The severity of pleuritis was gradually scored as 0 (no pleuritis), 1 (> 0–25% infected lung surface), 2 (> 25–50% infected lung surface) or 3 (> 50–100% infected lung surface). The percentage pleuritis within a batch is defined as a weighted measure of the percentage of pigs with lesions and the severity of pleuritis. The results were received from the slaughterhouse as a total score for that batch of pigs delivered on the day of slaughter.