M. hyopneumoniae is a main causative pathogen of porcine respiratory disease complex (PRDC) [1]. PRDC is characterized clinically by anorexia, slow growth, lethargy, fever, dyspnea and an antibiotic treatment-resistant cough in growing and finishing pigs [18, 19]. To prevent PRDC, detection of M. hyopneumoniae during the early stages of infection is critical but technically challenging. The colonization of pigs by M. hyopneumoniae starts from tracheal cilia as the initial postexposure infection site. Therefore, direct access into the trachea cilia could be useful for sensitive detection of M. hyopneumoniae. And the tracheobronchial lavage fluid may be regarded as the best sample to detect the infected microorganism in this case [20]. However, in previous reports, laryngeal swabs have demonstrated the highest sensitivity for M. hyopneumoniae DNA detection among oral, nasal, and tracheobronchial samples as well as antibody detection in serum samples [16, 21, 22]. Although the larynx is not considered the typical location for M. hyopneumoniae infection or colonization, it may function as a vestibular site that may incur lower airways. Consequently, the larynx could be the place where M. hyopneumoniae is concentrated, and it is a preferred sampling site for favorable access and proximity to the lower airways [16, 23]. As a field study, our results show that laryngeal swabs for M. hyopneumoniae detection are practical and reliable.
As a result, M. hyopneumoniae DNA was detected by nested PCR in 14 farms (63.6%) and 127 piglets (average 6.5%, range 0–28.8%). The prevalence (11.1%) of sows likely to transmit M. hyopneumoniae in herds was calculated as the ratio of total sows to sows with pathogen-positive piglets. Compared to previous reports, it was similar to the results obtained in other countries. Using laryngeal swabs, 7% of weanling pigs and more than 50% of sow herds tested positive for M. hyopneumoniae in the USA. In Germany, 18.7% of weanling pigs and 75% of sow herds were PCR positive [24].
There was a significant difference depending on herd size and gilt replacement rate. Farms with more than 550 sows had a significantly higher detection rate than those with lower sow numbers. Farms where more than 40% gilts were replaced also had a significantly higher detection rate than those where gilts were less replaced. In general, larger farms tend to replace gilts at a higher rate. Larger farms or high gilt replacement rates may support the transmission of pathogens due to management factors such as biosecurity, vaccination practices, and herd density [25]. However, larger farms may have additional factors, such as cross fostering in the farrowing unit, contamination by teeth grinding, tail docking, castration and housing, that have not been investigated in this survey, influencing the results.
There was no significant difference in the prevalence of M. hyopneumoniae detection in piglets among farms with regard to the serological status of introduced gilts. Similarly, the detection rate in farms with self-replacing gilts was not significantly different from the rate in farms with gilts brought from grandparent farms. With regard to the long shedding period, sows exposed to M. hyopneumoniae at the age of 50–100 days are expected to develop immunity early enough so that the possibility of transmission is reduced at their first farrowing [26].
Notably, farms where gilts were naturally exposed to M. hyopneumoniae had a significantly higher detection rate in piglets than farms with vaccinations or without acclimation. Considering that experimentally infected pigs shed M. hyopneumoniae for up to 200 days [27], natural exposure for gilt acclimation at an age of 100 days or older may support vertical transmission. According to the gilt acclimation survey of 22 farms, no farms had diagnostic verification after gilt acclimation. Seventeen farms introduced gilts over 140 days old, and 16 farms had acclimation periods below 10 weeks. Inappropriate acclimation resulted in a high risk of sow stability.
Amoxicillin, enrofloxacin, tilmicosin, ceftiofur, penicillin, streptomycin, gentamicin, tulathromycin, lincomycin, and spectinomycin were used on most farms. Treatments were applied to sows, piglets, or both. Interestingly, the detection rate was significantly higher in farms that used antibiotics in the sow herds than in those that did not use antibiotics. The detection rate was also significantly higher in farms using antibiotics in suckling pigs than in farms where antibiotics were not used. Although antibiotics have been demonstrated to be effective at controlling enzootic pneumonia [28], only limited information is available with regard to the reduction of transmission. It may be interpreted that the farms were facing serious PRDC problems when there was a higher detection rate of M. hyopneumoniae in the farms where antibiotics were actively used. Farms with severe enzootic pneumonia outbreaks tend to use antibiotics more frequently as a control measure. In addition, the symptoms may reappear after cessation of treatment or development of antimicrobial resistance [2].
As a result of subpopulation in a batch, there was a great deal of variability in the prevalence between farrowing batches within a farm. The mean prevalence at the first sampling time was 13%. In the second sampling time, a mean prevalence of 4.9% was reported. For the third sampling time, the mean prevalence was 2.2%. These results suggest that the time of infection and shedding of M. hyopneumoniae is not consistent in the different farrowing batches. In addition, a minimum of three sampling time points is necessary to assess sow herd stability. The necessity of multiple samplings has already been suggested [29].