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.