Internal parasites kill fewer sheep than goats. In 2014, 71.9 percent of adult sheep death losses were due to non-predator causes. For lambs, non-predator death losses accounted for 63.4 percent of all losses.
The top three causes of non-predator death loss in adult sheep were old age (24.3 percent), unknown non-predator causes (13.2 percent), and lambing problems (12.1 percent). The top three non-predator causes in lambs were weather-related causes (13.2 percent), unknown non-predator causes (12.6 percent), and lambing problems (11.7 percent).
In 2014, internal parasites accounted for 8.6 percent of non-predator deaths in adult sheep (13,612 sheep) and 9.2 percent of non-predator deaths in lambs (21,373 lambs). In contrast, worms killed 87,000 goats in 2015: 9.8 percent of adult goats and 19.2 percent of kids. The biggest difference is in the number of lambs/kids killed by worms.
One reason why parasite losses are lower in sheep than goats is because many sheep are raised under range conditions (in the West) where fewer parasite problems are experienced. Rangelands are more arid and grazing is more migratory.
Source: Sheep and Lamb Predator and Nonpredator Death Loss in the United States, 2015