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Acanthamoeba Infection

by Dr. Ramaz Mitaishvili

Eye inflammation


Early inflammation due to Acanthamoeba keratitis.
(Photo courtesy of Dan B. Jones, M.D.)

Acanthamoeba is a microscopic, free-living ameba that is relatively common in the environment. This ameba has been isolated from water (including natural and treated water in pools or hot tubs), soil, air (in association with cooling towers, heating, ventilation and air conditioner [HVAC] systems), sewage systems, and drinking water systems (shower heads, taps). Most people will be exposed to Acanthamoeba during their lifetime and will not get sick. However, Acanthamoeba is capable of causing several infections in humans.

   * Acanthamoeba keratitis – A local infection of the eye that typically occurs in healthy persons and can result in permanent visual impairment or blindness.
   * Granulomatous Amebic Encephalitis (GAE) – A serious infection of the brain and spinal cord that typically occurs in persons with a compromised immune system.

   * Disseminated infection – A widespread infection that can affect the skin, sinuses, lungs, and other organs independently or in combination. It is also more common in persons with a compromised immune system.

Epidemiology and Risk Factors

Acanthamoeba keratitis primarily affects otherwise healthy people, most of whom wear contact lenses. In the United States, an estimated 85% of cases occur in contact lens users. The incidence of the disease in the U.S. is approximately one to two cases per million contact lens users. Contact lens wearers who practice proper lens care and non-contact lens wearers can still develop the infection. However, there are several practices among contact lens users that increase the risk of getting Acanthamoeba keratitis including:

   * Improper storage and handling of lenses
   * Improper disinfection of lenses (such as using tap water or homemade solutions to clean the lenses)
   * Swimming, using a hot tub, or showering while wearing lenses
   * Coming into contact with contaminated water
   * Having a history of trauma to the cornea

No known cases of Acanthamoeba keratitis being spread from one person to another have been reported.

Granulomatous Amebic Encephalitis (GAE) and disseminated disease are very rare forms of Acanthamoeba infection and primarily affect people with compromised immune systems. While unusual, disseminated infection can also affect immunocompetent children and adults. Conditions that may increase a patient’s risk for GAE and disseminated infection include:

   * AIDS
   * Organ/Tissue transplant
   * Steroids or excessive use of antibiotics
   * Diabetes Mellitus
   * Cancer
   * Disorders in which white blood cells in the lymphatic tissue are over-produced or abnormal
   * Disorders in which blood cells or blood clotting mechanisms do not function properly or are abnormal
   * Liver cirrhosis
   * Lupus

Causal Agents: Naegleria fowleri and Acanthamoeba spp., are commonly found in lakes, swimming pools, tap water, and heating and air conditioning units. While only one species of Naegleria, N. fowleri, is known to infect humans, several species of Acanthamoeba, including A. culbertsoni, A. polyphaga, A. castellanii, A. astronyxis, A. hatchetti, A. rhysodes, A. divionensis, A. lugdunensis, and A. lenticulata are implicated in human disease. An additional agent of human disease, Balamuthia mandrillaris, is a related free-living ameba that is morphologically similar to Acanthamoeba in tissue sections in light microscopy.


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Free-living amebae belonging to the genera Acanthamoeba, Balamuthia, and Naegleria are important causes of disease in humans and animals. Naegleria fowleri produces an acute, and usually lethal, central nervous system (CNS) disease called primary amebic meingoencephalitis (PAM). N. fowleri has three stages, cysts , trophozoites , and flagellated forms , in its life cycle. The trophozoites replicate by promitosis (nuclear membrane remains intact) . Naegleria fowleri is found in fresh water, soil, thermal discharges of power plants, heated swimming pools, hydrotherapy and medicinal pools, aquariums, and sewage. Trophozoites can turn into temporary non-feeding flagellated forms which usually revert back to the trophozoite stage. Trophozoites infect humans or animals by entering the olfactory neuroepithelium and reaching the brain. N. fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are occasionally found in CSF. Acanthamoeba spp. and Balamuthia mandrillaris are opportunistic free-living amebae capable of causing granulomatous amebic encephalitis (GAE) in individuals with compromised immune systems. Acanthamoeba spp. have been found in soil; fresh, brackish, and sea water; sewage; swimming pools; contact lens equipment; medicinal pools; dental treatment units; dialysis machines; heating, ventilating, and air conditioning systems; mammalian cell cultures; vegetables; human nostrils and throats; and human and animal brain, skin, and lung tissues. B. mandrillaris has only recently been isolated from the environment and has also been isolated from autopsy specimens of infected humans and animals. Unlike N. fowleri, Acanthamoeba and Balamuthia have only two stages, cysts and trophozoites , in their life cycle. No flagellated stage exists as part of the life cycle. The trophozoites replicate by mitosis (nuclear membrane does not remain intact) . The trophozoites are the infective forms and are believed to gain entry into the body through the lower respiratory tract, ulcerated or broken skin and invade the central nervous system by hematogenous dissemination . Acanthamoeba spp. can also cause severe keratitis in otherwise healthy individuals, particularly contact lens users . Acanthamoeba spp. and Balamuthia mandrillaris cysts and trophozoites are found in tissue.

