Selective treatment strategies can combat antimicrobial resistance while protecting foals from R. equi

Rhodococcus equi is a hardy bacterium that lives in the soil and horse feces. It replicates in horse manure, so on densely stocked farms such as breeding operations, the environment can become heavily contaminated. When the bacterium becomes aerosolized—primarily due to environmental factors such as wind, dry conditions, and soil disturbances—foals can inhale it, potentially becoming infected.

Two forms of the bacterium exist: an avirulent form that is essentially benign for foals and another that is a virulent form. The virulent, disease-causing form has a specific genetic element called a plasmid with a gene that codes for a protein called the virulence-associated protein or VapA.

“The VapA protein enables R. equi to replicate inside immune cells in the lungs of foals, alveolar macrophages, leading to abscesses forming in the lung. This disease process is similar to what happens in tuberculosis, where the bacterium known as Mycobacterium tuberculosis is able to replicate in alveolar macrophages of humans to cause pneumonia,” explains Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, of Texas A&M University’s College of Veterinary Medicine & Biomedical Sciences, in College Station. Cohen is professor of equine internal medicine, Glenn Blodgett Chair in Equine Studies, and associate department head for research and graduate studies in the college’s Department of Large Animal Clinical Sciences