A heavy infestation with these parasites is always associated with a reduction in performance. This causes the ewes' milk production to decrease, and the lambs' daily weight gain is also lower. In the course of the project "Parasite burden in sheep during the grazing period" we are investigating, on the one hand, whether the genetics of the sheep have an influence on the burden and, on the other hand, whether environmental influences - this means, for example, the body condition of the individual sheep and the different seasons - also have a different influence can cause stress. A genetic influence can exist both within one and between two breeds.