Parasitic Diseases of Cattle Parasitic diseases impair health, reproduction, and productivity. Parasitic diseases of cattle impair health, reproduction, growth, and productivity. In severe cases, parasitic diseases may even cause death. These diseases are caused by internal helminths (roundworms, tapeworms and flukes) as well as external arthropods (mites, lice, ticks, and flies). Transmission of helminths is through oral ingestion or direct skin penetration by larval parasites on pasture. Transmission of arthropods is through direct contact (mites and lice) or exposure to larval stages on pasture (ticks) or fly-in by adult flies or skin penetration by larval flies (grubs).