International Journal for Parasitology

Established adult infections of Haemonchus contortus in sheep were found to exclude the establishment of H. placei given at dose levels of either 500 or 1500 larvae per week for at least 8 weeks. However, established infections of adult H. placei could not exclude H. contortus. At dose levels of 500 H. contortus larvae per week the existing H. placei infections in sheep were gradually replaced, during which time considerable species hybridization occurred. Dose levels of 1500 H. contortus larvae per week led to the rapid dislodgement of H. placei and much less opportunity for hybridization. A second experiment confirmed that existing H. contortus infections could exclude H. placei and that incoming H. contortus larvae could dislodge H. placei. Furthermore, the exclusion or dislodgement of H. placei was abrogated by injecting the host with dexamethasone. It is proposed that in sheep H. contortus is using a host mediated response to limit their competitor H. placei and that the resulting exclusion and dislodgement of H. placei acts as a major pre-mating barrier to species hybridization.