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21-01: Free-Living Ameba Infection

Shannon M. Langston; Brian D. Bales

Introduction

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Rural Health Clinic in Uganda. (Photo contributor: Seth W. Wright, MD.)

 

The authors acknowledge Seth Wright, Andrew Pfeffer, and Meg Jack for portions of this chapter written for the fourth edition.

Clinical Summary

Free-living amebas, usually harmless protozoan residents of soil and water, can cause three distinct, occasionally devastating, human illnesses. Primary amebic meningoencephalitis (PAM) is a disease of the previously healthy and is caused by Naegleria fowleri. Granulomatous amebic encephalitis (GAE) is caused by Acanthamoeba species or Balamuthia mandrillaris, and occurs in both healthy and immunocompromised persons. In wealthier countries, contact lens users may suffer from chronic amebic keratitis, also caused by Acanthamoeba. While these diseases are found worldwide, they are more common in tropical and subtropical regions.

PAM is devastating, usually fatal, and found mostly in children or young adults with a history of recent freshwater exposure. The organism enters through the nose and penetrates the cribriform plate to the subarachnoid space and brain. The acute illness 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.

Management and Disposition

The mainstay of management is consideration of these uncommon diseases. 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 can be cured, but brain involvement is fatal and often diagnosed at autopsy. Patients with suspected amebic keratitis should have immediate ophthalmologic referral.

FIGURE 21.1

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.)

In a patient with Balamuthia mandrillaris, there is redness and significant swelling of the upper lip and the area below the nasal fold.
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Pearls

  1. Lack of response to usual antimicrobials in a patient with severe meningitis symptoms should lead to the suspicion of PAM, particularly with recent freshwater exposure.

  2. Global warming might increase the rate of N fowleri infection as the organism thrives in freshwater over 30°C.

  3. Fatal cases of PAM have been reported following tap water nasal irrigation using a “neti pot” (a device used to clean nasal passages). Use of sterile or previously boiled water when irrigating eliminates the risk.

  4. Consider Acanthamoeba infection in all contact lens wearers with a corneal infection. Early amebic keratitis can mimic the dendritic pattern of herpes simplex infection.

  5. Immunocompromised patients with space-occupying lesions should have GAE on the differential.

FIGURE 21.2

Ameba Imaging. MRI showing extensive cerebral involvement in a fatal case of Balamuthia mandrillaris. (Photo contributors: Rob Greidanus, MD, ...

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