Rhodococcus equi

Overview

• Gram-positive aerobic saprophyte found worldwide
• Found in soil and intestinal tract of animals
• Replicates at warm temperatures in soils contaminated by faeces of herbivores
• Opportunistic respiratory pathogen of foals under 6 months causing suppurative bronchoneumonia

Characteristics

• Grows on non-enriched media
• Salmon-pink mucoid colonies due to capsule and pigment production
• No haemolysis on blood agar
• No growth on MacConkey agar
• Aerobic, non-motile
• CAMP test-positive
• Catalase positive, oxidase negative, weakly acid-fast
• Found as cocci and rods
• Intracellular pathogen

Pathogenesis and pathogenicity

• Organism present in faeces or healthy foals as well as adult horses
• Pastures can become heavily contaminated, leading to outbreaks
• Transmission via inhalation of dust contaminated with R. equi
• Organisms survive inside cells
• Virulence associated with specific surface antigens encoded by a large plasmid
• Capsular polysaccharide and mycolic acids in cell wall prevent phagocytosis
• Cell mediated response required to clear infection

Clinical infections

• Bronchoneumonia and lung abscessation in foals less than 4 months
• Infection associated with under-developed cell-mediated immunity
• Acute disease in 1 month-old foals: acute fever, anorexia, bronchopneumonia
• Insidious disease in 2-4 month-old foals with coughing, dyspnoea, weight loss, exercise intolerance, loud, moist rales on lung auscultation
• Occasionally diarrhoea
• Granulomatous ulcerative enterocolitis, mesenteric lymphadenitis and peritonitis in some foals following ingestion of contaminated sputum
• Foals over 6 months refractory to pulmonary infection
• Superficial abscesses in horses over 6 months
• Occasional infections in pigs and cattle, causing cervical lymphadenopathy
• Subcutaneous abscesses and mediastinal granulomas in cats
• Pneumonia in HIV patients

Diagnosis

• History of disease on the farm, age of foals affected and clinical signs
• Auscultation and rediography of thorax confirms pulmonary disease
• Specimens: tracheal aspirates and pus from lesions
• Culture aerobically on blood and MacConkey agar for 24-48 hours
• Colony characteristics and biochemical profile
• Quantitative faecal culture on selective media

Treatment/control

• Oral rifampin and erythromycin for 4-10 weeks
• Rehydration therapy; bronchodilators; expectorants
• Prevent build-up of bacteria in environment by removing manure from pastures regularly and rotating foals and mares onto clean pastures regularly
• Dusty conditions in paddocks should be avoided
• Hyperimmune serum from the dam may be given to the foal in the first month of life

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

• Causative agent Rhodococcus (Corynebacterium) equi
• Important cause of sever, often fatal granulomatous pneumonia in foals
• Clinical signs include depression, cough, weight loss, respiratory distress, diarrhoea, arthritis, subcutaneous abscesses
• Bacterium survives phagocytosis and multiplies
• Bacterial toxins -> caseous necrosis in lungs -> attracts inflammatory cells -> pyogranulomatous pneumonia
• Grossly:
  • Multiple firm nodules, usually no encapsulation
  • Partial atelectasis
• Histologically:
  • Pyogranulomatous lesions
  • Macrophages with ingested microorganisms in the alveoli
  • Necrosis spreading through parenchyma

• pathogen of the equine lung (and intestine)
• cutaneous abscesses, cellulitis in young horses
• CELLULITIS = deep suppurative infection often dissecting through tissue planes