Subcutaneous abscessation: An atypical presentation of Rhodococcus equi infection in a group of foals

BACKGROUND: Integumentary manifestations of Rhodococcus equi infection in foals are infrequently reported in the literature. Subcutaneous abscessation as the primary clinical presentation in temporally clustered cases of foal rhodococcosis in an equine farm has not, to the authors' knowledge, been previously described. OBJECTIVES: To describe an outbreak of R. equi infection where foals presented with subcutaneous abscessation as the primary clinical sign. STUDY DESIGN: Clinical case series. CLINICAL SUMMARY: Three Thoroughbred foals aged 6-11 weeks developed subcutaneous abscesses over a 3-week period in July 2024, out of a cohort of 11 foals. Thoracic ultrasound examination was performed on all foals on the farm, and 6/11 (54%) had evidence of subclinical pneumonia. No foals other than the three cases with subcutaneous abscesses were clinically affected. Purulent aspirates from abscesses in two foals were subjected to microbiological diagnosis and yielded a pure culture of R. equi. Two of the three clinically affected foals received 4 weeks of systemic antimicrobial treatment, and all abscesses resolved. Whole-genome sequencing of one isolate demonstrated near identity to a previous UK isolate associated with pulmonary rhodococcosis, and did not reveal genetic features that could explain the atypical presentation. MAIN LIMITATIONS: Financial constraints on the farm precluded microbiological confirmation of diagnosis in the third foal. Sequencing of a single isolate precluded assessment of strain relatedness within the outbreak. CONCLUSIONS: R. equi infection should be considered in foals presenting with subcutaneous abscesses, even in the absence of overt respiratory signs. Exposure-related factors likely contributed to this presentation, and the possibility of a transcutaneous route of entry should be considered.