LONDON (Reuters) - A British animal genetics firm, working with U.S. scientists, has bred the world's first pigs resistant to a common viral disease, using the hot new technology of gene editing. Genus, which supplies pig and bull semen to farmers worldwide, said on Tuesday it had worked with the University of Missouri to develop pigs resistant to Porcine Reproductive and Respiratory Syndrome Virus (PRRSv). The condition, also known as blue-ear disease, can be fatal as it affects the animals' immune system and costs farmers hundreds of millions of dollars a year. There is no cure.