It’s not much fun being a broiler chicken. Not only will you eventually end up on someone’s plate but you get tarnished with the same brush as all of bird-kind and blamed as a harbinger of an infectious microorganism (bird flu) that could cause the next great flu pandemic. However, new research shows that the spread of pandemic pathogens also jumps from humans to poultry, according to a study by Lowder and colleagues published online in the early edition of the journal PNAS.
Staphylococcus aureus is a bacteria commonly found on our skin (it is perhaps more notoriously known for MRSA, the strain of S. aureus which is resistant to the antibiotic methicillin and is a serious nosocomial (acquired in hospital) infection). S. aureus infection is also a major cause of infectious disease in poultry. It causes skeletal infections (or bacterial chondronecrosis osteomyelitis, BCO) that leads to lameness in chickens. BCO emerged in the 70’s and now has a huge economic impact on the global broiler chicken industry (a multibillion dollar industry controlled by a small number of multinational companies who supply a limited number of breeds to the global market).
Scientists from the University of Edinburgh and Robert Koch Institut (Germany) examined the population genetics of S. aureus isolates from the past 54 years, which were taken from healthy and diseased chickens in 8 countries across 4 continents and also different species of reared game and wild birds. The scientists used a type of sequencing called MLST which arranges the isolates into groups with similar sequence identity called clonal complexes. They found that most of the isolates were from a single sequence type, ST5, which is commonly associated with humans and MRSA strains. Further analysis traced the evolution of the poultry ST5 group; the isolates had descended from a single S. aureus that had jumped from a human host to infect poultry ~ 38yrs ago in Poland. The poultry ST5 group has rapidly spread across the world, probably due to distribution by the global poultry industry. However, the related human ST5 group is present in distinct geographical clusters and is rarely spread intercontinentally. Furthermore, the poultry ST5 group evolved to be successful poultry pathogens by acquiring novel mobile genetic elements from other poultry S. aureus strains and losing genes which are important for human disease. The poultry ST5 group are successful at causing disease because they are more resistant to killing by chicken heterophils (an immune cell found in chickens, equivalent to human neutrophils). This makes it harder for the chicken to fight the infection since heterophils are important for the innate immune response which protects against S. aureus infection.
This data highlights the huge influence human activity has on the emergence of animal pathogens and demonstrates just how globalisation helps spread infectious microorganisms across the world. Additionally, it suggests that farm livestock should be regularly screened to identify newly emerging bacterial pathogens.