varieties of the parasite and under most conditions. Researchers are trying to discover exactly how this protection works.
Like other amoebas, Acanthamoeba is frequently referred to as a protozoan. Amoeboid protozoans move by extending part of their body and then slowly flowing into it. The extension from their body is known as a pseudopod. Pseudopods, or pseudopodia, are extended in different directions as the organism moves. This behaviour may remind some people of the amoebas that they observed in school. The word "amoeba" is sometimes used as a general term for all organisms that move by extending pseudopods.
Acanthamoeba feeds on bacteria, yeasts, and organic particles. It extends pseudopods around its prey to trap it. The prey then enters a food vacuole, where digestion occurs. The amoeba also extends fine, spine-like pseudopods from its body, which are often called acanthopodia. They can be seen in the first photo in this article and in the video above.
The nucleus of the parasite is spherical and contains a nucleolus in its centre. The cell has one or more contractile vacuoles. These absorb water that enters the cell and then release it through a temporary pore in the cell membrane.
There are two stages in Acanthamoeba's life cycle: the trophozoite and the cyst. The trophozoite is the amoeboid stage and the one that feeds. The cyst is a double-walled stage that is inactive. It forms when conditions are potentially dangerous for the cell. Examples of these conditions include lack of food, a change in pH, temperature extremes, and the presence of harmful chemicals.
Giant viruses have been found in some types of Acanthamoeba. Giant viruses are huge compared to their smaller relatives and have some distinct features.
Structure and Function of the Cornea
The cornea is the outermost, transparent layer at the front of the eye. Light rays reflected off objects pass through the cornea and then travel to the retina at the back of the eyeball. The retina sends a signal along the optic nerve to the brain, which creates an image. If the cornea becomes cloudy and no longer transmits light rays to the retina, we will be unable to see.
The cornea consists of five layers. Starting at the front of the eye, these layers are as follows:
- epithelium - a surface layer that is five to seven cells thick and protects the cornea
- Bowman's layer - a thin layer made of collagen, a major protein in our body
- stroma - the thickest part of the cornea; contains collagen fibres and cells called keratocytes
- Descemet's membrane - a thin layer made of collagen fibres that are in a different form from those of the stroma
- endothelium - the thin, innermost layer
The collagen fibres in the cornea have a specific arrangement. This arrangement is vital in order to maintain the transparency of the cornea. If the alignment of the fibres and the spaces between them are changed, the cornea becomes cloudy.
Possible Causes of Acanthamoeba Keratitis
Keratitis is inflammation of the cornea. An Acanthamoeba infection is one cause of the problem, though other microbes can also cause keratitis. The infection may develop due to one of the following factors.
- Washing the hands but not completely drying them before touching contact lenses (Water drops on the hands may contain the parasite.)
- Cleaning lenses incorrectly by using tap water or a homemade solution
- Wearing the lenses while showering, swimming, using a hot tub, or participating in any other activity that may expose the eyes to contaminated water
- Storing the lenses in an unsterile environment
- Experiencing repeated trauma to the cornea (Wounds on the cornea make it easier for the parasite to enter.)
Photo A above shows the cornea of a patient with Acanthamoeba keratitis. Photo B shows the cornea of a patient after sodium fluorescein administration. The chemical is used as a diagnostic tool.
Effects of the Parasite
The actions of the parasite and the ways in which it exerts its effects are still being studied. The basic steps of the infection are described below.
- The parasite adheres to the surface of the cornea.
- It then breaks down the outer layer of the cornea, or the epithelium.
- Next, it enters the cornea.
- Once inside the cornea, the parasite begins to destroy it. The destruction involves the loss of the stroma (which forms the bulk of the cornea), including the keratocytes. These cells make the materials in the cornea and repair the structure when it's damaged or inflamed.
Acanthamoeba cysts may form within the stroma. These sometimes survive the treatment for the disease, releasing new trophozoites afterwards. This is one reason why the disorder can sometimes be hard to treat.
Possible Symptoms and Treatment
Some possible symptoms of Acanthamoeba keratitis are listed below. A patient may not have all of them. In addition, the symptoms may indicate the existence of a different problem.
- eye pain, which may be severe
- red eyes
- a feeling that something is in the eyes
- blurred vision
- an excessive production of tears
- light sensitivity
The infection is generally treated with antimicrobial chemicals that kill the parasite. Current medications sometimes take a considerable time to work, however, because the parasite is becoming resistant to certain drugs. If the cornea becomes seriously scarred, a corneal transplant may be needed.
The black scale bar in the photo above and in the first photo in this article represents ten micrometres. A micrometre is one thousandth of a millimetre and a millionth of a metre.
Preventing the Disease
The following steps are often recommended to prevent the development of Acanthamoeba keratitis.
- Wash and dry hands before touching contact lenses.
- Don't allow tap or other drinking water to contact the lenses.
- Remove the lenses in potentially harmful situations, such as showering.
- Clean the lenses with sterile solution recommended by an eye doctor.
- Follow the instructions about using the sterile solution carefully.
- Store the lenses in their case.
- Follow the instructions about cleaning the case and replacing it.
- Visit the eye doctor regularly so that any problems can be discovered in an early stage when they may be easier to treat.
Acanthamoeba keratitis is a rare infection, but when it occurs, it has a devastating effect for a long period of time on the patient's life. Only 70% of patients were cured within 12 months. For the remaining 30%, the treatment took over a year."— Dr. John Dart, Moorfields Eye Hospital, via CNN.
An Outbreak of Acanthamoeba Keratitis
The incidence of Acanthamoeba keratitis tripled between 2011 and 2016 in southeast England. The outbreak appears to be continuing. Researchers from University College London and Moorfields Eye Hospital have been studying the situation. After analyzing data from questionnaires, they say the reasons for the outbreak are likely one or more of the following:
- poor lens hygiene
- the use of a lens disinfectant containing a chemical called Oxipol (which is no longer used by the manufacturer)
- wearing contact lenses while swimming or in a hot tub
Acanthamoeba is more common in the UK water supply than in that of many other countries. This is due to the fact that the water often comes from a domestic supply instead of a major one. The water in local areas is frequently rich in lime, which supports the growth of the parasite population. Despite this fact, other countries are following the UK outbreak with interest because similar events have occurred in additional parts of the world, including the United States.
Moorfields Eye Hospital says that they have only seen cases where both eyes have been infected by Acanthamoeba at the same time instead of ones in which the parasite has spread from one eye to another. It's possible to have the infection in only one eye, however.
Further Studies About an Interesting Organism
Biologically, Acanthamoeba is an interesting organism. From a medical point of view, it's important to learn as much about it as possible. Further studies are needed to clarify its behaviour in our body and the way in which the body responds to its presence. We need to know how the different species and strains of the parasite behave, how people resist the infection, and how to prevent it in susceptible people. The results of the research could be very interesting as well as helpful.
Siddiqui, R., & Khan, N. A. (2012). Biology and pathogenesis of Acanthamoeba. Parasites & Vectors, 5, 6. http://doi.org/10.1186/1756-3305-5-6
The cornea and corneal disease from the National Eye Institute
Information about Acanthamoeba as a parasite from the CDC
Keratitis facts from the Mayo Clinic
Acanthamoeba keratitis facts from Moorfields Eye Hospital
Outbreak of eye infection in contact lens wearers from the MedicalXpress news service
Ongoing outbreak of eye infection in contact lens wearers from CNN
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
© 2018 Linda Crampton