Haematopinus quadripertusus, commonly called the cattle tail louse, get its name because, well, because it lives on the tails of cattle. These lice will actually take their blood meals from both ends of the cow, however, and can also be found on the mouth and in the ears as well as some of the soft bits on the backside. This louse was introduced into the Southern U.S. and is now a major pest of cattle in Florida, where they can cause anemia and weight loss - not something you want if you're trying to fatten up your cattle. Lice can spread from cow to cow when they are in close contact or via rubbing on the same thing such as a post. But, the lice can also colonize completely different herds of cattle by hitching rides on flies!
Image from this site, which has additional information on these parasites.