Parasitism is a well-known problem in sheep breeding. Originally, antiparasitics were very effective, but in recent years doubts have arisen about their effectiveness because the parasites are resistant to the active ingredients in the preparations and other side effects such as residues of these substances remain in the animals' meat. Parasite diseases have a considerable economic impact and lead to animal losses and additional costs. Recently, the selection of sheep for parasite resistance has proven to be a good solution to the problems associated with antiparasitics. There are several methods to determine the infestation of animals with parasites. The best known indicator is the FEC (faecal egg count) and the five-point check system, in which the nose, eye mucosa, head, back and tail are examined. In general, the selection of sheep for parasites is divided into resistance, resilience, number of treatments and immune response together with the effects of infection. In a Uruguayan study on two-way selection based on FEC, a genetic improvement of 15% in average breeding values between the first (2000) and the last generation (2019) of the resistant line and a genetic improvement of 11% between the resistant line and the rest of the population in 2019 was demonstrated.
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