Nematode parasites kill a lot of sheep

CROWD ANIMALS together and one likely outcome is parasitic infestation. This has been a problem since the beginning of animal husbandry. Many scholars, for instance, suspect that the origins of religious dietary laws forbidding the consumption of pig meat lie in pigs’ susceptibility to worms that are harmful to human beings. But they are also harmful to animals. They are, for example, the largest cause of natural death among the world’s sheep.Nowadays, at least where farmers have access to modern veterinary facilities, the usual approach to infestation is to administer vermicidal drugs (see picture). These often work. But, as with any such approach to parasites and pathogens, extensive use has encouraged the evolution of resistance. Many worms have become immune to at least one of the commonly used vermicides. Some are immune to all three.