Serum activities of complement 1q and antibodies to the virulence-associated protein A are lower in foals that develop rhodococcal pneumonia.

Objective: To determine the effects of transfusion of Rhodococcus equi hyperimmune plasma (REHIP) on serum concentrations of complement component 1q (C1q) and to examine the association of serum C1q and anti-rhodococcal antibodies of newborn foals with subsequent development of rhodococcal pneumonia.

Animals/samples: Foals (n = 205) from 2 Thoroughbred breeding farms in New York transfused with REHIP between January 1, 2022, and December 1, 2022.

Procedures: Blood was collected immediately before transfusion with REHIP and again from the contralateral vein immediately after transfusion.