Actinobacillus pleuropneumoniae, a Gram-negative, non-motile, facultatively anaerobic Pasteurellaceae, causes porcine pleuropneumonia, a highly contagious disease for which there is no effective vaccine. There are least twelve different serotypes, some of which produce no disease but others cause severe disease. Serotypes vary in different countries. Types 1, 5, 9, 11 and 12 are usually highly virulent and strains 3 and 6 are mild. The bacterium is carried in the tonsils and upper respiratory tract. It is transmitted short distances by droplet infection and only survives outside the pig for a few days.