Acanthamoeba species

Acanthamoeba species is an extra-intestinal ameba that is ubiquitous and is found in soil and water. They are the source of various infections. Some of which can be fatal or cause blindness.

Disease / Pathology

Acanthamoeba species are known to cause granulomatous amebic encephalitis (GAE) as well as amebic keratitis, cutaneous lesions, and sinusitis. Most at risk with infections with these ameba are those that are immunocompromised such as those undergoing immunotherapy or are AIDS patients. 

In the brain the ameba produce proteases that induce massive swelling with death in 95% of those infected. Those that survive will invariably have long term morbidity to contend with. Symptoms include neurological (headache – seizures – and mental status abnormalities). 

Granulomatous lesions are not normally seen in immunosuppressed or AIDS patients as they do not have the same response as other infected patients due to their suppressed T-cell state. In immunosuppressed or AIDS patients the finding is usually one of perivascular cuffing with ameba in necrotic tissue.

Keratitis due to Acanthamoeba spp can cause eye damage and/or blindness. The source of the organism is the use of contaminated water or saline solution in contact lens wearers.

Location in the Host

Acanthamoeba spp. can cause disease in the brain, eye, skin, and sinuses. Entry of the ameba into the skin can eventually involve the brain as the ameba spreads through the blood. Inhalation of the ameba can also lead to introduction into the brain as well.  

Geographic Distribution

Worldwide distribution. In cases of keratitis the use of unsterile contact lens washing solutions can be a source of the ameba. They are found in the soil, fresh water. 

Life Cycle

Image courtesy of CDC

Morphology & Diagnosis

Acanthamoeba species detection of the trophozoites and cyst forms in tissue or material from the infected site is diagnostic. Trophozoites are 15-25 u, in length and amoeboid in shape. The Cyst has a thick wall that is characteristic and stress resistant as seen in the image below. 

Keratitis is usually diagnosed by clinical presentation and history; visualization of the organisms with calcofluor white staining of corneal scrapings (calcofluor white stain has an affinity for the polysaccharide polymers of amebic cysts); and the culture of the ameba in E. coli inoculated media. The culture method is rarely done as the turn around time is not acceptable, methodology is complicated for most laboratories, and technologist expertise in performing the assay is rarely available. Because of the risk of vision loss, treatment is initiated based on high suspicion and rapid testing methods such as in office staining by the physician.

Images

Acanthamoeba species encephalitis – a magnified view of brain tissue within which was a centrally located Acanthamoeba sp. cyst. Image courtesy of DPDx
Wet mount of an Acanthamoeba spp. cyst – photo courtesy of CDC/Dr. George Healy