Equis ISSN 2398-2977

Rhodococcus equi infection

Contributor(s): Sarah Binns, Tim Brazil, Christopher Brown, Kate Hepworth-Warren, Timothy Mair, Carla Sommardahl


  • Severe pyogranulomatous bronchopneumonia, mainly in foals of 1-4 months old, rare in horses older than 6 months of age.
  • Cause:Rhodococcus equi  Rhodococcus equi   (nowPrescottella equi). Predisposing factors: stress, poor hygiene, environmental contamination, concurrent parasitism.
  • Signs: generally non-specific (fever, inappetence); some cases may show coughing or nasal discharge.
  • Diagnosis: bacteriology and PCR on transtracheal aspirate samples; radiology; ultrasonography; hematology.
  • Treatment: combinations of antibiotics based on susceptibility testing; macrolide and rifampicin.
  • Prognosis: fair to good with appropriate treatment; morbidity up to 80% in outbreaks.



  • Rhodococcus equi  Rhodococcus equi  .
  • Foals infected at an early age by inhalation or ingestion ofR. equi.
  • Fairly prolonged incubation period (9 days to 4 weeks experimentally).
  • R. equiis carried passively in the equine intestine and shed in feces.

Predisposing factors

  • Bacterial contamination of environment, especially in dry fecal material.
  • Decline in maternally derived immunity.
  • Immature local respiratory tract immunity in young foals.
  • Stress.
  • Poor hygiene.
  • Parasitism, especially migrating larvae.
  • Immune compromise.
  • Poor ventilation/dusty environment.


  • Virulent strains ofR. equi; presence of VaPA plasmid.
  • Equine herpesvirus   Equine herpesvirus  , may be a trigger for the development of rhodococcal pneumonia.
  • Genetic predisposition has been suggested.


  • Infection likely occurs shortly after birth, clinical signs rarely seen prior to one month of age.
  • Inhalation ofR. equiin dust   →   survives in alveolar macrophages   →    pyogranulomatous inflammation   →    abscesses in lungs and hilar lymph nodes   →    progressive destruction of lung tissue   →    acute or chronic suppurative bronchopneumonia.
  • Stimulates development of acute or chronic pyogranulomatous pneumonia.
  • Hematogenous spread from lungs or gastrointestinal system   →   septic arthritis/osteomyelitis.
  • Immune-complex deposition   →   polysynovitis and uveitis, hypopyon, anemia, thrombocytopenia.
  • Abscesses may form in intestines and mesenteric lymph nodes following ingestion of the organism or hematogenous spread from pulmonary infection.
  • Villous atrophy, intestinal ulceration and peritonitis contribute to diarrhea, weight loss and poor growth.
  • Organism may also localize in joints, bones, spleen, uterus or skin.
  • Rare manifestations:
    • Ulcerative lymphangitis.
    • Cellulitis.
    • Subcutaneous swellings and abscessation.
    • Uveitis.
    • Panophthalmitis.
    • Nephritis.
    • Hepatic and renal abscessation.
  • Vertebral or paravertebral body abscessation.


  • Incubation period: experimental data suggest 9 days to 4 weeks.
  • Foals are generally infected at a very young age.


