Viral co-infections of the porcine respiratory tract: Insight into the local cytokine response

Infections of the porcine respiratory tract are frequently multifactorial, with more than one pathogen involved. They have a significant impact on the efficiency of pig production. One example of such a mixed infection is the porcine respiratory disease complex (PRDC). PRDC can be caused by various viral or bacterial agents. The main viral agents associated with PRDC and considered the primary pathogens are porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), and porcine circovirus type 2 (PCV2). PRRSV, SIV, and PCV2 are known as inducers of inflammatory cytokines. Cytokines play an important role in all aspects of immune responses, but their uncontrolled release in virus-induced diseases may aggravate the course of the disease and the severity of pathological lesions. Although data regarding the kinetics of the local cytokine response in porcine lungs during mono-infection with these pathogens are abundant, their impact on each other during simultaneous infection in different combinations is not thoroughly understood. This paper aims to present the available data on interactions between SIV, PRRSV, and PCV2 in mixed infections of the porcine respiratory tract and the influence of co-infections on local cytokine profiles in the lungs.