2.5 Co-Infection with Multiple Viruses

Alongside single and dual infections, multiple (including triple or more viruses) infections are also prevalent among pigs(Opriessnig, Gimenez-Lirola, & Halbur, 2011). PRDC is caused by the combination of infectious pathogens, differences in production systems and environmental factors (Hansen et al., 2010). Previous study have demonstrated that PRRSV and Mhp were well known to potentiate PCV2-associated lesions, surprisingly, PRRSV vaccine was found to enhance PCV2 replication and it can be related to the failure of PRRSV vaccine in vaccinated co-infected pigs compared to non-vaccinated co-infected pigs (C. Park, Oh, Seo, Han, & Chae, 2013). Pallares et al have detected that the coinfection rate of PCV, PRRSV and Mhp was 16% (77/484) in 484 cases diagnosed as PMWS (Pallares et al., 2002). A survey also demonstrated that co-infection ratios with three and four pathogens were 17.3% and 7.3% respectively in 110 pneumocystis spp. positive lung samples of Austrian pigs with pneumonia (Weissenbacher-Lang et al., 2016). Vaccination against Mhp significantly protected pigs against multiple viral infections, suggesting that vaccination against Mhp decreases the risk of PMWS and PRDC, and reduces susceptibility of pigs to the other viral pathogens (Chae, 2011). These data indicate that Mhp plays an important role in the co-infection of dual or multiple pathogens in vivo .