Varying effects of infections with Mycoplasma hyopneumoniae on the weight gain recorded in three different multisource fattening pig herds

Pigs in three specialized fattening herds were studied with respect to the effect of infection with Mycoplasma hyopneumoniae on weight gain. Individua l pigs were weighed four times at 4-week intervals during the fattening per iod and their daily weight gain over the rearing period was calculated. A b lood sample was collected on each weighing occasion and analysed for the pr esence of antibodies to M. hyopneumoniae. The lungs of the principals were inspected at slaughter and the extent of pneumonic lesions was registered b y a specially developed technique that has been proven to warrant a high de gree of repeatability. No serum antibodies to M. hyopneumoniae were detecte d in one of the herds, and no pneumonic lesions were recorded at slaughter in that herd. In the other two herds, the prevalence of pigs with serum ant ibodies to M. hyopneumoniae increased from 6 to 54% and from 31 to 81%, res pectively, during the fattening period. The prevalence of pneumonic lesions at slaughter in these herds was higher the later the pigs seroconverted. O n the other hand, the extension of the lung lesions tended to be higher amo ng pigs that seroconverted early during the rearing period. Infections with M. hyopneumoniae acquired early during the rearing, presumably strengthene d by secondary infections and environmental errors, was found to decrease t he daily weight gain of the pigs. However, even non-complicated M. hyopneum oniae infections acquired late in the fattening period were associated with reduced daily weight gain. That growth reduction was estimated to be at le ast 60 g (about 6%) after adjusting for herd, pen, initial weight and sex.