The actiology of acute lameness in pigs, 3-5 months of age in nine Danish herds with high incidences of lameness was investigated. Eighty-seven acutely lame pigs, that exhibited lameness of varying degree in the hind legs, were selected. Non-lame pigs were matched on pen, sex and weight. The lame pigs had soft fluctuating joint swellings (odds ratio (OR), 7.21; 95% confidence interval (CI), 3.40-15.47). No indication of suppurative arthritis was observed. Joint infection with Mycoplasma hysoynoviae was found by culture in 20% (17 of 86) of the lame pigs and in 8% (seven of 83) of the non-lame pigs. Lameness and joint infection with M. hyosynoriae were significantly associated. Other ordinary bacteria were not found in any case. Macroscopic osteochondrotic lesions were observed at slaughter in 47% (37 of 78) of the previously lame pigs and in 35% (55 of 158) of an enlarged group without history of lameness. The cubital joints were most frequently affected and a history of hind leg lameness was not statistically associated with osteochondrotic lesions at slaughter (OR, 1.69; 95% CI, 0.94-3.05), or joint infection with M hyosynoviae at slaughter (OR, 0.88; 95% CI, 0.31-2.40). Arthritis due to M. hyosynoviae infection was the primary, cause of acute and severe lameness in grower-finisher pigs. Moreover, M. hyosynoviae was isolated from joints of several pigs without signs of lameness. This suggests that M. hyosynoviae may be present in joints without provoking clinical illness. The mean daily incidence of treatments due to lameness in the herds was 5.4 per 1000 pigs. joint disease implied 30-90 min extra labour for surveillance and treatment celery day per 1000 pigs, and 5% of the affected individuals were euthanized due to lameness. The average daily weight gains in the selected pigs until slaughter seemed unaffected by the lameness.
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