I am thinking of a Dollop episode that discussed pigs in early New York City. They were the trash clean up before there were people dedicated to the job. They were allowed to just run free and scrounge whatever offal they could find. It could have been less that pigs were unclean, rather pigs kept things clean. If people ate all the pigs, there would be corpses and fecal matter lying around for extended periods of time which would cause disease.