Romney Marsh sheep from New Zealand are a good example. Sheep carrying heavy worm burdens are often severely affected and lag behind the flock, while those with light infections are more active. This resistance is heritable, meaning that rams and ewes pass their ability to resist worm infection on to their lambs. By selecting worm-resistant sheep to breed from, the flock will, on average, become more resistant over time. Selective breeding can be a useful aid to parasite control.
Acquired resistance is the response of the immune system to parasites.
Even immune sheep remain infected and parasites are rarely all eliminated from immune sheep. This may be because parasites, being complex organisms, produce an array of proteins. The sheep recognises these as foreign and produces an array of responses. Only some responses help protect the sheep from worms but these are relatively weak because the responses are diluted by irrelevant ones. Nevertheless, host resistance is known to be important in lowering parasite burdens because when resistance wanes, parasite numbers are seen to increase.