Strangles, diagnosis and carriers

Streptococcus equi subspecies equi (S. equi) results in considerable suffering for the horses for weeks or even months and some horses develop life threatening complications. Despite quarantine efforts, this disease continues to plague the horse population with outbreaks every year. It is increasingly recognized that a key factor in spread of strangles is the occurrence of horses that have recovered from strangles but continue to carry bacteria in their upper airways, i.e. “silent carriers”. Recent research has focused on improving strangles diagnostics, and understanding of disease mechanisms, including spread of strangles (1,2). In chronic carriers, bacteria persist in guttural pouches, and are difficult to detect by cultivation (3,4). PCR is currently used to identify chronic carriers. Bacteria cultured from carriers often differ in colony appearance, which may be related to loss of virulence factors (5). Whether these bacteria are attenuated, or if they can revert to their original, highly virulent phenotype is not known. Our earlier studies showed that persistently PCR positive carriers are intermittently culture positive (4).
We have conducted long term follow up studies of strangles outbreaks with the aim to identify silent carriers of S. equi. Our overarching goal has been to identify tools that enable a reliable certification of horse establishments free of S. equi following an outbreak. Unfortunately, in addition to finding the silent carrier state more common and longer lasting than earlier reported, we found that carriage in the upper airway often occurs in absence of visible abnormalities (3, 4), rather than (6) as linked to empyema or chondroids in the guttural pouch. Equally worrisome, it appears that silent carriers spread S. equi bacteria to uninfected stall mates (4), and despite treatment of silent carriers we observed that the S. equi remained detectable in a large proportion of treated horses. The majority of clinically healthy silent carriers remain PCR positive but culture negative for S. equi months after clinical disease, and their infectious capacity is so far unknown.