Analysis of the antitrypsin activity and immunoglobulin G concentration in equine colostrum and milk

BACKGROUND: Immunoglobulins ingested via colostrum must remain intact to be absorbed by the neonate. Equine colostrum contains antitrypsin activity, which likely protects these proteins from proteolysis in the gastrointestinal tract. AIMS/OBJECTIVES: To quantify antitrypsin activity in equine colostrum and milk, describe its temporal changes during the first five days after parturition, compare actitvity between left and right mammary glands, evaluate differences between nulliparous and pluriparous mares, and examine associations with immunoglobulin G (IgG) concentrations and mare age. METHODS: This retrospective study included 67 mares after 73 parturitions, yielding 145 colostrum samples and 207 milk samples. Samples were obtained from both mammary glands when available. Antitrypsin activity was measured using a colorimetric trypsin inhibition assay, and IgG concentrations were determined by enzyme-linked immunosorbent assay (ELISA). Results are reported as median and range. RESULTS: Median antitrypsin activity in colostrum was higher in nulliparous mares (1.09, range 0.65) than in pluriparous mares (0.78, range 0.72). Activity declined over five days postpartum (median 0.49, range 0.81). No differences were observed between left and right mammary glands. Colostral antitrypsin activity correlated strongly with IgG concentration (rs = 0.77), whereas no correlation was detected in milk (rs = -0.04). Mare age was not associated with colostral antitrypsin activity (rs = -0.09). CONCLUSION: Antitrypsin activity in equine colostrum is highest immediately postpartum, particularly in nulliparous mares, and is closely associated with IgG concentrations. These results suggest that antitrypsin activity contributes to the protection of IgG during intestinal absorption and should be considered when assessing colostrum quality for neonatal care.