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The objective of this study was to determine the occurrence of Mycoplasma hyopneumoniae in the lower respiratory tract of suckling and recently weaned pigs from herds with a history of endemic respiratory disease in this age group. Bronchoalveolar lavage fluid was obtained from 500 clinically affected piglets originating from 50 herds (10 piglets per herd). The presence of M. hyopneumoniae was examined by application of a nested polymerase chain reaction. Mycoplasama hyopneumoniae was detected in lavage fluid in 12.3% of the suckling piglets and 10.6% of the weaned piglets (mean 11.2%). The percentage of herds in which M. hyopneumoniae could not be detected was 72% and in two of the herds all samples were tested positive. Herd level information on relevant hygiene, housing and management factors was obtained by means of a structured questionnaire. The univariable data analysis (odds ratio) showed that the detection of M. hyopneumoniae in young piglets is associated with one- and two-site production and inappropriate gilt acclimatization.

Summary: Objective: Few studies exist that concentrate on the detection of M. hyopneumoniae by PCR in piglets. Most of these studies do not permit the differentiation of latent infections from infections with typical pathomorphological lesions. The aim of the study was to characterize the occurrence of M. hyopneumoniae infections in piglets by combining and evaluating the results of macroscopical, histological, molecularbiological and immunohistochemical examinations. The evaluation of the suitability of the immunohistochemistry (IHC) as an alternative or as a supplement to other tests was a further aim. The study-design does not allow any conclusions regarding the prevalence of early M. hyo - pneumoniae-infections in piglets. Material and methods: 96 clinically affected piglets from eight pre-selected herds with an increased risk for M. hyopneumoniae infection were chosen for necropsy. Macroscopical and histological examinations of the lungs were performed. BALF and lung tissue were taken for the detection of M. hyopneumoniae by nested PCR (nPCR) and IHC. Results: 16 of 33 nPCR-positive piglets were positive for antigen by IHC while 63 nPCR-negative piglets were also negative for antigen by IHC (kappa index = 0.55). Of the 17 piglets with a nPCR-positive but IHC-negative test result, 12 originated from one herd leading to the assumption that not all field strains were detectable with the monoclonal antibody utilized. Fisher’s exact test showed a significant association (p < 0.0001) between detection of M. hyopneumoniae by nPCR in BALF or by IHC in lung tissue and the occurrence of typical pathological lesions. Conclusion: IHC with the monoclonal antibody used in this study is a suitable additional method but no alternative to the direct detection of M. hyopneumoniae by nPCR. Clinical relevance: In the case of the detection of M. hyopneumoniae in BALF by nPCR, typical pathological lesions can be expected with a higher probability than in nPCR-negative piglets.