Impact of infection sequence among porcine circovirus type 2, porcine reproductive and respiratory syndrome virus and Mycoplasma species on clinical outcomes.

This review focuses on divergent clinical outcomes resulting from different infection sequences involving 3 major swine pathogens; porcine circovirus type 2 (PCV2), porcine reproductive and respiratory syndrome virus (PRRSV), and Mycoplasma species. These three pathogens cause substantial economic losses in the global swine industry and are major contributors to porcine respiratory disease complex (PRDC) as well as a broad spectrum of clinical manifestations. Clinical outcomes vary depending on the sequence of infection between PCV2 and either PRRSV or M. hyopneumoniae. PRRSV potentiates PCV2-associated lymphoid lesions in pigs infected with PRRSV prior to PCV2d or in pigs simultaneously infected with PCV2d and PRRSV, but not in pigs infected with PCV2d prior to PRRSV. Similarly. sequential infection with M. hyopneumoniae followed by PCV2d, as well as simultaneous infection with these pathogens, enhances PCV2-associated lymphoid lesions. Clinical outcomes are influenced by the sequence of infection between PRRSV and M. hyopneumoniae. Potentiation of PRRSV-induced pneumonia by M. hyopneumoniae was detected only when M. hyopneumoniae infection preceded PRRSV infection. Mycoplasma hyorhinis potentiated PCV2 infection and exacerbated PRRSV-induced pneumonia regardless of the sequence of infection. Understanding pathogen interactions through infection sequence provides swine producers and veterinarians with clinically relevant information for preventing and controlling PRDC and other clinical manifestations in swine herds.