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Neurocysticercosis
474
NEUROCYSTICERCOSIS
PATHOGENESIS AND CLINICAL FEATURES
Neurocysticercosis Encyclopedia of the Neurological Sciences Copyright 2003, Elsevier Science (USA). All rights reserved.
HUMAN cysticercosis is invasion of tissue by the
larval stage (a cysticercus) of the pork tapeworm Taenia solium. Cysticerci of the central nervous system (CNS) are the most common of neurological parasitic infections in the world, rendering cysticercosis one of the most frequent neurological disorders. Cysticercosis is endemic to Latin America, Asia, and Africa, but it is also commonly seen in eastern and southern Europe and in countries with high rates of immigration from endemic areas, such as the United States. In many neurological centers in Latin America, for instance, cysticercosis is the most frequent cause of late-onset epilepsy, adult hydrocephalus, and chronic meningitis. Cysticercosis is far more common in areas with domestic breeding of pork and poor human waste sanitation.
TRANSMISSION Cysticercus is the embryo of the intestinal cestode T. solium. Both humans and pigs may act as intermediate hosts for cysticercus, but humans are the only host of the adult intestinal cestode and are required to complete the life cycle. Intestinal tapeworms are acquired by eating undercooked pork containing cysticerci, whereas human cysticercosis is a fecal–oral infection caused by ingestion of eggs excreted in the feces of a human tapeworm carrier. Eggs ingested by either humans or swine lose their coat in the gastrointestinal tract, pass into the blood circulation, and are carried to the brain, muscles, or eye, where the cysticerci will evolve. Should humans eat insufficiently cooked pork meat infected with cysticerci, the embryo evaginates from the cyst within the initial portion of the small intestine and firmly attaches to the intestinal wall. In a few weeks, this embryo, initially measuring 1 or 2 mm, will grow to a very large worm 2 or 3 m long. The adult tapeworm will produce, on a daily basis, mature, fertile proglottides, thus closing the life cycle of the parasite. A single carrier of T. solium can become an asymptomatic, continuous, and extensive source of cysticercosis for the community. Taenia eggs are infective even after long exposures in the environment.
Onset of symptoms may be from 1 to 35 years after exposure. Symptoms result from migration or arrest of larval forms in host tissues, particularly the CNS. In approximately 35% of neurocysticercosis cases, a single parasite occurs in brain parenchyma. In these patients, the infection may go unnoticed or the patient may experience occasional seizures. In another 35% of cases, various parasites infect the brain parenchyma producing, in addition to epilepsy, a protean mixture of neurological and psychiatric manifestations related to the topography of lesions. The number of parasites may vary from few to countless, and the clinical picture may include a discrete symptom, such as headache or dizziness, or a life-threatening disorder such as cysticercotic encephalitis. Lesions may be scattered throughout the brain parenchyma, or cysts may grow in clusters, producing a clinical picture similar to that of a brain tumor. The stage of the infection can also vary from live cysticerci to quiescent infections with calcified granulomas, in which the parasites degenerated and the active cyst was replaced by a remnant granuloma. In other patients (approximately 30% of cases), cysticerci lodge in the meninges, producing chronic arachnoiditis whose clinical manifestations are due to widespread inflammation, vasculitis, hydrocephalus, and multiple small brain infarctions. This is the most severe form of neurocysticercosis. The inflammatory process is distributed throughout the nervous system by cerebrospinal fluid (CSF) circulation, and the meningitis may outlast effective destruction of the parasites. The prognosis in these cases is closely related with the intensity of the inflammatory process in the subarachnoid space. Sustained results on periodic CSF analysis of more than 100 mg/dl of protein and more than 50 cells/mm3 may produce hydrocephalus secondary to inflammatory and fibrotic obstruction to CSF absorption. Ventricular or eye cysticercosis is rare (o1% of cases).
DIAGNOSIS Diagnosis is by neuroimaging studies—computed tomography or magnetic resonance. Immunodiagnostic tests in serum are unreliable, mostly due to a high percentage of false positives (approximately 30%) among healthy subjects from endemic areas and also to a high percentage of false negatives (approximately 30%) in serum from neurocysticercosis (NCC) patients with radiological evidence of
NEUROCYSTICERCOSIS
PATHOGENESIS AND CLINICAL FEATURES
Neurocysticercosis Encyclopedia of the Neurological Sciences Copyright 2003, Elsevier Science (USA). All rights reserved.
