Free-living ameba, usually harmless residents of soil and water, can occasionally cause devastating illness in humans. There are three distinct human illnesses caused by these protozoa. Primary amebic meningoencephalitis (PAM) is a disease of previously healthy individuals and is caused by Naegleria fowleri. Granulomatous amebic encephalitis (GAE) is caused by Acanthamoeba species and Balamuthia mandrillaris and occurs in both healthy and immunocompromised persons. PAM and GAE are found worldwide but are more common in tropical regions. Acanthamoeba also causes chronic amebic keratitis, which is related to contact lens use and is thus a disease primarily of wealthier countries.
PAM is a devastating illness that is usually fatal and often occurs in children with a history of recent fresh-water exposure. The organism enters through the nose of the victim and penetrates the cribriform plate to the subarachnoid space and brain. Patients present with an acute illness that is indistinguishable from bacterial meningitis. Patients with GAE often present with an initial focus of infection in the skin or respiratory tract followed by neurologic changes reflective of extensive brain involvement. Amebic keratitis from Acanthamoeba has been increasingly recognized in recent years and is typically a result of contaminated contact lens saline solutions.
Emergency Department Treatment and Disposition
The mainstay of management is consideration of these uncommon diseases in emergency department patients. PAM is almost always fatal but one survivor was successfully treated with amphotericin B, miconazole, and rifampin. Isolated cutaneous disease from Acanthamoeba and B mandrillaris has been cured, but brain involvement is fatal and usually diagnosed at autopsy. Patients with suspected amebic keratitis should have immediate ophthalmologic referral.
Ameba. Twenty-one year old patient from South America with 1 year of symptoms from Balamuthia mandrillaris. The primary site often involves the mid-face and oral cavity. This patient did not have intracranial involvement. (Photo contributors: Seth W. Wright, MD and Universidad Peruana Cayetano Heredia, Lima, Peru.)
Ameba Imaging. MRI showing extensive cerebral involvement in a fatal case of Balamuthia mandrillaris. (Photo contributors: Rob Greidanus, MD and Universidad Peruana Cayetano Heredia, Lima, Peru.)
Lack of response to usual antimicrobials in a patient with severe meningitis symptoms should lead to the suspicion of PAM, particularly with recent fresh-water exposure.
It is thought that global warming might increase the rate of N fowleri infection as the organism thrives in water over 30°C.
Consider Acanthamoeba infection in all contact lens wearers with a corneal infection.
Early amebic keratitis can mimic the dendritic pattern of herpes simplex infection.
Anemia in the tropics is a common consequence of various nutritional deficiencies, infections, parasites, genetic disorders, ...