Short review: Geographical distribution of equine-associated pVAPA plasmids in Rhodococcus equi in the world.

Overview

• This research article reviews the geographical distribution and molecular epidemiology of virulence plasmids, specifically pVAPA, found in pathogenic Rhodococcus equi strains affecting horses worldwide.
• It focuses on the diversity of pVAPA plasmid subtypes, their geographical preferences, and how the movement of horses influences their spread.

Introduction to Rhodococcus equi and Virulence Plasmids

• Rhodococcus equi (R. equi) is a bacterium that causes infection primarily in foals and is a concern in equine health.
• Virulence in R. equi is largely driven by the presence of the virulence-associated protein A (VapA), a 15–17 kDa protein encoded by the vapA gene on a large virulence plasmid called pVAPA.
• The typical size of pVAPA plasmids is approximately 85-90 kb, and strains harboring this plasmid type are pathogenic for horses.
• Beyond pVAPA, two other host-associated virulence plasmid types in R. equi have been identified:
  • pVAPB: a circular plasmid linked to porcine (pig) isolates, discovered in 1995.
  • pVAPN: a more recently detected linear plasmid associated with bovine (cattle) and caprine (goat) isolates.

Molecular Epidemiology of pVAPA

• Over the last three decades, significant molecular epidemiological work has been conducted worldwide, especially in horse-breeding farms in Japan and other countries.
• These studies focus on virulent R. equi isolates from foals, analyzing plasmid DNA patterns using restriction enzyme digestion.
• Restriction enzyme digestion profiles reveal genetic variability leading to identification of at least 14 distinct pVAPA subtypes.
• Such plasmid subtyping provides important clues about the diversity of infectious strains and their epidemiological relationships.

Geographical Distribution and Host Movements

• Distinct pVAPA subtypes show geographical preferences, i.e., some subtypes are more prevalent in certain regions of the world.
• The distribution of pVAPA subtypes is influenced by horse movement patterns, including trade, breeding, and transportation across countries and continents.
• Understanding these distributions helps in tracking sources of infection as well as evaluating risks of spread between farms and regions.
• The article summarizes previous studies that map these distributions globally, illustrating how equine-associated strains and their virulence plasmids effectively travel with horses.

Significance of the Review

• It consolidates knowledge on the diversity and global spread of equine-specific virulence plasmids in R. equi.
• Highlights the importance of molecular typing of plasmids in understanding disease epidemiology and implementing biosecurity measures on horse farms.
• Provides a perspective on how international horse movement contributes to the dissemination of virulent R. equi strains, informing veterinary surveillance and control strategies.