Occupational exposure and human carriage of Streptococcus equi subsp. zooepidemicus resulting in multiple livestock outbreaks

Background: Streptococcus equi susbsp. Zooepidemicus is a cause of septcaemia and an occupational hazard. It was previously thought to only infect humans, and no evidence is available that humans can become long-term carriers of this pathogen. Methods: Over 3 years two different, both naïve, independent swine farms experienced outbreaks of S. zooepidemicus. Site A was depopulated three times, and had three outbreaks. Site B only had one outbreak. Isolates were genetically profiled through whole genome sequencing. Potential carriers and environmental load were tested by a strain-specific real time PCR. Findings: Environmental samples and non-human carriers tested negative throughout outbreaks. Isolates recovered from pigs in all outbreaks from site A and site B had >99.9 % average nucleotide identity. Phylogenetic analyses suggested that all isolates are related. Patient 1, who transited between site A and B immediately before site B experienced their first swine case, was present in site A after removal off all pigs and before each outbreak. They also shed S. zooepidemicus on their mask and were positive by real time PCR from nasopharyngeal swabs. Interpretation: A human carrier of S. zooepidemicus was likely colonized during the first outbreak in pigs on site A. They shed the agent which resulted in multiple outbreaks in Site A, and introduction of the pathogen to Site B. This is the first recorded case of amphixenosis due to S. zooepidemicus, evidencing that humans can become colonized and spread the agent to animals. Funding: Natural Sciences and Engineering Research Council, Canada. Saskatchewan Agriculture Ministry, Canada. Results Driven Agriculture Research, Canada.