Monitoring herds for Streptococcus suis type 2 by sampling tonsils of slaughter pigs.

Heads were removed soon after slaughter from the dressed carcases of 155 pigs belonging to 12 herds with a history of streptococcal meningitis and from 180 pigs from four herds believed to be free from this disease. Deep scrapings from both tonsils were sown on two selective media. Streptococcus suis type 2 was detected in a proportion of pigs from the 12 herds with a history of the disease, including three herds in which no cases were noted during the year this study was made. Pigs in six of these herds had received therapeutic levels of certain antibiotics in their feed as growers. The confirmed detectable tonsillar carrier rates varied between these 12 herds from 20 to 90 per cent and between batches of heads from one herd from 0 to 100 per cent. Carrier rates could not be correlated with disease levels, herd size or husbandry system. S suis type 2 was also detected in pigs from two herds thought to be free of the disease, at rates of 20 and 1.5 per cent.