Self-cure and immunity following infection and reinfection in ovine haemonchosis

Egg expulsion and post mortem examination were used to compare self-cure and resistance to reinfection with Haemonchus contortus in female Merino sheep, AA and BB hemoglobin types. Animals were infected by 500 or 5000 larvae of H. concortus once, or several times, before being challenged by 500 (low dose) or 5000 (high dose) larvae.In AA sheep infected once by 500 larvae self-cure was detected by a challenge of either 500 or 5000 larvae while in multi-infected sheep only a challenge of 5000 L₃ induced the self-cure. In BB sheep a mild self-cure was detected only by a challenge of 5000 larvae.In AA sheep receiving several infections, challenge by 500 larvae was not effective in producing resistance while 5000 L₃ was effective in producing a state of resistance for up to 35 days post challenge.At post mortem examination, a strong state of resistance was evidenced in multi-infected animals by the low number of L₄ and adults present in the abomasum up to the 35th day after challenge.In BB sheep no state of resistance was demonstrated by challenge with either 500 or 5000 L₃, even in multi-infected animals.