Factors regulating tapeworm populations: the changing opportunities of lambs for ingesting the eggs of Taenia hydatigena.

Some intrinsic factors that may modify the transmission of cysticercosis caused by Taenia hydatigena in lambs have been assessed and the age at which the infective pattern should no longer be modified by the grazing-learning process and by colostrum-derived antibody, estimated. Lambs of 0-1, 2-3, 5-6, 8-9, 11-12 weeks of age, were placed for one week on a pasture experimentally contaminated with eggs of T hydatigena. The lambs were allowed to suck their mothers from birth. About half the lambs up to three weeks of age were free from larval tapeworms at autopsy; the rest had only a small number of cysts. Some of the lambs grazed on the pasture at or after five weeks of age, became heavily infected. The light infections in the very young lambs could have resulted from either a limited contact with eggs or from passively transferred antibody or, more likely, from a combination of these factors. The high cyst counts observed in some of the older lambs were consistent with the ingestion of eggs in clusters. The rise in the total cyst counts was concomitant with increasing pasture intake. The higher survival rates for the cysts found in the older rather than the younger animals were compatible with the waning before five weeks of age of colostrum-derived antibody.