Equine respiratory disease: a causal role for Streptococcus zooepidemicus.

This research article investigates the role of Streptococcus zooepidemicus bacteria in respiratory diseases affecting a large proportion of young horses worldwide. Using Multilocus Sequence Typing to identify S. zooepidemicus strains, the study examines the link between specific strains of the bacteria (ST-307 and ST-24) and respiratory disease outbreaks in horses.

Introduction to Equine Respiratory Disease and Streptococcus Zooepidemicus

• Respiratory disease is a significant issue in the equine industry, as it affects a high number of young horses worldwide and tends to recur in individual animals, causing disruptions in the industry.
• A large-scale study over three years found associations between respiratory diseases in UK racehorses and several pathogens, including Streptococcus equi subspecies zooepidemicus.
• However, the role of S. zooepidemicus in these respiratory disease outbreaks has been challenging to pin down due to the diversity of the bacteria group and various strains’ ability to establish persistent infections in horses.

Using Advanced Typing Methodologies

• Multilocus sequence typing (MLST) is a technique that differentiates strains based on the DNA sequence, permitting a finer resolution in identifying various S. zooepidemicus strains.
• Previous research in 2013 used MLST to identify the cause of a respiratory disease outbreak in Sweden, linking it to a ST-24 strain of S. zooepidemicus.

Historical Outbreak and the ST-307 Strain

• This research aims to investigate a historic respiratory disease outbreak in New Caledonia’s horses from 1997 to 1998.
• During this outbreak, attempts to find a viral causal agent proved unsuccessful, as did efforts to implicate common viruses as the cause.
• Subsequently, the researchers found substantial evidence pointing to an ST-307 strain of S. zooepidemicus as the causal agent behind the outbreak.

Clonal Relationships and Host Species

• The clonal nature of the S. zooepidemicus isolated from disease outbreaks in dogs, humans, and horses suggests variations in the capacity of different strains to cause diseases in these host species.
• The study also suggests that horses can harbour specific S. zooepidemicus strains even after developing immunity to them, thereby pointing to sequential infection by different pathogenic strains as a possible cause for recurring respiratory diseases in horses.