Egg in freshwater taken up by snails. Cercariae released into water and penetrate skin. With this first infection, urticaria, discrete raised lesions 1-3cm and immune response (to dying larvae, rather than living!). These then migrate into lungs, so acute schistosomiasis causing immune complex deposition, lymphadenopathy, eosinophilia, pulmonary infiltrates.
Larvae mature in liver. Adult worms migrate to mesenteric vessels of bowel, where eggs are laid. Chronic blood loss from gut lesions. Hepatomegaly, splenomegaly, eventually varices if fibrotic.
Worms can live 3-10 years. Immunity only really develops where lots of dead worms, not necessarily high worm burden! Eggs only start appearing 8 weeks after infection, and multiple samples required (intermittent excretion, perhaps every few days).
Serology only really useful in travellers as persists? Praziquantel often causes abdo pain, rash, headache. Only acts on adult worms.