Recurrent experimental evidence indicates that schistosomal egg granuloma formation at least in the murine model results from a host response generated against both egg- and worm-derived antigens. Further experiments aimed at identifying the existence in vivo of cross-sensitization between Schistosoma haematobium worms and S. mansoni-derived egg antigens were performed with respect to S. mansoni egg antigen-induced granuloma formation and fibrogenesis in the liver. Male OF1 mice bisexually infected with S. haematobium or S. mansoni were hepatically challenged (cecal vein injection) with S. mansoni SEA (soluble egg antigen)-coupled Sepharose beads at the end of prepatent infection (8-10 days prior to the start of egg deposition). The mean granuloma volume (MGV) of in-vivo-generated synchronized hepatic granulomas (8 days old) and the fibrotic response were estimated. Just like S. mansoni-infected rodents, mice carrying an S. haematobium infection generated an accelerated hepatic granulomogenesis [respective MGVs 4.72 +/- 0.56 and 5.41 +/- 0.75 x 10(6) microm3; P < 0.0001 versus unsensitized (MGV 3.00 +/- 0.40 x 10(6) microm3) mice] and an enhanced fibrotic response against S. mansoni SEA. They also had significantly enlarged spleens (P < 0.0001) and moderately enlarged livers (P = 0.02) as compared with S. haematobium-infected mice that were not challenged with SEA. From these observations we infer that in vivo, S. haematobium worms can positively modulate S. mansoni egg antigen-induced granuloma formation and hepatic fibrogenesis, resulting in more severe liver pathology.