Swine Respiratory Disease (SRD) The leading cause of swine mortality in the U.S. Swine Respiratory Disease (SRD) is the prevalent cause of nursery pig and grower/finisher deaths, according to the National Animal Health Monitoring System.1Often, multiple infectious agents are involved. Primary pathogens include Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae (APP) and Bordetella bronchiseptica, as well as the viral agents, such as porcine reproductive and respiratory syndrome (PRRS) virus and swine influenza virus (SIV). Common secondary invaders include Pasteurella multocida, Streptococcus suis, Haemophilus parasuis, Actinobacillus suis and Salmonella choleraesuis. These infectious agents act together to increase the severity and duration of the disease. 1. National Animal Health Monitoring System. 1996. Swine `95: Grower/Finisher. Part II. Reference of 1995 U.S. Grower/Finisher Health and Management Practices, p. 14–16. U.S. Department of Agriculture, Fort Collins, Colo.