Further studies on the subclinical carrier state of Streptococcus suis type 2 in pigs

Tonsil swabs were taken from 626 pigs of all ages in four infected herds. Streptococcus suis type 2 was isolated from 106 pigs aged between three weeks and 18 months. Detectable carrier rates in different groups of pigs varied between 0 and 80 per cent and were highest in weaned pigs aged between four and 10 weeks. Deep scrape cultures carried out post mortem suggested that the actual carrier rate was higher than that detected by tonsil swabs before death. Detection was improved by using two selective media. S suis type 2 persisted in the tonsils in the presence of circulating opsonic and binding antibodies and in pigs receiving penicillin-medicated feed. It was detected in the tonsils of individuals for up to 521 days. It was rarely detected in the nose. Some carriers appeared to eliminate the infection. Weaned detectable tonsil carriers transmitted the infection to previously uninfected pigs with which they were mixed. Titres of serum binding antibodies assessed by a micro enzyme-linked immunosorbent assay were relatively low and results from infected and non-infected pigs were not always distinguishable.