Rhodococcus equi infections

Previous authors: J F PRESCOTT AND S GIGUERE

Current authors:
L HUBER - PhD Student, DVM, MS, Veterinary Medical Center, University of Georgia, 2200 College Station Road, Athens, Georgia, GA 30602, USA
S GIGUÈRE (Deceased) - Professor and Hodgson Research Chair in Equine Studies, DVM, PhD, Diplomate ACVIM, 2200 College Station Road Athens, Georgia, 30505, USA

GENERAL INTRODUCTION: ACTINOMYCETES

Introduction

Rhodococcus equi is one of the most important causes of disease in foals between one and six months of age with most foals showing clinical signs between five and 12 weeks of age. Infection is characterized by a subacute to chronic abscessating bronchopneumonia, sometimes accompanied by ulcerative typhlocolitis, but other manifestations in foals include polyarthritis, osteomyelitis, mesenteric lymphadenitis, uveitis, and ulcerative lymphangitis. Subclinical disease has been reported where foals have pulmonary abscesses identified by thoracic ultrasonography. Infection in adult horses is rare and may follow immunosuppression. In cattle and pigs, tuberculosis-like lesions caused by R. equi may occur in the submandibular and other lymph nodes, whereas in goats the organism may cause granulomatous lesions and systemic infections. Infection in other species is rare and sometimes associated with immunosuppression.

Rhodococcus equi has emerged as an important cause of pneumonia or systemic infections in humans infected with HIV or undergoing immunosuppressive therapy.

Rhodococcus equi has a world-wide distribution, but is most common on large farms in regions with long, hot summers.

Aetiology

Rhodococcus equi shares the characteristics of other actinomycete members of the Mycolata (e.g. Corynebacterium, Mycobacterium, Nocardia) including the presence of lipid-rich cell envelope components dominated by the presence of mycolic acids.¹⁴¹ It is a Gram-positive, obligately aerobic coccus to coccobacillus. The organism grows well on nonselective media, achieving its characteristic flowing, mucoid colonies after 48 hours of culture at 37°C. Characteristic colonial variants occur. The organism is usually recognized in clinical bacteriology laboratories by its microscopic and colonial appearance, its strong urease activity, and its production of synergistic haemolysis (‘CAMP reaction’) with Corynebacterium pseudotuberculosis or Staphylococcus aureus. Salmon-pink or darker red colonies may develop after a week or longer of incubation, or during storage, but are often difficult to recognize in younger colonies. Semi-selective media have been developed for isolation of the organism from soil, air, or faeces.^{98, 173} These semi-selective media might also be useful for isolation of R. equi from biological samples heavily contaminated with other bacteria. Polymerase chain reaction (PCR) assays for rapid identification of R. equi have been described.^{8, 85}

Epidemiology

Although R. equi is likely present in the environment of all horse farms, the clinical disease is endemic and devastating at some farms, sporadic at others, and unrecognized at many. Differences in the prevalence of the disease might reflect variation in environmental (temperature, dust) and managemental conditions, as well as differences in the number of virulent isolates in the environment.¹⁵⁴ Rhodococcus equi is a soil saprophyte with simple growth requirements. The bacterium is found in greater numbers where horses are present since the volatile fatty acids in their manure enhance its growth.⁷⁰ Rhodococcus equi pneumonia does not appear to be associated with lack of attention to routine preventive health practices.²⁶ In contrast, breeding farms with a large acreage, a large number of mares and foals, a high foal density, and a large population of transient mares and foals have greater odds of being affected by R. equi pneumonia.^{25, 35}

The highest numbers of R. equi are found in surface soil whereas the organism cannot be found at depths exceeding 30 cm.¹⁴⁶ The bacterium can be isolated from the soil of virtually all horse breeding farms at numbers of 10² to 10⁵ CFU per gram of soil.¹⁴⁵ In the same study, the proportion of virulent plasmid encoded virulence-associated protein A (containing pVAPA) R. equi represented 1.7 to 23.3 per cent of all isolates.¹⁴⁵ However, the presence or the concentration of virulent R. equi in soil are not positively associated with increased cumulative incidence of R. equi pneumonia at breeding farms.^{32, 105, 109} Rhodococcus equi can be cultured also from the soil of areas not inhabited by horses. In one study, it was isolated from approximately 74 per cent of soil samples collected from 115 parks and 49 household yards. The number of R. equi in those samples ranged from 10 to 10⁵ colony forming units (CFU) per gram of soil.¹⁴⁸ None of the 1,294 isolates recovered from those samples contained pVAPA or pVAPB.¹⁴⁸ In contrast to soil concentration, airborne concentration of virulent R. equi is associated with disease incidence at breeding farms.^{33, 34, 84, 109} The fact that exposure to R. equi preceded development of pneumonia suggests that the higher concentration of airborne virulent R. equi is likely the cause of pneumonia rather than an effect.³³

Currently, there is no compelling evidence that R. equi is contagious among foals and that the affected foals should be isolated from other foals. In one study, air samples from the breathing zone of...