  • R. equipresent in feed   →   passes through (and may proliferate in) intestinal tract of healthy horses and other herbivores   →   contamination of pasture and bedding with infected fecal material   →   transmitted by inhalation of dust or ingestion.
  • Congenital infection may also occur, via umbilical or mucous membrane exposure.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Giles Cet al(2014)Rhodococcus equi(Prescottella equi) vaccines: the future of vaccine development.Equine Vet J25,47(5), 510-518 PubMed.
  • Venner M, Credner N, Lammer M & Giguere S (2013)Comparison of tulathromycin, azithromycin and azithromycin-rifampin for the treatment of mild pnuemonia associated with Rhodococcus equi.Vet Rec173(16), 397 PubMed.
  • Boyen F, Pasmans F & Haesebrouck F (2011)Acquired antimicrobial resistance in equine Rhodococcus equi isolates.Vet Rec168(4), 101 PubMed
  • Giguere Set al  (2011)Diagnosis, treatment, control and prevention of infections caused by Rhodococcus equi.J Vet Intern Med25(6), 1209-1220 PubMed.
  • Giguere Set al(2011)Rhodococcus equi: Clinical manifestations, virulence and immunity.J Vet Intern Med25(6), 1221-1230 PubMed.
  • Reuss S M, Chaffin M K & Cohen N D (2009)Extrapulmonary disorders associated with Rhodococcus equi infection in foals: 150 cases (1987).JAVMA253(7), 855-863 PubMed.
  • Davis E (2008)Rhodococcus equi in adult horses.Equine Vet Educ20(2), 72-73 Wiley Online Library.
  • Waldridge B M, Morresey P R, Loynachan A T, Reimer J, Riddle W T, Williams N M & Bras R (2008)Rhodococcus equi pneumonia in an adult horse.Equine Vet Educ20(2), 67-71 VetMedResource.
  • Muscatello G, Leadon D, Klay M, Meijer W & Gilkerson J (2007)Rhodococcus equi infection in foals: the science of 'Rattles'.Equine Vet J39(5), 470-478 PubMed.
  • Janicek J Cet al(2006)Intracranial abscess caused by Rhodococcus equi infection in a foal.JAVMA228(2), 251-253 PubMed.
  • Cohen N Det al(2005)Study of serum amyloid A concentrations as a means of achieving early diagnosis of Rhodococcus equi pneumonia.Equine Vet J37(3), 212-216 PubMed.
  • Chaffin M K, Cohen N Det al(2003)Foal-related risk factors associated with development of Rhodococcus equi pneumonia on farms with endemic infection.JAVMA233(12), 1791-1799 PubMed.
  • Chaffin M K, Cohen N D & Martens R J (2003)Evaluation of equine breeding farm management and preventative health practices as risk factors for development of Rhodococcus equi pneumonia in foals.JAVMA222(4), 476-485 PubMed.
  • Chaffin M K, Cohen N D & Martens R J (2003)Evaluation of equine breeding farm characteristics as risk factors for development of Rhodococcus equi pneumonia in foals.JAVMA222(4), 467-475 PubMed.
  • Ainsworth D M (1999)Rhodococcal infection in foals.Equine Vet Educ11(4), 191-198 VetMedResource.
  • Baverud Vet al(1998)Clostridium difficile associated with acute colitis in mares when their foals are treated with erythromycin and rifampicin for Rhodococcus equi infection.Equine Vet J30, 482-488 VetMedResource.
  • Giguere S & Prescott J F (1997)Clinical manifestations, diagnosis, treatment and prevention of Rhodococcus equi infections in foals.Vet Microbiol56(3-4), 313-334 VetMedResource.
  • Paradis M R (1997)Cutaneous and musculoskeletal manifestations of Rhodococcus equi infection in foals.Equine Vet Educ9, 266-270 VetMedResource.
  • Takai S (1997)Epidemiology of Rhodococcus equi infections - a review.Vet Microbiol56(3-4), 167-176 VetMedResource.
  • Clarke A F (1989)Management and housing practices in relation to Rhodococcus equi infection of foals.Equine Vet Educ1(1), 30-32 VetMedResource.
  • Leadon D P (1989)An outbreak involving six cases of Rhodococcus equi on a studfarm.Equine Vet Educ1(1), 23-24 VetMedResource.
  • Keen P M (1989)Pharmacology and treatment of Rhodococcus equi infection.Equine Vet Educ1(1), 28-29 VetMedResource.
  • Hillidge C J (1987)Use of erythromycin-rifampin combination in treatment of Rhodococcus equi pneumonia.Vet Microbiol14, 337-342 PubMed.

Other sources of information

  • Althaus O & Venner M (2004) Ultrasonography of the Lung: A Help for Early Recognition ofRhodococcus equi Pneumonia in Foals. In:Proc 43rd BEVA Congress. Equine Vet J Ltd, Newmarket. pp 201.
  • Chaffin M K (1999)Rhodococcus equiPneumonia in Foals - Clinical Presentations, Treatment and Prevention.TNAVC 1999 Proceedings.pp 79-80.
  • Prescott J F & Giguere S (1998)Rhodococcus equi- The Disease and Prospects for its Control.In:Proc World Equine Airway Symposium.