Resistance is the ability of the sheep to resist infection with worms and reject established worms.

Resilience is the ability of sheep to maintain a relatively unchanged level of production (growth rate of the young animal, wool growth, milk production) despite a worm infection.

Resistance to worms
Resistance to worm infection can be thought of as developing in three stages:

1. Fewer incoming worm larvae establish.
2. Established female worms lay fewer eggs.
3. Established adult worms are rejected by the animal.

Improved nutrition is thought to have little effect on the establishment rate of incoming worm larvae.

Improved nutrition can help decrease the egg production of established female worms, but this effect is inconsistent among studies.

The greatest effect of improved nutrition is to reduce the time required for sheep to reject established adult worms, and to increase the efficiency of this.

How improved nutrition enhances resistance to worm infection is uncertain. It is thought it may enhance the immune responses of sheep against worms.

Resilience to worms
In non-resistant animals improved nutrition can increase the resilience of worm-infected animals.

In some instances, improved nutrition has resulted in worm-infected animals having identical rates of body and wool growth as similarly fed uninfected animals.