Christina Gale, BSc, Swine Marketing Manager, Eduardo Velazquez, MRCVS, Swine Veterinary Service Manager and Emma Pattison, BSc, Swine Field and Vaccination Services Manager, Ceva Animal Health, UK
Mycoplasma hyopneumoniae (M. hyopneumoniae) is one of the most important primary pathogens of the porcine respiratory system and is the causative agent of enzootic pneumonia (EP) (Fraile et al., 2010). M. hyopneumoniae is widespread throughout the pig population and is endemic on most farms worldwide. Pneumonic lung lesions due to M. hyopneumoniae are commonly observed in the slaughterhouse, with average herd-level prevalence reported as 24%, ranging up to 88% (Maes et al., 2001), demonstrating the large variability observed between units depending on individual farm situation.
Mycoplasma hyopneumoniae is 200–500 nm in size and requires complex media and aerobic and microaerophilic conditions to be cultured. Infection with M. hyopneumoniae occurs via inhalation of infected aerosols or via direct contact with infected animals. The pathogen establishes itself in the respiratory tract, attaching to the ciliated cells of the tracheal, bronchial and bronchiolar epithelium. Once in the respiratory tract, it can persist for weeks to months and causes pathological changes in the lungs. Lesions are thought to form in the lobes of the lung due to lack of normal clearance of secretory products from the respiratory system, due to infection of the ciliated epithelium (Taylor, 1995). Early lesions are observed as small dark red areas in the anterior lobes, which enlarge over time and after a few weeks lose their red colour and become more pink (Figure 1). Lesions resolve after 12 to 14 weeks with formation of interlobular fissures (Maes et al., 2008). This is important to note, as infections early in production may be resolved once the animal reaches the slaughterhouse, so any scarring (Figure 2) should be noted as losses may still have occurred during the growing and finishing stages.
Figure 1 - Pneumonic lung lesions caused by infection with M.hyopneumoniae
Figure 2 - Scarring of lobes (score 1) due to recovery of lung lesions caused by infection with M.hyopneumoniae
Histologically, inflammation occurs and neutrophils can accumulate in the airways and bronchioles. Lymphocytes and macrophages will also be present 5 days after infection, followed by presence of large mononuclear cells, polymorphs, lymphocytes and plasma cells in the alveoli from day 7 (Taylor, 1995). Immunoglobulin IgA forms in tracheal mucosa so may be detectable from day 30 on diagnostic testing of secretions, and IgG may also be present at a slightly later stage.
Enzootic pneumonia can clinically be acute or chronic in its form, with different clinical presentations of disease:
Diagnosis can be carried out by M. hyopneumoniae-specific seroconversion or laryngeal swabs tested by PCR (Pieters et al., 2017). Examination of the lungs at slaughter can also be useful to report presence of pneumonic lesions, characterised by consolidated areas especially in the cranial lobes (Maes et al., 2001). Seasonal patterns are often observed with M. hyopneumoniae, with lung lesion prevalence and severity increasing following the winter months. This was demonstrated by Ostanello et al. (2007) where, as scored using the Madec and Kobisch method (1982), mean lung lesion score was significantly increased in batches of animals raised in the winter months (October–March) compared to the summer months (April–September), with scores of 2.33 and 1.81 respectively. Decreased growth rate and feed conversion ratio (FCR) may also be observed, typically with no or low mortality (Sibila et al., 2009).
Mycoplasma hyopneumoniae is also a major contributor to development of the porcine respiratory disease complex (PRDC) (Garcia-Morante et al., 2015). The pathogen modifies the immune response and facilitates the expression of co-infections. It is described as an inhibitor of macrophage phagocytic activity, which may explain the chronicity of M. hyopneumoniae infections and the greater host susceptibility to other pathogens (Figures 3 and 4) (Saade et al., 2020). Co-infections are very important in respiratory disease, as interaction of multiple pathogens has been shown to increase the severity of clinical signs (Saade et al., 2020) and therefore the impact on performance.
Co-infections can occur with both viral and bacterial diseases. Regarding viruses, it is important to mention three very important diseases affecting the swine herd globally:
Figures 3 and 4 - Interactions between pathogens and the impact on clinical signs observed (Source: Saade et al., 2020)
In relation to bacterial co-infections, the importance of M. hyopneumoniae is as crucial as with viruses:
In general, M. hyopneumoniae has a large role in increasing the severity of respiratory disease on farms where other pathogens are also present. Other bacteria such as Bordetella bronchiseptica, Haemophilus parasuis, Trueperella pyogenes, Streptococci or Staphylococci are also commonly found in field outbreaks of EP (Maes et al., 2018).
M. hyopneumoniae is known to be one of the most prevalent swine pathogens worldwide, causing substantial economic losses within the swine industry (Maes et al., 2018). Enzootic pneumonia has a large economic impact on the pig industry, primarily due to the cost of treatment, reduced performance and increased mortality due to secondary infections (Holst et al., 2015).
The economic impact primarily derives from the decreased performance in production parameters, including:
Straw et al. (1989) demonstrated that pneumonia due to M. hyopneumoniae caused a decrease in ADG and feed efficiency by 17% and 14% respectively. This is reflected in increased days to slaughter, therefore increasing cost of production.
Calculating the economic impact due to M. hyopneumoniae specifically is difficult to estimate due to the contribution of numerous co-infections (Maes et al., 2018). The cost has been shown to be higher if other respiratory pathogens such as PRRS or SIV are also present along with M. hyopneumoniae (Haden et al., 2012). A recent study has demonstrated that the severity of the impact on productive performance, and therefore economic losses, is correlated with the percentage of lung consolidation caused by M. hyopneumoniae, reporting a negative correlation between lung consolidation lesions and ADG, where a 1% increase in lesion area corresponded to a decrease of 1.8 g (Ferraz et al., 2020). Therefore, these few studies are similar in the economic impact of M. hyopneumoniae noted; however, it is important to remember that this value will always depend on the extent and severity of lesions.
Correct angles of vaccination
Vaccination is commonly used at weaning to control EP and has been shown to improve the daily weight gain by 2–8% and the feed conversion ratio by 2–5%, therefore reducing losses due to EP (Segales et al., 2008). There are numerous vaccines available which vary in the vaccination schedule, but the ultimate goal should be reduction of lung lesions and performance losses due to M. hyopneumoniae.
Vaccination with an efficient vaccine has been shown to be effective at reducing both the percentage of pigs with EP-like lesions and the severity of these lesions, as demonstrated by the Madec index which is calculated using a modified Madec score. The Ceva Lung Program (CLP) is a tool for analysis of the lungs in the slaughterhouse and has been used in many swine-producing countries to demonstrate these reductions.
Vaccination is also associated with reduced use of antimicrobials to control disease (Maes et al., 2008), contributing to achieving targets in reducing antimicrobial use across the industry. Vaccination combined with good management practices will help to reduce the impact of the pathogen as well as co-infections on farms, working to reduce the lung lesion prevalence and severity, which has been shown to be correlated with production losses.