Despite the reported efficacy of M. hyopneumoniae vaccines in reducing lung lesions [28] and bacterial load from the respiratory tract [29], they are not capable of fully eliminate the bacteria from the animal, giving them the opportunity to cause typical MLL in a variable percentage of pigs at abattoir in different countries [12, 17, 19, 20]. To investigate the prevalence and severity of MLL at abattoirs in Spain and Portugal using a 0 to 5 scoring system adapted to fast slaughterlines, a study with an elevated number of pigs (199,678) involving a high number of farms (221) was performed. Additionally, a sample of 788 lungs was taken to corroborate microscopically the presence of lesions characteristics of the infection induced by M. hyopneumoniae.
In our study, no gross lesions were found in 50.26 % of all examined lungs at abattoir and roughly 31 % showed cranioventral consolidations compatible with mycoplasma lesions, showing similar frequencies in Spain (31.14 %) and Portugal (29.95 %). These prevalence rates of MLL were higher than those reported in Belgium (23.85 % and 24.00 %) [9, 17] but lower than those previously found in Spain (44.61 to 55.69 %) [11, 19], Italy (46.38 %) [12], France (69.30 %) [18] or Germany (72.60 %) [20]. The main difference among studies was the number of examined lungs, with nearly 200,000 lungs evaluated in this study in comparison with numbers ranging from 600 [19] to 10,404 in other studies [11]. However, other factors such as the epidemiological scenario in each country also play a role in these differences. Consequently, considering the high number of farms and the different pig production areas herein reported, we consider that our data provide an accurate percentage of prevalence of MLL in Spain and Portugal. Moreover, if we consider the histopathological results and the percentage of selected lungs with MLL compatible with M. hyopneumoniae infection (78.17 %), the real prevalence of gross lesions caused by this bacterium would be reduced from 30.97 % to 24.21 %. Other lesions, such as suppurative bronchopneumonia, as represented in Fig. 1, exhibited a similar gross appearance [30, 31] and, hence, this type of pneumonia could be mixed up with lesions caused by M. hyopneumoniae. Thereby, those studies, in which a microscopic confirmation has not been performed, could bias the results, overestimating the prevalence. In the study carried out by Luhers et al. [20], 78.30 % of 400 lungs with MLL collected at the abattoir were positive to M. hyopneumoniae by PCR, a percentage very similar to the one found with compatible microscopic lesions in our study (78.17 %). These results highlight the interest of performing additional studies on lungs with MLL to confirm the diagnosis of EP or infection by M. hyopneumoniae.
Pleurisy, associated with dorsocaudal infarcts or alone, was the second most prevalent gross lesion of our study, affecting 18.77 % of the examined lungs, a percentage very similar to that found in France (15.00 %) [18] or Belgium (16.00 %) [9], but lower than those previously reported in Italy (25.10 %) [12] or Spain (26.80 %) [11]. In our case, most of the pleurisy (66.65 %) was associated with dorsocaudal infarcts, a hallmark of A. pleuropneumoniae infection, lesion which has been associated with decreased ADG during the grower-finisher period, leading to lower economic return [16]. Therefore, the measures to control this pathogen (management, therapeutic or vaccination) should be revised and/or implemented in the farms included in the study.
According to our findings, most of the lesions of bronchointerstitial and interstitial pneumonia, characteristics of mycoplasma and viral infections, respectively, had a chronic course, by contrast, the majority of the lesions of suppurative and fibrinous bronchopneumonia, characteristics of bacterial infection, had an acute course and a higher degree of severity. These results suggest that viruses and mycoplasmas could have acted in an earlier stage of the life of piglets, probably during nursery, and later on, during the fattening period, bacteria would have taken action. Ruggeri et al. [32] reported pleurisy, followed by pleuropneumonia, catarrhal bronchopneumonia and bronchointerstitial pneumonia, as the most prevalent microscopic lesions in fattening pigs, but animals included in that study died because of respiratory diseases, that is, samples were not collected from healthy animals at abattoir as in the present study. However, to a certain extent, it agrees with our observations, since most of the deaths were consequence of an acute process of fibrinous and suppurative bronchopneumonia. A recent study carried out in Brazil by Galdeano et al. [15] also found the characteristic lesions of M. hyopneumoniae infection as the most prevalent one, being detected in 63.75 % of the lungs examined microscopically, but different to our study, the second most frequent lesion was chronic bronchopneumonia (57.14 %) whereas other lung lesions common in our study such as suppurative bronchopneumonia, interstitial pneumonia or fibrinous bronchopneumonia, were observed in a lesser extent (15.63 %, 3.61 % and 0.15 %, respectively). Nevertheless, it must be taken into account that porcine reproductive and respiratory syndrome virus (PRRSV) has never been detected in Brazil [33], thus, one of the main primary agents involved in the PRDC [5], alone or in combination with other pathogens, is not taken place in those farms. Therefore, the clinical and lesional picture at abattoir would be totally different in Brazil, when compared with those countries where PRRSV is endemic, such as Spain or Portugal.
The histopathological study provided us a more accurate idea of the real prevalence of lesions caused by M. hyopneumoniae but also information about the concomitant action of other pathogens involved in the PRDC that sometimes go clinically unnoticed. In addition, some lesions are unspecific and could be caused by different pathogens, but in association with other techniques, such as serology, bacteriology or PCR, histopathology could provide a more accurate information about which agent is causing the lesion, since a positive result in those diagnostic techniques against some pathogens [i.e. M. hyopneumoniae or porcine circovirus type 2 (PCV2)] does not necessarily always mean that they are causing any lesion.
Lesions of bronchointerstitial pneumonia compatible with M. hyopneumoniae infection were found in the six more frequent combinations of lesion patterns in our study, representing 66.13 % of the lungs examined, what support the relevant role of this agent in the PRDC as a primary agent as well as enhancing the action of other pathogens involved in as PRRSV [34], PCV2 [35] or swine influenza virus [36].
The scoring system from 0 to 5 points used in this study for MLL, adapted from a previous one [37], has been showed as a simple and repeatable method that can be easily applied in cases of abattoirs with fast slaughterline, sometimes more than 500 pigs per hour. By contrast, it is not as precise as other methods which express the proportion of affected lung area in percentages [22, 23, 24, 25].