Rhodococcus equi

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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