Efficacy of treatment and survival rate of foals with pneumonia: Retrospective comparison of rifampin/azithromycin and rifampin/tulathromycin

Abstract

Rhodococcus equi (R. equi), a Gram-positive facultative intracellular pathogen is one of the most common causes of pneumonia in foals. The retrospective study compares the efficacy of rifampin/azithromycin versus rifampin/tulathromycin for the treatment of R. equi pneumonia in foals. Weekly medical data of foals born in the seasons 2012 to 2016 (i.e. five foaling seasons) that developed pneumonia were collected and analysed. Foals 21 days of age or older with a pulmonary consolidation of more than 1 cm (abscess score ^ 1 cm) were enrolled for participation in the study. All foals meeting the criteria for inclusion were listed in a spreadsheet and 330 foals affected with pneumonia each year were randomly selected by an online research randomizer. Foals with incorrect or missing data and those that were included in previous treatment studies were excluded from the analysis, resulting in a total enrolment of 1544 foals with pneumonia. The farm established o screening program in 2003 in order to detect foals in the early course of pneumonia, so all foals were examined once weekly starting at the age of two weeks until four and a half months of age. A physical examination of the respiratory tract, body temperature, haematology and an ultrasonographic examination of the lungs were included. Sonography areas with visible consolidation were measured and added to calculate an "abscess score" which represents the extent of pulmonary damage. Weekly medical data were analysed retrospectively. The risk of therapy failure, which means either death or worsening of pneumonia requiring that ihe foal be switched to another therapy, showed in a univariate comparison of the two treatments, that there was a higher incidence of treatment failure in foals treated with RIF/AZM (32/353, 9.1 %) than in foals that were treated with RIF/TUL (19/406, 4.7%; Pearson's x2, P = 0.016). In a Kaplan-Meier analysis, there was also a significant difference between the two treatments with respect to their survival functions (Log-rank test, P = 0.045; Figure 1). The incidence of mortality due to R. equi pneumonia was higher in foals that were treoted with RIF/AZM (8/353, 2.3%) than in those that were treated with RIF/TUL (0/406, 0.0%; Fisher's exact test, P = 0.002). After adjusting for year, age of foals at the beginning of treatment, and abscess score at the beginning of treatment, the risk of treatment failure was not significantly lower for foals that were treated with RIF/TUL compared to those that were treated with RIF/AZM [Hazard Ratio (95% CI) = 0.75 (0.38, 1.5)]. Age of foals was negatively associated with treatment failure, with the risk of failure decreasing by 30% for every one-month increase in age at the beginning of treatment [HR (95% CI) = 0.70 (0.54, 0.91)]. Abscess score was positively associated with treatment failure, with the risk of failure increasing by 4% for every one cm increase in score [HR (95% CI) = 1.04 (1.01, 1.07)]. Year was also associated with the risk of treatment failure but it did not meet the proportional hazards assumption, so the model was adjusted for the effects of year by stratification. In conclusion Rifampin/tulathromycin is an effective therapy against mild to moderate R. equi pneumonia in foals as this treatment has similar survival rates compared to rifampin/azithromycin. In terms of costs and dosage form, the combination of tulathromycin with rifampin is on adequate therapeutic option for the treatment of fools with R. equi pneumonia.