Equine Internal Medicine Q&A 17

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A 10-week-old foal in good body condition showed signs of acute dyspnoea.

Question	Answer		Article
Describe the radiographic findings illustrated in the image.	Lateral view of the thorax demonstrates nodular lesions in the cranioventral lung fields. These lesions, some of which are cavitary, obscure the cardiac and caudal vena cava silhouette. Tracheobronchial lymphadenopathy is suggested by the elevation of the trachea.		Link to Article
What is the most likely aetiologic agent?	Radiographic lesions of this type seen in a 10-week-old foal are almost pathognomonic for Rhodococcus equi infection.		Link to Article
What is the source of the infection for the foal?	Rhod. equi is a Gram-positive, pleomorphic, facultative, intracellular, obligate aerobic bacteria. Rhod. equi lives in faeces and soil and is resistant to most chemical and environmental conditions. The organism is found in the intestines of many normal mammals including horses. Foals are most often infected by inhalation of the organism from the soil or faeces of the animals, but ingestion or umbilical entry are also possible.		Link to Article
What is the recommended treatment?	Treatment of Rhod. equi has been difficult due to the intracellular characteristics of the organism. The combination of erythromycin and rifampin is excellent in vitro and has decreased the mortality rate associated with Rhod. equi pneumonia. Erythromycin is a non-polar, macrolide antibiotic that penetrates caseous material readily. Several oral forms have been available for use in horses. Erythromycin causes gastrointestinal irritation, which may cause mild diarrhoea in horses. Erythromycin phosphate or stearate is dosed at 37.5mg/kg p/o bid, while erythromycin estolate is used at 25mg/kg p/o tid or qid. Rifampin is synergistic with erythromycin and penetrates macrophages, neutrophils and caseous material readily. Rifampin is dosed at 5mg/kg p/o bid, although some clinicians use 10mg/kg p/o sid.		Link to Article