A bovine serocolostrum to transfer the passive immunity in foals and calves

Ongulates are born agammaglobulinaemic; their survival depends on the passive transfer of maternal immunoglobulins (IgG1) by colostrum ingestion. Currently, there is still 10 to 12 % of neonatal mortality, half is a the consequence of the failure in IgG1 absorption through the immature gut. During the 24 first hours the gut is able to transfer a wide proportion of native IgG1 by micropinocytosis. This transport should be enhanced by molecules present in the aqueous phase of the colostrum (serocolostrum). Hence, four serocolostrums, were produced by membrane fractionation with modification in soluble molecules composition except for IgG1 concentration. They were expected to contain factors enhancer of the IgG1 intestinal absorption in order to be used as a colostrum replacer for foals and calves. Two serocolostrums were freeze-dried to be used in addition to whole and skimmed milk (foals and calves) or to IgG1 deficient equine colostrum (DEC, foals). Animals were fed in the 7 first hours after birth with 4.1 g of IgG1/kg body weight. Control groups received equine (foals) or bovine (calves) colostrum by tube feeding. Serum samples were taken versus time and serum concentration in equine and bovine IgG1 was measured by single radial immunodiffusion. The half life and the absorption coefficient (AC) of the IgG1 were calculated. The maximal IgG1 concentration in animals’ serum was systematically at 18 hours after feeding. Regardless the composition of the serocolostrums, when given as a substitute, it was observed exactly the same failure in IgG1 absorption with an IgG1 serum concentration 2.4 times (~ 8 gkg-1, AC = 16±5 %) and 3.5 times (~ 5 gkg-1, AC = 25±7 %) lower in calves and foals respectively, when compared to the control groups. When serocolostrum was mixed with whole or skimmed bovine milk, IgG1 serum concentration was only increased by ~1.5-1.8 for both species. When It was mixed to the DEC, the bovine IgG1 concentration in foal serum was increased by 3 ([IgG1] ~15 gkg-1 (AC = 45±5 %). Hence, it seems that molecular fractions (caseins and/or fat) present in colostrum but lacking in serocolostrum are involved in IgG absorption either by transfer activation or by avoiding a transfer limitation. Additional experiments are ongoing to test this assumption. The identification of these factors should permit to better understand the cellular mechanism of absorption in immature enterocytes. Our results also showed that bovine serocolostrum was efficient enough to compensate the failure in passive immunization in calves and foals even if the half time of bovine IgG1 is reduced in foals.