Application of quantitative PCR for Streptococcus suis serotypes 2 and 9 on tonsil and saliva samples in pigs under field conditions

Abstract

IntroductionStreptococcus suis infections are important indications for antimicrobial treatment in pigs. To control S. suis infections, it will be helpful to develop a practical test to assign a S. suis-status to farms. In this research we addressed the following questions:• Can qPCR be used for detection of S. suis serotype 9 infection in life pigs in the field?• What is the sensitivity of a convenient sampling method (saliva-swabs) compared to tonsil-brushing under field conditions?• What is the effect of enrichment in the culture procedure prior to qPCR on sensitivity?Material & MethodsOn two farms 60 piglets (Farm A: 15x6 weeks, 45x9 weeks; Farm B: 60x9 weeks old) were sampled. Farm B reported episodes of clinical S. suis infections, Farm A did not. Per piglet 4 samples were taken:• Tonsil-brush.• Tonsil-brush, enriched in Todd-Hewitt-medium (eTH).• Saliva-swab.• Saliva-swab, eTH.All samples were tested by a previously validated qPCR for serotypes 2 and 9.ResultsWhereas the estimated serotype 2 prevalence was 0% in both farms, serotype 9 was found in both; in Farm A 98% (95%CI:95-100%) and in Farm B 32% (95%CI:20-43%) of the animals tested positive. Serotype 9 positive test results were observed in tonsil-samples of 74 and in saliva-samples of 63 piglets. Tonsil- and saliva-samples test results showed substantial correlation, especially for enriched samples (Kappa=0.71). Enrichment resulted in a 5-fold (saliva) or 10-fold (tonsil) increase in number of positives.Discussion & ConclusionAlthough Farm A never reported clinical signs due to S. suis serotype 9, the estimated prevalence was significantly higher than on Farm B. Current ongoing research suggests that this may be explained by differences in pathogenicity between serotype 9 strains. Saliva-swabs (eTH) could be an effective sampling method for assessing S. suis serotype 9 farm status.