Neuroschistosomiasis

Abstract

Schistosomiasis is an infection caused by digenetic trematode platyhelminths of the genus Schistosoma. These blood flukes use man and other mammals as definitive hosts and aquatic and amphibious snails as intermediate hosts. Of the schistosomal species, S. mansoni, S haematobium and S. japonicum are the most important to man and the most widely distributed. The infection affects about 200 million individuals in 74 countries of Latin America, Africa and Asia. Far less commonly, schistosomes reach the central nervous system (CNS). This may occur at any time 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 clinical forms of schistosomal infection. The presence of eggs in the CNS induces a cell-mediated periovular granulomatous reaction. When eggs reach the CNS during the early stages of the infection or during evolution of the disease to its chronic forms, large necrotic-exudative granulomas are found. In-situ egg deposition following the anomalous migration 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 effect produced by the heavy concentration of eggs and the presence of large granulomas in circumscribed areas of the brain and spinal cord explains, 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 lumbosacral segments of the spinal cord. Most of the cases of CNS involvement associated with the hepatosplenic and cardiopulmonary chronic forms, or with severe urinary schistosomiasis, though more frequent, are asymptomatic. In the patients with these clinical forms, the random and sparse distribution of eggs in the CNS indicates that the embolization of eggs from the portal mesenteric system to the brain and spinal cord constitutes the main route of CNS invasion by Schistosoma. The discrete inflammatory reaction elicited by the sparsely distributed eggs in the CNS explains the lack of neurological symptoms that could be produced by egg deposition.

Research development of the pathogenesis pathways for neuroschistosomiasis

“…There are 2 main mechanisms of cerebral NS: egg embo-lization (indirect deposition) and adult worm migration (direct deposition) [6][7][8][9] . haematobium can also induce similar symptoms.…”

“…However, disorder caused by S. mansoni is not so common as that by S. japonicum, and disorder caused by S. haematobium is even rare [5] . Pittella [8] has reported that the schistosoma adult worms begin laying eggs after mature, and the majority of the eggs deposit in the portal system and the pulmonary system through the arteriovenous anastomosis. Then the eggs enter the pulmonary vein, reaching the brain through the arterial system.…”

“…However, some authors propose that the cerebral focal lesions and vascular necrosis in autopsy could be interpreted by the vein embolization mechanism. They prefer the cerebral venous egg thrombosis mechanism [8,11,15] , although the tumor-like mass granulomas by S. mansoni eggs may be located in any lobe of the brain. According to their reports, the most common site is the cerebellum, which is consistent with the hypothesis that the eggs reach the CNS through retrograde venous flow into the non-valve Batson's venous plexus that connects the portal venous system and venae cavae to the spinal cord and cerebral veins [1,12] .…”

“…The autopsy studies find that S. haematobium and S. mansoni adult worms could invade the leptomeningeal or the pia mater in the subarachnoid space of spinal cord [6,8,13,14] . Besides, S. japonicum adult worms have also been found in the cerebral vein in the experimental monkeys [9,17] .…”

“…The female adult worms begin to shed eggs. Totally, it may take 4-6 weeks for migration and maturation, depending on the response of the host and the species involved [8] . As a result of egg-induced chronic presinusoidal inflammation, fibrous material deposition and vascular destruction are induced, which is called "Symmer's pipe-stem fibrosis", indicating a periportal fibrosis without bridging and nodular formation [18] .…”