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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Rhizopods

Amebic Meningoencephalitis

Primary amebic meningoencephalitis is caused by free-living amebas belonging predomi-nately to the Naegleria and Acanthamoeba genera.



Primary amebic meningoencephalitis is caused by free-living amebas belonging predomi-nately to the Naegleria and Acanthamoeba genera. The disease produced by the former has been better defined; it affects children and young adults, appears to be acquired by swimming in fresh water, and is almost always fatal. Acanthamoebameningoencephalitis is a subacute or chronic illness that also is usually fatal. Naegleria species are found in large numbers in shallow fresh water, particularly during warm weather. Acanthamoeba species are found in soil and in fresh and brackish water, and they have been recovered from the oropharynx of asymptomatic humans.

Approximately 140 cases of Naegleria meningoencephalitis have been reported, pri-marily in Great Britain, Belgium, Czechoslovakia, Australia, New Zealand, India, Nige-ria, and the United States. Serologic studies suggest that inapparent infections are much more common. Most cases in the United States have occurred in the southeastern states. Characteristically, the patients have fallen ill during the summer after swimming or water-skiing in small, shallow, freshwater lakes. The Czechoslovakian cases followed swim-ming in a chlorinated indoor pool, and several have occurred after bathing in hot mineral water. A recent report from Africa suggests the disease may have been acquired by inhal-ing airborne cysts during the dry, windy season in the sub-Sahara.

Histologic evidence suggests that Naegleria traverses the nasal mucosa and the cribriform plate to the central nervous system. Here the organism produces a severe purulent, hemorrhagic inflammatory reaction that extends perivascularly from the olfactory bulbs to other regions of the brain. The infection is characterized by the rapid onset of severe bifrontal headache, seizures, and at times, abnormalities in taste or smell. The disease runs an inexorably downhill course to coma, ending fatally within a few days.

A careful examination of the cerebrospinal fluid often provides a presumptive diagno-sis of Naegleria infection. The fluid is usually bloody and demonstrates an intense neu-trophilic response. The protein level is elevated and the glucose level decreased. No bacteria can be demonstrated on stain or culture. Early examination of a wet mount preparation of unspun spinal fluid reveals typical trophozoites. Staining with specific flu-orescent antibody confirms the identification. The organism can usually be isolated on agar plates seeded with a Gram-negative bacillus (to feed the amebas) or grown axeni-cally in tissue culture. To date, there are reports of only four patients who have survived a Naegleria infection. All were diagnosed early; and treated with high-dose amphotericin Balong with rifampin.

The epidemiology of Acanthamoeba encephalitis has not been clearly defined. Infec-tions usually involve older, immunocompromised persons, and a history of freshwater swimming is generally absent. The ameba probably reaches the brain by hematogenous dissemination from an unknown primary site, possibly the respiratory tract, skin, or eye. Metastatic lesions have been reported. Histologically, Acanthamoeba infections produce a diffuse, necrotizing, granulomatous encephalitis, with frequent involvement of the mid-brain. Both cysts and trophozoites can be found in the lesions. Cutaneous ulcers and hard nodules containing amebas have been detected in AIDS patients.

The clinical course of Acanthamoeba disease is more prolonged than that of Naegle-ria infection and occasionally ends in spontaneous recovery; the disease in immunocom-promised hosts is invariably fatal. The spinal fluid usually demonstrates a mononuclear response. Amebas can occasionally be visualized in or cultured from the cerebrospinal fluid or biopsy specimens. Fluorescein-labeled antiserum is available from the Centers for Disease Control and Prevention. Definitive diagnosis is usually made histologically after death. Acanthamoeba species are sensitive to a variety of agents, but studies of clinical efficacy have not been performed.

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