Say ‘parasites’ to a sheep farmer and they’ll have plenty to say. If in Britain, they might well mention the industry-led group Sustainable Control of Parasites in Sheep. Sheep are affected by parasitic internal helminths, a huge group including nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes). The farmer might also mention ectoparasites which infect the outside of the sheep’s body, maybe its skin, and intracellular parasites which infect the the sheep’s cells.
Farmers can get information from EBLEX (the English Beef and Lamb Executive) in the guide Controlling Parasites in Sheep.
Good science about sheep parasites is being done at the Scottish Agricultural College and at Moredun. There are academic journals including Veterinary Parasitology. There’s a World Association for the Advancement of Veterinary Parasitology. Sheep parasites are one of the topics addressed by the British Society for Parasitology.