Porcine Respiratory Disease Complex

This chapter discusses porcine respiratory disease complex (PRDC) in the broader sense of mixed respiratory infections that occur in swine of any age. It demonstrates a clear trend towards increasing case numbers of pneumonia due to porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), porcine circovirus type 2 (PCV2), Mycoplasma hyopneumoniae, Pasteurella multocida, Streptococcus suis, Bordetella bronchiseptica, and Actinobacillus suis. The increased incidence of PRRSV, PCV2, and A. suis-induced pneumonia is most remarkable. The sixfold increase in SIV can in large part be explained by the introduction of subtype H3N2 into the United States in 1998. The number of cases of pneumonia due to pseudorabies virus (PRV), A. pleuropneumoniae, and Salmonella choleraesuis is steady or decreasing. The chapter discusses some of the successful models of mixed infection and theories as to the pathogenic mechanisms of interaction. Epidemiological evidence that secondary bacterial infections often follow viral respiratory diseases is well documented. Although some of these superinfections have been replicated experimentally, specific mechanisms of interactions are poorly understood and often inferred based on pathogenesis of the viral infections. PRV, possibly porcine respiratory coronavirus (PRCV), SIV, and PRRSV are all common viral respiratory pathogens of pigs, and all have been variously implicated in increasing the incidence of bacterial respiratory disease in herds which circulate the viruses. The chapter also talks about methods of treatment or prevention of respiratory disease in swine herds, which fall into the categories of management changes, vaccination, and antibiotic usage.