Long term dynamics of a Streptococcus equi ssp equi outbreak, assessed by qPCR and culture and seM sequencing in silent carriers of strangles.

This study investigates the long-term spread and mutation of Streptococcus equi subspec. equi (S. equi), a bacteria in carrier horses, using culture, qPCR and seM sequencing. The research specifically follows a strangles outbreak in 41 Icelandic horses over 13 months, highlighting the persistence and mutation of S. equi in carrier animals.

Outbreak Monitoring and Sample Collection

• The researchers monitored an outbreak of strangles, a disease caused by the S. equi bacterium, in 41 Icelandic horses.
• Throughout the study, the researchers collected nasal and guttural pouch lavages from the horses on eleven separate occasions over a period of 13 months.
• 10 horses persistently carrying S. equi were selected for the study, from which 115 samples were collected.

Sample Processing and Bacterial Detection

• Quantitative PCR (qPCR) testing revealed 61 of the samples were positive for S. equi.
• A culture test showed that 32 of these S. equi positive samples were also culture-positive, solidifying the presence of the bacterium.

Genetic Analysis of the Bacteria

• The researchers conducted an amplification, or copying, of the seM gene part of the S. equi bacteria. The seM gene encodes the M-protein, a major virulence factor of the bacterium.
• The seM gene sequence was determined successfully in six samples that were solely qPCR positive.
• The outbreak was linked to a S. equi strain of seM type 72.

Evidence of Mutation and Persistence

• Three months after initial sampling, the S. equi bacteria in two horses showed one amino acid change in the seM gene sequence.
• After six months, S. equi with truncated, or shortened, seM genes were found in two horses. This showed evidence that the S. equi strain in these horses was mutating.
• In persisting carriers, the S. equi bacterium not only remained qPCR positive, but was also intermittently culture positive, indicating the persistence of the presence of the bacterium. Variants of S. equi were identified in additional horses.

Significance of Findings

• The study concludes that many horses remain persistently qPCR positive for S. equi after acute strangles outbreaks and are also intermittently culture positive, highlighting the persistent nature of this bacterium in carrier horses.