The acute encephalitis syndrome may be associated with a wide variety of parasitic infections. With the exception of cerebral toxoplasmosis, parasitic encephalitis is rare in the United States and Western Europe. Encephalitides caused by tapeworms, nematodes, and schistosomes remain important causes of morbidity in populations throughout the developing world (T.a..ble,,3). In most of these diseases, however, the burden of parasitic involvement is outside the central nervous system. Because effective pharmacotherapy is available against many parasitic infections, it is always important to consider this possibility in patients presenting with an acute encephalitis syndrome who may have risk factors for parasitic infestation.
Table 3 Parasitic infections causing acute encephalitis syndrome
Toxoplasmosis is caused by the protozoan Toxoplasma gondii, and is one of the most frequent opportunistic central nervous system infections in patients with AIDS. After ingestion of raw meats or soil contaminated with protozoan oocysts (typically from feline fecal material), organisms are disseminated hematogenously, invade cerebral and retinal blood vessels, and form miliary granulomas. Most cases of central nervous system toxoplasmosis are believed to represent reactivation of a latent infection. In the immunocompromised host, recrudescent parasites replicate unchecked. The most common clinical presentation is that of an awake patient, with or without fever or evidence of systemic illness, presenting with signs of a focal mass lesion (single or multiple toxoplasmic abscesses). Early initiation of specific pharmacotherapy can result in dramatic improvement in the neurological picture. Up to 80 per cent will respond to a combination of pyrimethamine and sulfadiazine. Chronic pharmacological suppression of the parasitic infection is required.
Neurocysticercosis is caused by encystment of larvae of the pork tapeworm Taenia solium in brain tissue. It is a very common central nervous system infection throughout India, Southeast Asia, China, Central and South America, Mexico, and to a lesser extent the southwestern United States. Central nervous system involvement occurs in the majority of cases, and most commonly presents with focal seizures or a circumscribed neurological deficit. A more dramatic acute presentation featuring stupor or coma may result from the development of obstructive hydrocephalus or rupture of a cyst, provoking an intense inflammatory response.
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