Introduction

Mycoplasma hyopneumoniae (M. hyopneumoniae , Mhp) forms part of the etiology of a chronic insidious lympho-histiocytic bronchopneumonia, also described as mycoplasmal pneumonia of swine (MPS) or enzootic pneumonia (EP) (DeBey, Jacobson, & Ross, 1992), is a strong impediment to the efficiency and profitability of the global pig industry(Simionatto, Marchioro, Maes, & Dellagostin, 2013). This disease is characterized by high morbidity but low mortality (Maes et al., 2008; Morris, Gardner, Hietala, & Carpenter, 1995), causing cough, asthma, anorexia, and many other symptoms among off-springs, as well as showing decreased daily weight gain and significant lung lesions in the field (Tao, Shu, Chen, Wu, & He, 2019). Mhp was first isolated from the trachea, bronchi, bronchioles and other mucosal surfaces (Blanchard et al., 1992), and it’s difficult to be isolated from fields directly because of their slow growth and easy contamination. Despite the continuous improvement of mediums, in vitro culture of Mhp still remains difficult (Calus et al., 2010; Cook, Beddow, Manso-Silvan, Maglennon, & Rycroft, 2016). This limitation, to some extent has stymied the research and the development of improved methods for disease control.
Piglets may be infected with Mhp at the early stage, with prevalence increment during the post-weaning period (Vangroenweghe, Labarque, Piepers, Strutzberg-Minder, & Maes, 2015), the main process involved in the Mhp infection is shown in Fig. 1. Mhp infection causes immunosuppression in pigs, which provides conditions for the secondary infections. Owing to the development of modern swine production, which has intensified the production of swine on a larger-scale, mixed infections by multiple pathogens are becoming more common and serious in swine. Although the exact mechanism of Mhp infection still remains unknown, many studies have reported co-infection with other pathogens for decades, these include Porcine Circovirus (PCV) (Opriessnig & Halbur, 2012), Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), Swine Influenza Virus (SwIV), Pseudorabies Virus(PRV) and other bacterial pathogens, they also serve as possible co-factors for triggering postweaning multisystemic wasting syndrome (PMWS), which have further economic and welfare implications (Ciprian et al., 1988; Krakowka et al., 2000; Maes et al., 2008; Maes, Verdonck, Deluyker, & de Kruif, 1996; Pogranichniy, Yoon, Harms, Sorden, & Daniels, 2002; Wang et al., 2016; Wellenberg, Stockhofe-Zurwieden, Boersma, Jong, & Elbers, 2004). The objective of this review was to summarize the interaction during the co-infection of Mhp with other viruses and bacteria together with other factors leading to infection in recent years, to provide the theoretical basis for studies in the future.