Immunogenicity of phospholipase A₂ toxins and their role in Streptococcus equi pathogenicity

Streptococcus equi subsp. equi (S. equi) is the causative agent of strangles, one of the most frequently diagnosed infectious diseases of horses worldwide. Phospholipase A₂ toxins (PLA₂) cleave phospholipid molecules at position sn-2 contributing to the production of leukotrienes that are important inflammatory mediators. Two homologous phospholipases, SlaA and SlaB are encoded by the S. equi genome suggesting that PLA₂ toxins may contribute to its pathogenicity. Here we report the immunogenicity and role of PLA₂ toxins during natural and experimental infection of horses with S. equi. The levels of anti-PLA₂ specific antibodies in serum from horses naturally exposed to S. equi or without exposure were measured by indirect ELISA. Furthermore, the importance of PLA₂ was determined during experimental infection of Welsh Mountain ponies with a mutant strain of S. equi lacking slaA and slaB. Our results show that PLA₂ toxins are immunogenic, which supports their production during natural S. equi infection, but that these toxins are not essential for the development of strangles in a susceptible natural host.