Mycoplasma hyorhinis is a member of the Mycoplasmatales family. This bacterium is often found as a commensal in the respiratory tract of pigs. It is thought to facilitate and exacerbate the development of diseases such as porcine enzootic pneumonia and porcine reproductive and respiratory syndrome (PRRS). Rarely, it may cause mycoplasma arthritis, mycoplasmal polyserositis or mycoplasma septicaemia in piglets without the involvement of other bacteria. This presents as polyarthritis or polyserositis.