A consideration on the relationship between worm age and mortality of Setaria marshalli recovered from the peritoneal cavity of calves born in Tottori prefecture, Japan.

Eight calves born in November-December of 1992 were necropsied at the age of 29-230 days in an attempt to estimate the life span of Setaria marshalli in Japan. Worm ages were estimated on the base of active season for mosquitoes. Thirty worms estimated 4-9 months old recovered from 4 calves were all alive in the peritoneal cavity. Thirty-five out of 50 worms estimated 7-13 months old were dead with being entrapped by fibrin on the peritoneum. It is speculated that this is a normal fate of this parasite at the end of its life, and therefore, the life span of S. marshalli would be approximately one year after prenatal infection.