Quantification of antibiotic usage against Streptococcus suis in weaner pigs in the Netherlands between 2017 and 2021

Streptococcus suis is an important pig pathogen that can cause severe disease in the post-weaning period. As there are no commercial vaccines available in the Netherlands, antibiotic treatment is often necessary to control disease. S. suis is regarded as one of the major causes of antibiotic prescription in weaned pigs, but scientific studies supporting this claim with quantitative data are lacking. The aim of this study was to obtain insight in, and to quantify usage of, antibiotics against S. suis in weaner pigs at sow farms in the Netherlands. Three sources of data were used, including 1) total antibiotic usage in weaner pigs in the Netherlands, as yearly reported by the Netherlands Veterinary Medicines Institute (SDa), and estimates from 2) pig veterinarians and 3) pig farmers on the relative amount of antibiotics prescribed against S. suis, via questionnaires. Information from the SDa on the total amount of antibiotic usage was combined with the estimates of the veterinarians to estimate the antibiotic usage against S. suis in weaner pigs. Our study shows that ∼90 % of the total amount of amoxicillin (AMOX) and ampicillin (AMPI) prescribed in weaner pigs is used for treatment of disease caused by S. suis (S. suis disease). Of all orally prescribed antibiotics against S. suis, AMOX comprises 85 % of the total absolute usage expressed by the mean defined daily dosage per animal farm (3.2 - 5.4 mean DDDA_F). Furthermore, veterinarians reported that at farms with structural S. suis disease problems second choice antibiotics (AMOX, AMPI) are more often prescribed than at farms with incidental S. suis disease. All together, we estimated that S. suis disease accounts for about 1/3rd of the total antibiotic usage in weaner pigs. This study is the first to quantify antibiotic usage against S. suis in weaner pigs and shows that S. suis is a hotspot of antibiotic usage. This underlines the severity and relevance of S. suis disease and the need for alternative control measures.