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Abstract
While Rhodococcus equi remains the most common cause of subacute or chronic granulomatous
bronchopneumonia in foals, development of a relevant model to study R. equi infection
has proven difficult. The objective of this study was to identify a challenge dose
of R. equi that resulted in slow progressive disease, spontaneous regression of lung
lesions and age-dependent susceptibility. Foals less than one-week of age were challenged
intratracheally using either 10(6), 10(5), 10(4), 10(3) or 10(2) cfu of R. equi. Two
doses (10(3) cfu and 10(5) cfu) were used to challenge 2 and 3-week-old, and 3 and
6-week-old foals, respectively. Physical examination, thoracic ultrasound and blood
work were performed. Foals were euthanized at the end of the study or when clinical
signs of pneumonia developed. All foals were necropsied and their lung lesions scored.
Foals challenged with low concentrations of R. equi developed slow progressive pneumonia
and approximately 50% of the foals recovered spontaneously. Likewise, macroscopic
(>1cm diameter) pyogranulomatous lesions were only observed when low doses of R. equi
were used. Clinical pneumonia was not seen after low dose challenge in the 3-week-old
foals or in the 6-week-old foals. This study demonstrates that the use of low doses
of R. equi to challenge neonatal foals provides an improved model for studying this
disease. Furthermore, susceptibility to R. equi infection was shown to diminish early
in the foal's life, as has been reported in the field.