Effects of infection with Echinostoma paraensei on the circulating haemocyte population of the host snail Biomphalaria glabrata.

Circulating haemocytes from Echinostoma paraensei-infected M line Biomphalaria glabrata snails, or from age- and size-matched control snails, were studied on plastic slides with phase-contrast optics. Granulocytes, hyalinocytes, and round cells were consistently present; granulocytes were further categorized as 'fully spread' (FS) or 'partially spread' (PS). Among control snails, the relative percentage and estimated number/mm3 of round cells declined significantly with increased snail size, and the corresponding values for both categories of granulocytes increased. At 1 day post-infection (p.i.) with E. paraensei, overall composition of the haemocyte population was relatively unaffected, but by 8 days p.i. infected snails had significantly higher relative percentages of round cells and PS granulocytes than controls. Because a marked increase in the number of circulating haemocytes is also evident by 8 days p.i., infected snails had approximately 12 times more round cells and PS granulocytes/mm3 of haemolymph than did controls. At 30 days p.i. the relative and absolute abundance of PS granulocytes was still significantly elevated, but otherwise haemocyte populations did not differ from control snails. Alterations in granulocyte size in infected snails were also noted. Infection with E. paraensei has a striking impact of circulating haemocyte populations and also increases the relative concentration of haemocytes with less ability to adhere to a foreign surface.