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