Individual animal-related factors

• Age - Adult sheep have better acquired immunity than lambs and recently weaned sheep.
• Sex - Dry adult ewes seem to have better immunity than males. The reasons for this are not known.
• Pregnancy and lactation - During late pregnancy and early lactation immunity in ewes diminishes.
• Liveweight - Animals less than 23kg usually have poorer immunity than heavier animals.
• Health - Sheep in poor health because of other diseases will have reduced immunity to worms.
• Breed and sire line - Some breeds and sire lines have better immunity to worms than others.  Using rams selected for worm resistance helps significantly.

External factors

• Poor nutrition - Sheep need adequate nutrition to develop and maintain immunity to worms. Poor seasons or grazing management may result in sheep that become susceptible to worms.
• Stress - Weaning, transport and inclement weather can all reduce immunity to worms.