Evidence of lateral gene transfer among strains of Streptococcus zooepidemicus in weanling horses with respiratory disease.

This research article looks at the evidence of lateral gene transfer among different strains of Streptococcus zooepidemicus (Sz) in young horses experiencing respiratory disease. The main finding of the study is that Sz strains may exchange genes, evolving into newer forms that can invade the lower respiratory tract, leading to outbreaks of respiratory diseases in horses and dogs.

Streptococcus zooepidemicus Disposition

• Sz is typically found living harmlessly in the tonsils of healthy horses, but can also opportunistically move to the lower respiratory tract.
• The bacteria vary genetically and show recombination across various gene sequences including szp, szm, and MLST data.
• Although many serovars (serological variants) of the protective SzP can live in a horse’s tonsils simultaneously, infections in the lower respiratory tract usually come from a single clone.
• Particularly, specific clones causing respiratory disease epidemics have been recently reported in horses and dogs across North America, Europe, and Asia.

Study Overview and Findings

• This research provides evidence of lateral gene exchange and recombination among Sz strains, derived from respiratory disease cases resulting from experimental equine herpes 1 virus infection in a group of young horses and ponies.
• Out of the 18 weanlings with respiratory disease, 13 yielded mucoid colonies of Sz upon nasal swab culture.
• Upon comparing seven genes (arcC, nrdE, proS, spi, tdk, tpi, yqiL) of the isolated Sz, three different clades were identified.

Clade Composition and Variation

• Clade-1 (ST-212) and Clade-2 (ST-24) comprised of seven and three isolates, respectively.
• ST-24 and ST-212 differed in all seven housekeeping genes as well as szp and szm alleles.
• Two isolates of Clade-1 were assigned to ST-308, a single locus variant of ST-212 containing the proS-16 allele sequenced in ST-24, suggesting gene transfer.
• One ST-308 isolate contained szm-2, the same allele found in Clade-2 isolates, whereas another was positive for the Clade-2 szp-N2HV2 allele.

Implications

• The findings suggest the possibility of gene transfer occurring among Sz strains within the host itself may lead to the formation of invasive novel clones.
• This adaptive evolution may explain the cause of epizootics or disease outbreaks of respiratory issues among horses and dogs.