5. Conclusions and Future Perspectives

In summary, many studies have demonstrated that Mhp alone is not enough to induce PRDC or PMWS (Krakowka et al., 2000; Posa et al., 2013a; Wellenberg et al., 2004). The major route of Mhp entry is through the mucosal surfaces of respiratory tract followed by adhering to the cilium of epithelial cells(N. Zhang et al., 2019), the cilium can be gobbled up and shed due to a large number proliferation of Mhp(Tao et al., 2019) and also the duration of Mhp infection can lasts for not less than 254 days (Pieters, Pijoan, Fano, & Dee, 2009). Mhp infection increases susceptibility of the pig to secondary infections, clinical sign of PCV2, PRRSV, SwIV, PRV and bacterial pathogens infections. Meanwhile, the secondary infection and worse habitat condition also provide a better environment for Mhp infection. The main target sites for these swine pathogens are shown in Table. 1. In addition, once infected with Mhp, pigs can excrete the pathogen lasting for 254 days post-infection (Pieters et al., 2009), this may be a high risk for off-springs infection. Moreover, farming system is a major factor influencing infection and sero-positivity for Mhp infection on pig farms, the virulence status of Mhp isolates may also affect the clinical pattern of infection (Vicca et al., 2002). Consequently, more researches are needed to improve our understanding of the interactions between different swine viruses and bacteria during co-infection with Mhp, encompassing how they interact with the host immune response and how they affect the efficacy of vaccination. At the same time, other experimental models should be developed and analyzed, and field trials can also be conducted to establish best practices for controlling this complex disease syndrome. The thrust of this review may provide point ideas in understanding of MPS and the developing of new strategies to control the disease.