Over the years, Acanthamoeba cases have been reported from many countries worldwide. In the United States, both CNS and Acanthamoeba eye infections have been reported. CNS infections occur primarily in patients who are immunocompromised or weakened.Contact lenses wearers, particularly those with soft contacts, may be at risk of contracting Acanthamoeba eye infections. Poor hygiene practices, especially the use of homemade saline rinsing solutions, are the major risk factors that can lead to these infections.Animals, including rabbits, beavers, cattle, water buffalo, dogs and turkeys, are known to contract Acanthamoeba infections. As in humans, immunocompromised animals appear to contract fatal CNS infections.
Morphology of Acanthamoeba species
The Acanthamoeba trophozoite averages 25 μm with a range of 12 to 45 μm. Motility is lenient or sluggish and there is little evidence of progressive motility.Spine Like pseudopods, known as acanthopodia, project from the base of the organism. Acanthamoeba trophozoites contain one nucleus, consisting of a large karyosome similar to Naegleria fowleri.Obvious peripheral chromatin is missing. The cytoplasm is granular and vacuolated.
The round- shaped Acanthamoeba cyst measures 8 to 25 μm. The cyst has a double cell wall. The inner smooth cell wall is surrounded by an external jagged cell wall.This appearance is characteristic and helps in the identification of Acanthamoeba cysts.The single nucleus is similar in appearance to that of the corresponding trophozoite, a large karyosome and no obvious peripheral chromatin.The nucleus is surrounded by a disorganized, granular and sometimes vacuolated cytoplasm.
Life Cycle of Acanthamoeba species
Acanthamoeba trophozoites and cysts convert between these two morphological forms in the external environment.Human beings can acquire Acanthamoeba in one of two ways.One route involves aspiration or nasal inhalation of the organisms.Trophozoites and cysts enter the lower respiratory tract or ulcers in the mucosa or skin.These organisms often migrate through the bloodstream and invade the central nervous system( CNS), causing severe CNS infections. The second route of infection is the direct invasion of the parasite in the eye.Two groups of people are at risk for direct eye invasion, wearers of contact lenses and those who have experienced trauma to the cornea.Contact lens wearers who use homemade, non- sterile saline solutions that are contaminated with Acanthamoeba usually suffer from a serious eye infection known as Acanthamoeba keratitis.It’s important to note that unlike Naegleria fowleri associated with swimming or bathing in contaminated water, Acanthamoeba spp. Infection is not associated with water, but with contaminated saline.
There are currently 10 Acanthamoeba species known to infect humans.Acanthamoeba castellanii has been identified as the species responsible for most CNS and human eye infections.The names of these species and the type of infection with which each is associated are listed in the table below.
|Species Name||Associated with CNS Infection||Associated with Eye Infection|
Clinical symptoms of Acanthamoeba species infections
Granulomatous amebic encephalitis
Acanthamoeba CNS infections are also known as granulomatous amebic encephalitis( GAE).The symptoms of this condition develop slowly over time and include headaches, seizures, stiff neck, nausea and vomiting.Granulomatous brain lesions are characteristic and may contain both Acanthamoeba trophozoites and cysts.Occasionally, Acanthamoeba spp invades other areas of the body, including the kidneys, pancreas, prostate and uterus, and forms similar granulomatous lesions.
Acanthamoeba infections in the cornea of the eye are known as amebic keratitis.Severe eye pain and vision problems are common symptoms.The infected corneal tissue may contain Acanthamoeba trophozoites and cysts.Perforation of the cornea and subsequent loss of vision may result.
Laboratory Diagnosis of Acanthamoeba infections
Just as with Naegleria fowleri, the specimen of choice for the diagnosis of Acanthamoeba spp trophozoites and cysts is CSF. Tissue of the brain can also be examined. Corneal scrapings are the choice specimens for the recovery of Acanthamoeba infections of the eye.On non- nutrient agar plates seeded with gram- negative bacteria( specifically, a viable strain of E. coli), suspected corneal scrapings may be grown.The bacteria serve as a food source for the parasites. As Acanthamoeba organisms feed, they produce a number of marks( known as tracks) on the agar. Histological examination of corneal scrapings may also recover Acanthamoeba.Although mainly used to detect fungi in clinical specimens, calcofluor white may be used to stain Acanthamoeba cysts present in corneal scrapings. Indirect immunofluorescent antibody staining is the technique of choice for speciation of Acanthamoeba.
Treatment of Acanthamoeba infections
Due to the slow progression of GAE, most patients who suffer from it die not only before an accurate diagnosis can be made, but also before experimental treatments can be administered and studied.There is some evidence to suggest that sulfamethazine may be a suitable treatment. Cases of Acanthamoeba keratitis have been successfully treated with several medications including itraconazole, ketoconazole, miconazole, propamidine isethionate and rifampin.Propamidine appears to have the best documented success record of all these agents. The key to successful treatment of eye infections is to start treatment immediately after the infection has been diagnosed.
Prevention and Control of Acanthamoeba infections
Strategies designed to prevent individuals from contracting Acanthamoeba CNS infections are difficult to determine because the life cycle of this ameba is poorly understood.However, Acanthamoeba eye infections can be prevented primarily by following all protocols established by the manufacturer associated with the use of contact lenses.One of the most important protocols for contact lens wearers is to avoid using homemade, non- sterile saline solutions.