Gastrointestinal parasites as a threat to grazing sheep

Despite pressure to increase farm productivity, a tendency for animal-friendly production systems, like the one that includes grazing, is becoming more common due to an increased consumer awareness of animal welfare. Pasture is generally the main food source for these animals. Grazing sheep are therefore exposed to a huge diversity of parasites. These parasites impact greatly on animal health, welfare and productivity, and can be responsible for high economic losses due to delayed development of lambs and low productivity levels of adult sheep, which can be especially threatening in endangered breeds. Nematodes of the genera Haemonchus, Trichostrongylus, Teladorsagia, Cooperia and Nematodirus can cause serious health problems in sheep. Haemonchus contortus is responsible for the main health problems in sheep, and represents a significant cause of mortality worldwide. Nematodirus battus is a common cause of lamb diarrhoea. Some trematodes, besides affecting animal health, are zoonotic and may have health implications for farmers and local communities. Prophylaxis plans are essential in order to keep acceptable infection levels. A detailed knowledge of parasite species involved, as well as their burden and prevalence, is necessary. Pasture management should be a primary tool to control parasites. Integration of more than one measure like good farming practices, and appropriate biological control measures is essential to achieve the sustainable control on the parasites. Anti-parasitic drugs are still an important part of parasite control in grazing sheep. Consumers worry about the quality of meat as well as about the animal welfare, but studies of the effects of parasites are scarce, and there is a concern that grazing management systems increase the prevalence of parasites. The question is whether to choose less productive breeds but well adapted to the local environmental conditions (autochthonous breeds) and more resistant to parasites, or high productive breeds but not adapted to the local environment and its parasites. The aim of this review was to understand the current situation of the prevalence of gastrointestinal parasites in grazing sheep, and the consequences on sheep management and the effects on meat quality.