Viral co-infections targeting the porcine respiratory system: Consequences and limits of the experimental systems

Coinfections affecting the porcine respiratory system have often been overlooked, in favor of mono-infections, even though they are significantly more common in the field. In pigs, the term 'porcine respiratory complex' is used to describe coinfections involving both viruses, such as, for example, the swine influenza type A virus (swIAV), the porcine respiratory and reproductive syndrome virus (PRRSV), and the porcine circovirus type 2 (PCV-2), as well as bacteria. Until recently, most studies were primarily focused on clinical aspects and paid little attention to the molecular consequences of coinfections. This narrative review addresses the consequences of coinfections in the porcine respiratory system involving viruses. When possible, interactions that can occur between viruses are briefly presented. Conversely, research involving bacteria, protozoa, and fungi has not been considered at all. Finally, the main limitations complicating the interpretation of results from coinfection/superinfection studies are considered, and prospects in this exciting field of health research are presented.