35-08: Infections with Pathogenic Free-Living Amoebas

ESSENTIALS OF DIAGNOSIS

The free-living amoebas that cause disease in humans are of the genera Acanthamoeba, Naegleria, Balamuthia, and Sappinia. The organisms are widely distributed in soil and fresh and brackish water. Acanthamoeba and Naegleria species have been found to harbor Legionella, Vibrio cholerae, and other endosymbiotic bacteria and may serve as a reservoir for these organisms.

1. PRIMARY AMEBIC MENINGOENCEPHALITIS

Primary amebic meningoencephalitis is a fulminating, hemorrhagic, necrotizing meningoencephalitis that occurs in healthy children and young adults and is rapidly fatal. It is caused by free-living amoebas, most commonly Naegleria fowleri, but also Balamuthia mandrillaris and Acanthamoeba species. N fowleri is a thermophilic organism found in fresh and polluted warm lake water, domestic water supplies, swimming pools, thermal water, and sewers. Patients may give a history of exposure to warm fresh water, but activities other than swimming, for example ritual or therapeutic nasal cleansing, may be responsible for infections. The incubation period varies from 2 to 15 days. Early symptoms include headache, fever, stiff neck, and lethargy, often associated with rhinitis and pharyngitis. Vomiting, disorientation, and other signs of meningoencephalitis develop within 1 or 2 days, followed by coma and then death within 7–10 days. No distinctive clinical features distinguish the infection from acute bacterial meningoencephalitis. CSF shows hundreds to thousands of leukocytes and erythrocytes per cubic millimeter. Protein is usually elevated, and glucose is normal or moderately reduced. A fresh wet mount of the CSF may show motile trophozoites. Staining with Giemsa or Wright stain will identify the trophozoites. Species identification is based on morphology and immunologic methods. Primary amebic meningoencephalitis is nearly always fatal. Amphotericin B appears to be the drug of choice; the four known survivors in North America were all treated with amphotericin B, rifampin, and other agents.

2. GRANULOMATOUS AMEBIC ENCEPHALITIS

Acanthamoeba species and B mandrillaris can cause an encephalitis that is more chronic in nature than primary amebic meningoencephalitis. One case of encephalitis caused by Sappinia has also been described. Neurologic disease may be preceded by skin lesions, including ulcers and nodules. After an uncertain incubation period, neurologic symptoms develop slowly, with headache, meningismus, nausea, vomiting, lethargy, and low-grade fevers progressing over weeks to months to focal neurologic findings, mental status abnormalities, and eventually coma and death. CT and MRI show single or multiple nonspecific lesions. Lumbar puncture is dangerous due to increased intracranial pressure. CSF shows a lymphocytic pleocytosis with elevated protein; amoebas are not typically seen. Diagnosis can be made by biopsy of skin or brain lesions. Information on the treatment of granulomatous amebic encephalitis is limited. Some patients have been successfully treated with various combinations of flucytosine, pentamidine, fluconazole or itraconazole, sulfadiazine, TMP-SMZ, and azithromycin.