Rhodococcus equi

Introduction

Pyogranulomatous lesions due to Rhodococcus equi (Image sourced from Bristol Biomed Image Archive with permission)

Rhodococcus equi is a gram-positive aerobic, intracellular, saprophyte found worldwide in the soil and in the intestinal tract of animals. It replicates at warm temperatures in soils contaminated by faeces of herbivores and is an opportunistic respiratory pathogen of foals under 6 months causing suppurative bronchoneumonia.

Pastures can become heavily contaminated leading to outbreaks of the disease. The organism can be present in the faeces of adult horses and also healthy foals, without causing clinical disease. Transmission can be via inhalation of dust contaminated with R. equi.  

Clinical Signs

Bronchoneumonia and lung abscessation in foals less than 4 months old. Acute disease in 1 month-old foals with signs such as acute fever, anorexia, bronchopneumonia. In older foals, 2 - 4 months the disease is more insidious with signs such as coughing, dyspnoea, weight loss, exercise intolerance, loud, moist rattles on lung auscultation. There is occasionally diarrhoea seen, due to granulomatous ulcerative enterocolitis, mesenteric lymphadenitis and peritonitis in some foals following ingestion of contaminated sputum.

Foals over 6 months are refractory to pulmonary infection, but superficial abscesses may occur.

Diagnosis

History of the disease on the farm, age of foals and clinical signs are used to make a presumptive diagnosis. Auscultation and radiography of thorax confirms pulmonary disease. Sonography of the lung shows small abscesses close to the pleura in a very early stage, even before clinical signs can be seen. White blood cell cound (WBC) is rising in an early stage, too.

Samples of tracheal aspirates and pus from lesions need to be taken and cultured aerobically on blood and MacConkey agar for 24-48 hours. Characteristic salmon-pink mucoid colonies grow, due to capsule and pigment production. 

PCR from the tracheobronchial mucus is a safe method to confirm diagnosis.

Treatment and Control

Starting the treatment in an early stage is important for the success! Affected farms should be screened regularly. Clinical examination, WBC and sonography of the lung are effective tools to catch the disease early.

Oral rifampin and erythromycin for 4-10 weeks is the treatment of choice for this disease. In Germany the treatment of choice is Tulathromycin (Draxxin by Pfizer), which is given i.m. every week for 4-8 weeks.

Rehydration therapy if diarrhoea is involved may be needed. Bronchodilators and expectorants should be used supportively.

Sonography of the lung and clinical examination are the best control for treatment´s success. Control measures include preventing build-up of bacteria in environment by removing manure from pastures often and rotating foals and mares onto clean pastures regularly. Dusty conditions in paddocks should be avoided. In sickly looking foals, hyperimmune serum from the dam may be given to the foal in the first month of life. 

{{Learning flashcards = Equine Internal Medicine Q&A 17 |full text = Rhodococcus equi infection in horses: an overview. Rajesh Agrawal; Nishi Pande; Rajesh Katoch; Anish Yadav; Ajitpal Singh; Veterinary Practitioner, c/o Dr. A. K. Gahlot, Bikaner, India, Veterinary Practitioner, 2009, 10, 2, pp 187-188, 16 ref.

Review of the epidemiology and ecology of Rhodococcus equi. Muscatello, G.; Lowe, J. M.; Flash, M. L.; McBride, K. L.; Browning, G. F.; Gilkerson, J. R.; Green, E. M. ; American Association of Equine Practitioners (AAEP), Lexington, USA, Proceedings of the 53rd Annual Convention of the American Association of Equine Practitioners, Orlando, Florida, USA, 1-5 December, 2007, 2007, pp 214-217, 27 ref.

References

There are many dissertations from Hannover University (summary in English):

Evaluation of the efficacy of Tulathromycin in the treatment of lung abscesses in foals

Comparison of the detection of Rhodococcus equi by microbiological culture and the polymerase chain reaction in tracheobronchial fluid from foals

Althaus, O.P., (2004) Sonographie der Lunge: Eine Hilfe zur Früherkennung der Rhodococcus equi-Pneumonie beim Fohlen, Diss. Tierärztliche Hochschule Hannover

Brown, C.M, Bertone, J.J. (2002) The 5-Minute Veterinary Consult- Equine, Lippincott, Williams & Wilkins Blackwell

Kerth, R., (2005) Untersuchung der Wirksamkeit von Tulathromycin bei der Behandlung von Lungenabszessen bei Fohlen, Diss. Tierärztliche Hochschule Hannover

Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) Equine Internal Medicine (Third Edition), Saunders

Reed, S.M, Bayly, W.M, Sellon, D.C. (2004) Equine Internal Medicine (Second Edition) Saunders

Robinson, N.E., Sprayberry, K.A. (2009) Current Therapy in Equine Medicine (Sixth Edition) Saunders Elsevier

Rose, R. J. and Hodgson, D. R. (2000) Manual of Equine Practice (Second Edition) Sauders