The Epidemiology of Streptococcus equi subspecies equi in New Zealand in Relation to Vaccine Efficacy

Strangles is caused by host restricted pathogen Streptococcus equi subspecies equi (S. equi) and is the most frequently diagnosed equine disease. This highly contiguous disease accounts for approximate 30% of recorded incidents of equine disease annually (Harrington, Sutcliffe, & Chanter, 2002). Strangles is characterized by abscessation of the lymph nodes of the head and neck. The severity of this disease varies largely depending on the immune status of the affected animal (Sweeney et al., 2005). Diagnosis of strangles can be complicated by the presence of other beta haemolytic streptococci, especially the closely related S. zooepidemicus. Moreover, the current method used to detect S. equi in the carriage state – via samples from the guttural pouches of apparently healthy horses after recovery from strangles is not always practical. Currently there are two vaccines for strangles available in NZ. One is a nonencapsulated and attenuated strain of S. equi, Pinnacle ® IN (Ford Dodge, USA), which is administered intranasally. Another is an inactivated bacterin, Equivac ® S (Pfizer, NZ), used intramuscularly. However, the efficacy of these two vaccines to the three NZ S. equi strains is not clear. Also the level of induced antibodies with the sera of inoculated animals is unkown. In this study, an enzyme-linked immuno-sorbent assay (ELISA) and an indirect fluorescent-antibody assay (IFA) were developed to detect specific antibodies in sera from horses either infected with S. equi, or vaccinated with Pinnacle or Equivac S, or unvaccinated. Four peptides synthesized by GenScript (GenScript, USA) were used for the ELISA study. Of these, three peptides targeted the Nterminal variable regions of S. equi strains 99, 100 and the vaccine strain. The other was a PEPK repeats peptide, corresponding to a region present in the Nterminal region of the S. equi protein SzPSe. Both ELISA and IFA results showed that relatively high levels of antibodies were induced following vaccination or infection. Also, the induced antibodies demonstrated cross-reactivity to all three S. equi strains tested. ELISA for the PEPK peptide produced higher ODs than seen with the three SeM peptides, suggesting that there were more antibodies against PEPK peptide within the sera. The IFA results showed a difference in titre ii between pre-vaccinated horses and post-vaccinated horses; whilst the ELISA results did not. This may be due to serological reactivity between those four peptides and ‘natural antibodies’ induced by exposure to the closely related S. zooepidemicus. Further evidence in support of this result was seen in western blotting, which showed that ‘natural antibodies’ in serum from pre-vaccinated horses reacted serologically with proteins from S. equi. IFA was shown to be the better method for the diagnosis of strangles and for the study of the antibody responses after vaccination or infection. ELISA and IFA results also showed that the observed antibody levels induced by Pinnacle and Equivac S were similar, indicating the comparable efficacy of these two vaccines. All of the vaccinated horses had high levels of pre-existing antibodies within their sera, even one year after vaccination. This may suggest that an increase in the interval of booster time could be acceptable. However, it is acknowledged that antibody titres do not necessarily equal protection from infection. Western blotting results indicated that a stronger immunity was formed after natural infection than seen in horses after. This is because after infection the entire immune system (both humoral and cell-mediated) will be stimulated.