The Influence of Gastrointestinal Nematodes on Modern Sheep Farming and Novel Control Strategies

Abstract

Gastrointestinal nematodes represent parasites of great importance in veterinary medicine, especially in grazing animals such as small ruminants. In most cases, these parasites cause subclinical diseases with losses in body weight, reduced feed intake and decreased production. However, high worm burdens may lead to severe clinical signs such as anorexia, anaemia, diarrhoea, protein loss, oedema, decreased immunity and fertility, which can lead to fatal outcomes. Therefore, gastrointestinal nematodes hinder sheep farming in different ways leading to high economic losses that are estimated at several hundred million euros in Europe, although it is difficult to quantify it precisely. An additional problem is the development of anthelmintic resistance in nematodes that occur as a result of irrational use of commercial drugs, whereby the main risk factors are overfrequent treatments, underdosing, the use of only one drug without combination or rotation etc. This leads to a decrease in their effectiveness, and consequently to even higher economic losses (an additional 35-40 million euros). Moreover, these costs will tend to increase in the future due to widespread of resistance and the occurrence of multiresistant strains. For these reasons, the focus of nowadays research is finding alternative solutions including genetic selection of naturally resistant animals, pasture management, dietary manipulation, biological control (use of nematophagous fungi, bacteria or earthworms and dung beetles) and the use of plant-based formulations such as extracts and essential oils. All of the above strategies have shown promise, although it appears that none of them could be used independently. Therefore, future strategies for the control of gastrointestinal nematodes should be based on an integrated approach. This implies the combination of mentioned alternatives with rational use of anthelmintics based on refugia strategies (target treatments and target selective treatments).