Transfusion with 2litres of hyperimmune plasma is superior to transfusion of 1litre for protecting foals against pneumonia attributed to Rhodococcus equi

BackgroundTransfusion of plasma hyperimmune against Rhodococcus equi (REHIP) is used to reduce the incidence of foal pneumonia caused by R. equi on endemic farms, but the optimal volume to transfuse is unknown.ObjectivesTo determine whether transfusion of 2 L of REHIP to foals was superior to transfusion of 1 L for reducing the incidence of pneumonia attributed to R. equi in foals.Study designRetrospective cohort design.MethodsMedical records of 158 foals from two breeding farms in Saratoga Springs, New York, were reviewed. Information collected from mares and foals included the date of birth, volume of REHIP transfused to foals, and whether the foal developed pneumonia attributed to R. equi. Data were analysed using random‐effects logistic regression with pneumonia as the outcome variable and farm modelled as a random effect to account for clustering of foals within farm.ResultsThere were 77 foals with evidence of clinical (n = 75) or subclinical (n = 2) pneumonia. The odds of pneumonia were 2.5‐fold greater (95% confidence interval [CI], 1.1–5.8; P = 0.0319) for foals transfused with 1 L than for foals transfused with 2 L. The odds of pneumonia were 2.7‐fold (95% CI, 1.4–5.5; P = 0.0046) greater for foals born in April or May than for foals born before April. Accounting for the effects of birth month, the odds of pneumonia were 2.4‐fold (95% CI, 1.1–5.6; P = 0.0457) higher for foals transfused with 1 L of REHIP than for foals transfused with 2 L.Main limitationsThe major limitations of this study are the observational study design and that diagnosis of R. equi pneumonia was presumptive.ConclusionsTransfusing foals with 2 L of REHIP is probably superior to transfusing 1 L for reducing the incidence of R. equi pneumonia and appeared safe.