Rhodococcus equi

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• Causes peritonitis
• Pneumonia in foals

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 bronchopneumonia

Characteristics

• Grows on non-enriched media
• Salmon-pink mucoid colonies due to capsule and pigment production
• No haemolysis on blood 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
• Virulence associated with specific surface antigens encoded by a large plasmid
• Capsular polysaccharide and mycolic acids in cell wall prevent phagocytosis

Clinical infections

• Bronchopneumonia and lung abscessation in foals less than 4 months
• 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 and mesenteric lymphadenitis 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