NEUROSCHISTOSOMIASIS

Schistosomiasis is an infection caused by digenetic trematode platyhel minths of the genus Schistosoma. These blood flukes use man and other mammals as definitive hosts and aquatic and amphibious snails as inter mediate hosts. Of the schistosomal species, S. mansoni, S haematobium and S. japonicum are the most important to man and the most widely dis tributed. The infection affects about 200 million individuals in 74 co untries of Latin America, Africa and Asia, Far less commonly, schistos omes reach the central nervous system (CNS). This may occur at any tim e from the moment the worms have matured and the eggs have been laid. For this reason, CNS involvement may be observed with any of the clini cal forms of schistosomal infection. The presence of eggs in the CNS i nduces a cell-mediated periovular granulomatous reaction. When eggs re ach the CNS during the early stages of the infection or during evoluti on of the disease to its chronic forms, large necrotic-exudative granu lomas are found. In-situ egg deposition following the anomalous migrat ion of adult worms appears to be the main, if not the only, mechanism by which Schistosoma may reach the CNS in these stages. The mass effec t produced by the heavy concentration of eggs and the presence of larg e granulomas in circumscribed areas of the brain and spinal cord expla ins, respectively, 1) the signs and symptoms of increased intracranial pressure and focal neurological signs; and 2) the signs and symptoms of rapidly progressing transverse myelitis, usually affecting the lumb osacral segments of the spinal cord. Most of the cases of CNS involvem ent associated with the hepatosplenic and cardiopulmonary chronic form s, or with severe urinary schistosomiasis, though more frequent, are a symptomatic. In the patients with these clinical forms, the random and sparse distribution of eggs in the CNS indicates that the embolizatio n of eggs from the portal mesenteric system to the brain and spinal co rd constitutes the main route of CNS invasion by Schistosoma. The disc rete inflammatory reaction elicited by the sparsely distributed eggs i n the CNS explains the lack of neurological symptoms that could be pro duced by egg deposition.