M. hyo

Mycoplasma hyopneumoniae infections

Mycoplasma hyopneumoniae (M. hyo) is the primary pathogen of enzootic pneumonia (EP), a chronic respiratory disease in pigs, and one of the primary agents involved in the porcine respiratory disease complex (PRDC) (Maes et al. 2017). EP is characterised by a chronic, nonproductive cough, decreased growth rate and feed conversion ratio (Sibila et al. 2009), typically with no or low mortality. PRDC develops as a consequence of coinfections of both bacterial and viral pathogens, especially porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV) and porcine circovirus type 2 (PCV2) (Sibila et al.2009). PRDC can result in an increased mortality and severe performance losses. The major threat for the farm economy is represented by the decrease in the daily weight gain and eventual increased medication cost. Infection with M. hyo often appears to have a subclinical course, where only the growth performance is reduced.

M.hyo bacteria

M.hyo is one of the smallest self-replicating microorganisms with a missing cell wall. The pathogen is not able to survive for a long time outside its host, but in aerosols its survival time increases as it can remain infective for up to 31 days in water at 2-7°C (Villareal 2010).

The clinical outcome of M.hyo infections is determined by different factors, such as housing conditions, management practices, co‐infections and also by virulence differences among M.hyo isolates. Variation across several isolates of M.hyo has been described at antigenic, proteomic, transcriptomic, pathogenic and genomic levels and also in their virulence (Meyns 2007, Vicca 2003, Betlach 2019). Although the pathogenesis is not yet fully understood, it has been found that some of the proteins (mainly adhesins), but also genes involved in metabolism and growth contribute to virulence.

High virulent strains induce faster and more severe disease signs than the low virulent strains (Villareal 2009). Low virulent strains also induce later seroconversion. However, previous exposure to the low virulent strain doesn’t reduce the lesions due to the subsequent high virulent M.hyo infection.

Co‐infection with more than one strain in a pig or batch of pigs might result in more severe lung lesions (Michiels et al., 2017)

Prevalence of nPCR positive results, average number of different strains, severity of Mycoplasma-like lesions ±SD, prevalence pneumonia, fissures and pleurisy expressed in percentages. (Michiels et al., 2017)

Disease

M.hyo is the primary pathogen of enzootic pneumonia (EP), a chronic respiratory disease in pigs, and one of the primary agents involved in the porcine respiratory disease complex (PRDC). Pneumonia induced by M. hyo is considered one of the most wide-spread and most important chronic diseases in pigs. The prevalence in estimated as high as 80% in the world-wide pig population (Batista 2006). Nearly all (more than 99%) of commercial swine herds have mycoplasmal pneumonia in United States.

Sows and piglets in the breeding herds are considered the reservoir of M.hyo infections for the entire production system. Circulation of M.hyo is thought to occur among existing sows and be transmitted to incoming gilts, which are capable of maintaining the pathogen within the farm and are responsible for the majority of bacterial shedding to newborn pigs. Infection with M. hyo has a long duration, reaching up to 240 days (Pieters 2009), complicating the already slow disease transmission scenario observed in sow herds.

The positivity is different in various age categories. Typically, the massive circulation occurs in growing-finishing pigs. The prevalence in weaned pigs is very low in most of conventional farms. (Krejci 2019)

EP is characterized by a chronic, nonproductive cough, decreased growth rate and feed conversion ratio (Sibila et al. 2009), typically with no or low mortality. PRDC develops because of coinfections of both bacterial and viral pathogens, especially porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV) and porcine circovirus type 2 (PCV2) (Sibila et al.2009). PRDC can result in an increased mortality and severe performance losses. The major threat for the farm economy is represented by the decrease of the daily weight gain and eventual increased medication cost. The infection with M.hyo often appears to have a subclinical course, where only the growth performance is reduced. Typical clinical picture of enzootic pneumonia includes non-productive, dry cough, anorexia and dyspnea. Dynamics and duration of coughing are important for the estimation of the impact of EP on growth retardation. It is difficult to assess the economic effect of mycoplasmal pneumonia because of the multifactorial origin of PRDC. It was reported a 17% decrease in daily weight gain and a 14% decrease in feed efficiency in herds with enzootic pneumonia (Straw 1989).

When healthy pigs free from M. hyo were mixed into M. hyo positive herds and were thus exposed to the natural infection, their performance was lower. The ADG of pigs with a non-complicated mycoplasmal pneumonia as compared with those which remained free from M.hyo was decreased by more than 60 g per day after adjusting for herd, pen, weight and sex (Rautiainen et al. 2000).

The presence of the infection is usually confirmed by M.hyo specific seroconversion or by the detection of germs by PCR in the laryngeal swabs (Pieters et al. 2017, Sibila et al. 2009). Lung tissue infected with M.hyo develops consolidation and catarrhal broncho-pneumonia with purple to grey regions of meaty aspect. The consolidation can be observed from 3-12 weeks post infection. The lesions are mainly localized in the apical and cardiac lobes, as well as in the anterior part of the diaphragmatic lobes and in the intermediate lobe. Lesions resolve after 12 to 14 weeks with formation of interlobular fissures (Maes et al. 2008). Considering the chronic type of such lesions, bronchopneumonia with the cranioventral consolidation of lungs is very indicative for EP also in slaughter pigs.

Example of a lung with lesions derived from an experimental M.hyo inoculation (Garcıa-Morante, 2016).