Section 2 | Respiratory Diseases in Horses: Rhodococcus equi

Ontario Equine
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How Does This Disease Spread?

Feces and the Environment

Fecal excretion by horses is an extremely common mode of transmission by which Rhodococcus equi is spread (within breeding farms in particular). Rhodococcus equi can live and replicate in soil, especially when the soil is contaminated with horse feces. It can easily be ingested by grazing herbivores, survive in their intestinal tract, and be excreted in manure to contaminate the soil.

Mares on affected breeding farms often shed (excrete) a small amount of Rhodococcus equi in their feces. However, affected foals can shed a significant amount of the bacteria (particularly those with pneumonia due to Rhodococcus equi)1.


Airborne Spread

The major route of infection for Rhodococcus equi in cases of pneumonia is through contaminated dust1. When the weather becomes warmer, drier, and windier, Rhodococcus equi becomes airborne and inhaled by susceptible foals.


Specific environmental factors that contribute to high levels of airborne bacteria include:

  • Low soil moisture content
  • Poor grass cover
  • High ambient temperature


Affected foals have also been shown to exhale a high concentration of Rhodococcus equi suggesting that airborne transmission may also serve as a source of disease spread on the farm2.


  1. Muscatello, G. 2012. Rhodococcus equi pneumonia in the foal – Part 1: Pathogenesis and epidemiology. Vet J. 192:20-26.
  2. Muscatello, G., J.R. Gilkerson, and G.F. Browning. 2009. Detection of virulent Rhodococcus equi in exhaled air samples from naturally infected foals. J Clin Microbiol. 47:734-737.