Survival of foals with experimentally induced Rhodococcus equi infection given either hyperimmune plasma containing R. equi antibody or normal equine plasma.

The research explores whether administering hyperimmune plasma containing Rhodococcus equi antibodies to young horses reduces the severity and death rate of R. equi pneumonia, compared to normal equine plasma. Results showed no significant difference in disease severity or death rate between the two treatments, suggesting both types of plasma may have a protective role, though the exact protective component requires further study.

Study Design and Methodology

• The study involved a controlled trial using 16 foals (young horses born from mares) that had been deprived of colostrum – the first milk a mare produces that’s rich in antibodies for the newborn.
• All foals were given normal equine plasma within the first 24 hours of life. Characteristics of this plasma included an R. equi titer (an indication of the amount of antibodies against the bacteria in the plasma) of less than 20% and a virulence-associated protein A (VapA) level of less than 160.
• At 14 days old, categorization of the foals occurred: six were given normal equine plasma and 10 received hyperimmune (HI) plasma. The HI plasma had an R. equi titer of at least 100% and a VapA level of at least 10,000.
• Subsequently, the foals were intentionally infected with a pathogenic strain of R. equi at 21 days of age to induce R. equi pneumonia.

Data Collection and Findings

• Foals underwent repeated physical examinations, as well as weight measurements, complete blood cell counts, fibrinogen measurements and thoracic radiographs to assess the severity of the disease.
• Data revealed foals given HI plasma had significantly higher R. equi ELISA titers (42.4%) than those given normal plasma (20.9%) at the time of experimental infection.
• Regardless of this, disease severity and mortality rates were similar for both groups.
• Some foals with severe R. equi pneumonia recovered without the use of antibiotics.

Conclusion and Implications

• The research suggests potential protective effects of both normal equine plasma and HI plasma against R. equi pneumonia in colostrum-deprived foals.
• Notably, the exact constituent in the plasma that provides protection remains undetermined and therefore calls for additional comprehensive research.
• The absence of adverse effects to plasma administration in the early weeks of life implies safety of this procedure in this relatively vulnerable population.