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