Geographic Distribution: While infrequent, infections appear to occur worldwide.

Epidemiology and Risk Factors Acanthamoeba keratitis primarily affects otherwise healthy people, most of whom wear contact lenses. In the United States, an estimated 85% of cases occur in contact lens users. The incidence of the disease in the U.S. is approximately one to two cases per million contact lens users. Contact lens wearers who practice proper lens care and non-contact lens wearers can still develop the infection. However, there are several practices among contact lens users that increase the risk of getting Acanthamoeba keratitis including:

   * Improper storage and handling of lenses
   * Improper disinfection of lenses (such as using tap water or homemade solutions to clean the lenses)
   * Swimming, using a hot tub, or showering while wearing lenses
   * Coming into contact with contaminated water
   * Having a history of trauma to the cornea

No known cases of Acanthamoeba keratitis being spread from one person to another have been reported.

Granulomatous Amebic Encephalitis (GAE) and disseminated disease are very rare forms of Acanthamoeba infection and primarily affect people with compromised immune systems. While unusual, disseminated infection can also affect immunocompetent children and adults. Conditions that may increase a patient’s risk for GAE and disseminated infection include:

   * AIDS
   * Organ/Tissue transplant
   * Steroids or excessive use of antibiotics
   * Diabetes Mellitus
   * Cancer
   * Disorders in which white blood cells in the lymphatic tissue are over-produced or abnormal
   * Disorders in which blood cells or blood clotting mechanisms do not function properly or are abnormal
   * Liver cirrhosis
   * Lupus

Early diagnosis is essential for effective treatment of Acanthamoeba keratitis. The infection is usually diagnosed by an eye specialist based on symptoms, growth of the ameba from a scraping of the eye, and/or seeing the ameba by a process called confocal microscopy.


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Granulomatous Amebic Encephalitis (GAE) and disseminated disease are more difficult to diagnose and are often at advanced stages when they are diagnosed. Tests useful in the diagnosis of GAE include brain scans, biopsies, or spinal taps. In disseminated disease, biopsy of the involved sites (e.g., skin, sinuses) can be useful in diagnosis. Treatment

Early diagnosis is essential for effective treatment of Acanthamoeba keratitis. Several prescription eye medications are available for treatment. However, the infection can sometimes be difficult to treat and the best treatment regimen for each patient should be determined by an eye doctor. If you suspect your eye might be infected with Acanthamoeba, see an ophthalmologist immediately.

Skin infection without involvement of the central nervous system can be successfully treated. Because this is a serious infection and those affected typically have weakened immune systems, early diagnosis offers the best chance at cure.

Prevention

These guidelines should be followed by all contact lens users to help reduce the risk of eye infections, including Acanthamoeba keratitis:

   * Visit your eye care provider for regular eye examinations.
   * Wear and replace contact lenses according to the schedule prescribed by your eye care provider.
   * Remove contact lenses before any activity involving contact with water, including showering, using a hot tub, or swimming. Extended-wear contact lens users should discuss concerns with their eye care provider.
   * Wash hands with soap and water and dry before handling contact lenses.
   * Clean contact lenses according to the manufacturer's guidelines and instructions from your eye care provider.
        1. Use fresh cleaning or disinfecting solution each time lenses are cleaned and stored. Never reuse or top off old solution.
        2. Never use saline solution and rewetting drops to disinfect lenses. Neither solution is an effective or approved disinfectant.
   * Store reusable lenses in the proper storage case.
        1. Storage cases should be rinsed with sterile contact lens solution (never use tap water) and left open to dry after each use.
        2. Replace storage cases at least once every three months.

Given that Acanthamoeba is relatively common in the environment, and unlike infection of the eye, prevention of Granulomatous Amebic Encephalitis (GAE) and disseminated disease is more challenging. Since persons with weakened immune systems are more susceptible to infection, they should follow the advice of their treating physician carefully.
 
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