Mycoplasma hyopneumoniae is the primary pathogen causing Enzootic Pneumonia (EP), one of the most prevalent respiratory diseases in pigs resulting in high morbidity but low mortality. It is also one of the main pathogens involved in the porcine respiratory disease complex (PRDC), however other bacteria and viruses are also often contributing factors.

Acute inflammation, due to Mycoplasma hyopneumoniae infection, leads to a catarrhal or even purulent exudate in the airways. The accumulation of neutrophils and mono-nuclear cells around the bronchi and bronchioles results in airway collapse and consolidation of the lungs.

Figure 1: Pig lungs - Lesions of broncho-pneumonia and cranial pleurisy*

Lung tissue infected with Mycoplasma hyopneumoniae develops consolidation and catarrhal broncho-pneumonia with purple to grey regions of meaty appearance. The consolidation can be observed from 3 - 12 weeks post infection. The lesions are mainly localized in the apical and cardiac lobes, in the anterior part of the diaphragmatic lobes, as well as in the intermediate lobe. Lesions resolve after 12 to 14 weeks with formation of interlobular fissures, also referred to as scarring.

Mycoplasma hyopneumoniae and related viral and bacterial pathogens are also responsible for the development of pleuritis, which is typically localized in the cranio-ventral part of the lungs, most often between the apical and cardiac lobes.

Consolidation of the apical and cardiac lung lobes indicating the acute to sub-acute bronchopneumonia is suggestive of Mycoplasma hyopneumoniae induced EP. Pleuritis at the junction of the cardiac and diaphragmatic lobes can still be assigned to the cranio-ventral area. Pleurisy in this area are most likely due to Mycoplasma hyopneumoniae and associated bacterial or viral infections.

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