Resistance to Fasciola hepatica in sheep harbouring primary Schistosoma bovis infections.

A substantial level of resistance to a heterologous challenge with Fasciola hepatica was demonstrated in sheep harbouring primary non-patent (two- to three-week-old) and newly patent (seven- to eight-week-old) Schistosoma bovis infections, the liver-fluke burdens being reduced by 70% and 93%, respectively, (p less than 0.01) in these groups compared with that of the challenge control group. The resistance was also reflected in less pronounced F. hepatica-induced liver damage and presumably also in a reduction of the egg-production capacity per established liver-fluke. In contrast, a corresponding heterologous resistance to challenge with F. hepatica could not be demonstrated in sheep harbouring primary 16- to 17-week-old S. bovis infections. Possible, presumably immunological, mechanisms involved in the observed cross-resistance are discussed, as is the possible practical relevance of this cross-resistance phenomenon in attempts to vaccinate sheep against F. hepatica.