Porcine lungs were macroscopically and microscopically examined at slaughter, with special regard to different stages of lesions similar to those caused by Mycoplasma hyopneumoniae.There was good conformity between the macroscopical and microscopical findings.In an extended abattoir survey, lesions were found in 4210 out of 4508 lungs examined. The majority of lungs with pleuritic lesions (274 out of 369) revealed by the extended examination were registered by the official procedure. No correlation between pleuritis and time for seroconversion, or with the levels of antibodies to Actinobacilluspleuropneumoniae, was found.Among lungs affected with pneumonic lesions (n = 3841), lesions similar to those caused by Mycoplasma hyopneumoniae were predominant (n = 3769). Only 15 ' YO of these lesions were revealed by official registration at slaughter. This figure is explained by the fact that only 35 'Yo of the infections were still active at the time of slaughter and that only ongoing lesions exceeding a certain magnitude were recorded according to the official regulations. By following the development of antibodies to Mycoplasma hyopneumoniae through the fattening period, the duration of the active infection was estimated to be approximately 12 weeks. Consequently, infections with Mycoplasma hyopneumoniae gained during the early fattening period will, in general, escape detection at slaughter.

The present investigation comprised six integrated herds (100-150 sows each) and one specialized fattening herd. Serologic tests by ELISA were performed on blood samples from piglets and growing pigs during various rearing phases in order to determine the presence of Actinobacillus infection and to estimate the age of the animals at onset of the disease. Using this information, herdspecific vaccination programs were designed against Actinobadlus pleuropneumoniae serotype 2. The weaned piglets in the integrated herds were vaccinated twice before being moved to the fattening unit. In the specialized fattening herd, the pigs were vaccinated on arrival at the growing unit and just before transfer to the finishing unit. The frequencies of chronic pleuritis at slaughter demonstrated that a vaccination program adapted to the herd-specific problem, in combination with environmental improvements, reduces the prevalence of chronic pleuritis. O n the other hand, only a moderate reduction of the prevalence of chronic pleuritis occurred as a result of vaccination if no environmental improvements were made. In two of the herds during the vaccination period, the number of feeding days and food consumption decreased, while growth per day increased. There was also an apparent correlation between higher serum titres and a decreased prevalence of pleuritis at slaughter in three of the herds.

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Summary The present investigation comprised 5 specialized herds of slaughter pigs, each with a production of 1,500 to 4,000 fatteners per year. High prevalence of pleuritis was demonstrated in all herds at slaughter. Serological investigations by means of ELISA were carried out to determine antibody titres to Actinobacillus pleuropneumoniae (A. pl.) serotype 2 during the fattening period and at slaughter. The investigations showed that this agent was a predominant cause of the subclinical pleuritis although A. pl. was difficult to isolate. An environment study was performed in all herds and major environmental deficiencies were corrected during the course of the investigation. Similarly, all herds changed to a strict routine of batch rearing. In 4 herds out of 5, the animals were vaccinated on arrival to the growing‐finishing units and again generally 3–4 weeks later with an inactivated vaccine against A. pl. serotype 2. The prevalence of pleuritis decreased with 25–75% during the three‐year observation period. Also the prevalence of pneumonia decreased at the same time. Conversely, daily weight gain and feed conversion improved. The prevalence of pleuritis and pneumonia registered at slaughter was lower in the three‐breed crosses (including Hampshire, Landrace and Yorkshire) than in the two‐breed crosses with Landrace and Yorkshire.

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