COLOSTRUM PRODUCTION

The transfer of immunoglobulins from the maternal circulation to the mammary gland is termed colostrogenesis. In mares the concentration of immunoglobulins in the mammary gland increases in the last 10 days before foaling.¹ This appears to be an active process because immunoglobulin concentrations are higher in colostrum than in maternal serum. The hormonal influences on colostrogenesis in the mare are not well understood. During the period of colostrogenesis, serum progesterone and prolactin increase. Mares exposed to ergopeptides that have abnormally low progesterone and prolactin fail to develop udders or produce normal mammary secretion.² At parturition, serum progesterone falls and serum prolactin increases dramatically, possibly due to loss of opioid-induced suppression.³ These changes may initiate the transition from colostrum to milk production. By 24 hours post partum, immunoglobulin levels in colostrum have decreased fortyfold.⁴

COLOSTRUM CONTENT

Colostrum contains immunoglobulins, maternal immune cells, and cytokines. In the mare, IgG is the major immunoglobulin class in colostrum, with approximately 60% isotype IgGb, 25% isotype IgGa, and 15% isotype IgG(T).⁴ Normal colostrum has greater than 3000 mg/dl of IgG, with excellent quality colostrum having IgG levels greater than 6000 mg/dl.^5,⁶ Stimulation of the maternal immune system by intramuscular injection of levamisole or 1,3/1,6 glucan significantly increases colostral immunoglobulin levels.⁷ Intramuscular vaccination of mares in the last trimester of pregnancy against rotavirus, Streptococcus equi, tetanus, and influenza increases colostral immunoglobulin levels against these pathogens.^8,⁹ During late pregnancy, mares should be housed on the farm where they will foal to stimulate colostral antibodies to local pathogens.

Colostrum contains more than 1 million cells per milliliter, including macrophages, lymphocytes, neutrophils, and epithelial cells. Other factors, including complement, lactoferrin, epidermal growth factor, interferon-γ, and insulin-like growth factor, are also present. In the mare the importance of the nonimmunoglobulin portions of colostrum has not been studied; however, they likely act both locally in the gut and systemically as modulators of the neonatal immune system.