HUMAN cysticercosis is invasion of tissue by the
larval stage (a cysticercus) of the pork tapeworm Taenia solium. Cysticerci of the central nervous system (CNS) are the most common of neurological parasitic infections in the world, rendering cysticercosis one of the most frequent neurological disorders. Cysticercosis is endemic to Latin America, Asia, and Africa, but it is also commonly seen in eastern and southern Europe and in countries with high rates of immigration from endemic areas, such as the United States. In many neurological centers in Latin America, for instance, cysticercosis is the most frequent cause of late-onset epilepsy, adult hydrocephalus, and chronic meningitis. Cysticercosis is far more common in areas with domestic breeding of pork and poor human waste sanitation.
TRANSMISSION Cysticercus is the embryo of the intestinal cestode T. solium. Both humans and pigs may act as intermediate hosts for cysticercus, but humans are the only host of the adult intestinal cestode and are required to complete the life cycle. Intestinal tapeworms are acquired by eating undercooked pork containing cysticerci, whereas human cysticercosis is a fecal–oral infection caused by ingestion of eggs excreted in the feces of a human tapeworm carrier. Eggs ingested by either humans or swine lose their coat in the gastrointestinal tract, pass into the blood circulation, and are carried to the brain, muscles, or eye, where the cysticerci will evolve. Should humans eat insufficiently cooked pork meat infected with cysticerci, the embryo evaginates from the cyst within the initial portion of the small intestine and firmly attaches to the intestinal wall. In a few weeks, this embryo, initially measuring 1 or 2 mm, will grow to a very large worm 2 or 3 m long. The adult tapeworm will produce, on a daily basis, mature, fertile proglottides, thus closing the life cycle of the parasite. A single carrier of T. solium can become an asymptomatic, continuous, and extensive source of cysticercosis for the community. Taenia eggs are infective even after long exposures in the environment.
Onset of symptoms may be from 1 to 35 years after exposure. Symptoms result from migration or arrest of larval forms in host tissues, particularly the CNS. In approximately 35% of neurocysticercosis cases, a single parasite occurs in brain parenchyma. In these patients, the infection may go unnoticed or the patient may experience occasional seizures. In another 35% of cases, various parasites infect the brain parenchyma producing, in addition to epilepsy, a protean mixture of neurological and psychiatric manifestations related to the topography of lesions. The number of parasites may vary from few to countless, and the clinical picture may include a discrete symptom, such as headache or dizziness, or a life-threatening disorder such as cysticercotic encephalitis. Lesions may be scattered throughout the brain parenchyma, or cysts may grow in clusters, producing a clinical picture similar to that of a brain tumor. The stage of the infection can also vary from live cysticerci to quiescent infections with calcified granulomas, in which the parasites degenerated and the active cyst was replaced by a remnant granuloma. In other patients (approximately 30% of cases), cysticerci lodge in the meninges, producing chronic arachnoiditis whose clinical manifestations are due to widespread inflammation, vasculitis, hydrocephalus, and multiple small brain infarctions. This is the most severe form of neurocysticercosis. The inflammatory process is distributed throughout the nervous system by cerebrospinal fluid (CSF) circulation, and the meningitis may outlast effective destruction of the parasites. The prognosis in these cases is closely related with the intensity of the inflammatory process in the subarachnoid space. Sustained results on periodic CSF analysis of more than 100 mg/dl of protein and more than 50 cells/mm3 may produce hydrocephalus secondary to inflammatory and fibrotic obstruction to CSF absorption. Ventricular or eye cysticercosis is rare (o1% of cases).
DIAGNOSIS Diagnosis is by neuroimaging studies—computed tomography or magnetic resonance. Immunodiagnostic tests in serum are unreliable, mostly due to a high percentage of false positives (approximately 30%) among healthy subjects from endemic areas and also to a high percentage of false negatives (approximately 30%) in serum from neurocysticercosis (NCC) patients with radiological evidence of