1.Histological sections of livers from white mice infected with male or female Schistosoma mansoni, S. japonicum or S. haematobium, and with both sexes of S. mansoni or S. japonicum prior to egg deposition, were examined to study lesions associated with the presence of worms in the portal-mesenteric vessels in the absence of lesions produced by eggs. Most of this material came from S. mansoni infections.
2.In unisexual S. mansoni infections periportal infiltration of mononuclear and polymorphonuclear leucocytes appeared after 3 to 4 weeks in the majority of unisexual infections and increased in intensity in infections up to about 12 weeks in duration, after which it tended to become less intense and was predominantly mononuclear.
3.Cellular infiltration in the parenchyma of the liver, predominantly polymorphonuclear, occurred in some animals, apparently independent of extensions from the periportal infiltrations. These increased in frequency and intensity in infections up to about 12 weeks in duration, but were rare in infections 20 weeks or more in duration.
4.Areas of coagulative necrosis in the parenchyma of the liver were found in a considerable number of animals. They became invaded by polymorphonuclear leucocytes with later disappearance of liver cells and invasion of mononuclear and multinuclear cells. Worms were sometimes present in venules immediately adjacent to these lesions, giving the impression that they may have been infarcts initiated by temporary obstruction of the blood supply by the worms.
5.No evidence of development of cirrhosis of the liver was found in unisexual infections up to 95 weeks in duration.
6.Some of the unisexual infections with S. japonicum or S. haematobium showed the same types of lesions as those infected with S. mansoni. In general the lesions were less intense, but accurate comparison is not warranted.
7.In bisexual S. mansoni infections up to 5 weeks in duration periportal and parenchymal cellular infiltration and coagulative necrosis appeared earlier and were more frequent and intense than in unisexual infections of the same duration. Occasional mural thrombi were found in portal venules.
8.Some uninfected mice had mild degrees of periportal and parenchymal cellular infiltration, but areas of coagulative necrosis were not seen.
9.Although some infected mice failed to show any lesions in the liver, it seems possible that the intense cellular infiltrations and areas of necrosis were causally related to the presence of worms in the mesenteric-portal blood vessels.