The gold standard for laboratory diagnosis of schistosomiasis is the presence of 
typical eggs in stool or urine. The laboratory diagnosis of schistosomiasis and 
Katayama syndrome in returning travellers is difficult because the number of 
excreted eggs is often very limited. In early infections and in patients with 
only a few contacts with contaminated water, the total number of parasites, 
migrating larvae or schistosomulae, and adult worms, is very low. Eggs can only 
be found in faeces or urine when there is at least one pair of adult worms at 
the final location. The number of parasites increases as a function of the 
number of contacts with infected water. The exact latency between contamination 
and egg production is unknown. It is estimated that excretion of eggs starts 
after 40-50 days. The specific diagnosis of early schistosomiasis and Katayama 
fever relies essentially on serologic tests or preferably on PCR (if available). 
These assays are much more sensitive (up to four times) in the early phase of 
schistosomiasis than microscopic examination for typical eggs. Eosinophilia 
(sometimes exceeding 50%) is often present in patients with acute 
schistosomiasis (Katayama fever), but may be limited or absent in late fibrotic 
manifestations of the disease.