Relationship of Mixed Bacterial Infection to Prognosis in Foals with Pneumonia Caused by Rhodococcus equi

Background Isolation of multiple bacterial species is common in foals with Rhodococcus equi pneumonia. Hypothesis There is no association between isolation of other microorganisms and outcome. Animals 155 foals with pneumonia caused by R. equi . Methods Case records of foals diagnosed with R. equi pneumonia based on culture of the respiratory tract were reviewed at 2 referral hospitals (University of Florida [ UF ] and Texas A&M University [ TAMU ]). Results R. equi was cultured from a tracheobronchial aspirate ( TBA ) in 115 foals and from lung tissue in 38 foals. Survival was significantly higher at UF (71%; 70/99) than at TAMU (50%; 28/56). R. equi was significantly more likely to grow in pure cultures from samples obtained from foals at UF (55%; 54/99) than from foals at TAMU (23%; 13/56). Microorganisms cultured with R. equi included Gram‐positive bacteria in 40, Gram‐negative bacteria in 41, and fungi in 23 foals. The most common bacteria isolated were beta‐hemolytic streptococci (n = 26) and Escherichia coli (n = 18). Mixed infections were significantly more likely to be encountered in TBA than in lung tissue. Only foals from which R. equi was cultured from a TBA were included in the analysis for association between mixed infection and outcome. After adjusting for the effect of hospital using multivariate logistic regression, mixed culture, mixed bacterial culture, Gram‐positive bacteria, beta‐hemolytic streptococci, Gram‐negative bacteria, enteric Gram‐negative bacteria, nonenteric Gram‐negative bacteria, and fungi were not significantly associated with outcome. Conclusions and Clinical Importance Isolation of multiple bacteria or fungi from a TBA along with R. equi does not negatively impact prognosis.