Amebic Infections

Naegleria and Acanthamoeba are free-living amebae that infect the CNS. Naegleria fowleri causes an acute fulminant meningoencephalitis, known as primary amebic meningoencephalitis (PAM), and is acquired by swimming or waterskiing in small, shallow freshwater lakes as a result of direct contamination through the nasal cavity. Infection with this organism has also been acquired from swimming in chlorinated pools; presumably, the organism gets into the pool through cracks in the pool. In Africa, cases have been reported during the dry and windy season by inhalation of dust containing amebic cysts. '109'

Acanthamoeba CNS infections are generally not water related and occur in immunosuppressed and debilitated individuals. This organism has been isolated from the nasopharynx of asymptomatic individuals. Acanthamoeba keratitis has been reported in healthy contact lens wearers. '109'

N. fowleri invades the nasal mucosa and ascends to the brain through the cribiform plate via the fila olfactoria, extensions of the olfactory nerves. '121' Amebae can then spread throughout the meninges. The temporal and frontal lobes are most severely affected. Acanthamoeba spp. reach the nervous system through the bloodstream from a distant site of infection or from a corneal infection of the eye and typically cause a subacute or chronic meningoencephalitis, or inflammatory lesions in multiple areas of the brain, particularly the white matter, the basal ganglia, the brain stem and the cerebellum. A necrotizing vasculitis is common owing to invasion of arterial walls by trophozoites. '121'

The incubation period for infection by N. fowleri is usually 4 to 7 days. The patient presents with severe bifrontal headache, high fever, nausea, vomiting, and meningismus, a clinical presentation resembling bacterial meningitis. Twenty-four to forty-eight hours later, signs of encephalitis with focal neurological deficits, confusion, delirium, stupor, and finally coma occur. Meningoencephalitis due to Naegleria is typically a fatal disease with death occurring within 2 to 4 days. Because of involvement of the olfactory bulbs and tracts, early symptoms of abnormal smell or taste sensation have been reported. The incubation period of Acanthamoeba is longer than that of Naegleria infections and is typically weeks to months. The insidious onset of symptoms and signs of focal encephalitis or focal mass lesions follows. The presentation may include seizures, hemiparesis, visual disturbances, headache, low- grade fever, and ataxia.

A diagnosis of N. fowleri meningoencephalitis is rarely made while the patient is alive. Examination of the CSF demonstrates a leukocytosis with a neutrophilic predominance, an increased protein concentration, and a decreased glucose concentration. The ameboid movements of the trophozoites can be seen on fresh or wet unstained specimens of CSF. Trophozoites can also be demonstrated in the brain by light or electron microscopy. Serological tests for Naegleria are of limited value because patients usually die before the results are available. In contrast, serological tests for Acanthamoeba infections are helpful in diagnosis. Neuroimaging scans may demonstrate one or multiple abscesses of Acanthamoeba. Examination of the CSF shows a mild lymphocytic pleocytosis with a normal to decreased glucose concentration and a mild increase in protein concentration. '10?'

The drug of choice for acute N. fowleri meningoencephalitis is intravenous amphotericin B (1 mg/kg/d) for at least 2 to 3 weeks. A few survivors of this infection have been documented, but one survivor required treatment with a combination of intravenous and intrathecal amphotericin B, intravenous and intrathecal miconazole, and rifampin, sulfisoxazole, and dexamethasone. The outcome may be improved by adding intrathecal amphotericin B (0.5 mg on alternate days) to parenteral amphotericin B.[10?] , '122' , '123' Acanthamoeba abscesses are treated with surgical excision

and amphotericin B. Although amphotericin B is amebicidal for N. fowleri, it is much less active against Acanthamoeba. '121' No effective vaccine for free-living amebae infections exist. Children should be prevented from swimming in contaminated fresh water, and Acanthamoeba keratitis can be prevented by using appropriate antimicrobial contact lens cleaning solutions. '121'


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