Streptococcus equi with truncated M-proteins isolated from outwardly healthy horses

The M-protein genes of Streptococcus equi isolated from 17 outwardly health y horses after 4 strangles outbreaks had ended, including a quarantined ani mal, were compared with those of 5. equi isolates from 167 active cases of strangles across 4 countries. The healthy horses included 16 persistent 5. equi carriers, at least one from each of the four outbreaks. These carriers , despite being outwardly healthy, had empyema of the guttural pouch(es), a n enlargement of the equine Eustachian tube. A persistent carrier from two of these outbreaks, the quarantined animal and a healthy animal with normal guttural pouches, from which 5. equi was isolated only once, were colonize d by variant 5, equi with truncated M-protein genes (24% of outwardly healt hy animals with 5. equi). The truncated M-protein genes had in-frame deleti ons in slightly different positions between the signal sequence and the cen tral repeat region, equivalent to approximately 20% of the mature expressed protein. Immunoblotting with antibody to recombinant M-protein confirmed t hat the variants expressed a truncated form of the M-protein. In contrast t o the outwardly healthy 5. equi carriers, only 1/167 of 5. equi isolates fr om strangles cases possessed a truncated M-protein gene (<1%; Fisher's exac t test, P = 0.0002). Compared with isolates from healthy horses with a trun cated WI-protein, much more of the N terminus of the truncated M-protein wa s retained in the variant 5. equi from a strangles case. Variant 5 equi fro m outwardly healthy animals were more susceptible to phagocytosis by neutro phils in vitro than typical isolates. This is the first report of detection of 5, equi with a truncated M-protein. The distribution of the variants be tween strangles cases and carriers suggests that the 80% of the M-protein r etained in the variants may contribute to colonization whilst the deleted p ortion of the gene may be needed for full virulence.