Although eosinophilic bar- or droplet-like inclusions are frequently
detectable inside eggs deposited in the livers of
Schistosoma japonicum-infected animals, little is known of their
exact nature. In the livers of mice implanted with freshly
laid eggs, inclusion-positive eggs were found in 28·7 and 46·2%
of deposited eggs at 2 and 4 weeks, respectively, after
implantation, but in 4·3% at 5 weeks when most of the eggs had already
degenerated. When the extent of granuloma
formation was investigated, granulomas around inclusion-positive eggs were
smaller than those around negative eggs.
Host factors associated with the formation of inclusion were sought using
in vivo and in vitro studies. Following the
administration of anti-egg antigen serum into egg-implanted mice, no increase
in occurrence of inclusion-positive eggs
was seen. In a co-culture of mature eggs with infected rabbit or mouse
serum, inclusions were rarely found. In contrast,
they were found in 17·9% of eggs in the presence of splenic cells.
The present study is the first to show that there is
decreased granuloma formation in the presence of eosinophilic inclusions
inside eggs and our in vitro study suggests that
host cell–egg interaction is responsible for the formation of inclusions.