Failure of Passive Transfer
Large animal neonates are born with limited energy reserves and are considered immunocompetent but immunologically naive at birth (ie, agammaglobulinemic). Thus, ensuring the provision of good-quality colostrum by the dam and adequate colostrum intake by the neonate are critical influences on neonatal survival. Good-quality colostrum tends to be sticky, yellow, and thick with a specific gravity of >1.060 in mares. All factors that might compromise colostrum quality, volume, and delivery to the neonate should be recognized; these include maternal factors (disease during gestation, premature lactation, maiden dam), delivery factors (abnormal parturition, placental abnormalities), and neonatal factors (prematurity, dysmaturity, maternal rejection, multiple birth, or any other condition limiting neonatal mobility and strength). Manually stripping the teats of the dam to remove wax plugs and to check for the presence of colostrum may facilitate successful nursing attempts. The neonate should be examined for any obvious congenital problems that may inhibit the ability to stand or to nurse effectively, including signs of prematurity, musculoskeletal abnormalities, and cleft palate (and in crias, choanal atresia).
Healthy foals begin sucking colostrum within 1–2 hr after birth, and maternal antibodies are detectable in the foal’s blood within 6 hr. Although adequacy of passive transfer of immunoglobulin is generally assessed at 18–48 hr, neonates at high risk of failure of passive transfer (FPT) and/or sepsis may be tested as early as 6–12 hr after birth. Weak or abnormal neonates that do not stand or nurse successfully within 2–4 hr should be supplemented with colostrum early (commonly tube or bottle fed). A 50-kg foal requires a minimum 1.5–2 L (ideally 5%–12% of body wt) of good-quality colostrum (~70–75 g of IgG), fed over multiple feedings in the first 12–18 hr after birth. Colostrum of other species has been used (such as cow colostrum for foals, crias, kids, and lambs) but will not necessarily provide pathogen-specific immunity. Commercial colostrum substitutes have similar limitations and may increase the risk of adverse immunologic reactions if a subsequent plasma transfusion is needed. Oral administration of colostrum or colostral substitutes has only minimal benefit in neonates >18–24 hr after birth because of the limited capacity of the neonatal small intestine to absorb macromolecules.
Complete FPT in foals is defined as a serum IgG concentration of <400 mg/dL at 24 hr of age, with partial FPT being associated with a serum IgG concentration of 400–800 mg/dL. Healthy, vigorous foals with complete FPT (<400 mg/dL) and high-risk foals with partial or complete FPT (IgG ≤800 mg/dL) should receive equine plasma (IV) that contains an IgG concentration of >1,200 mg/dL. The average IgG concentration in 1 L of equine plasma will typically increase the serum IgG concentration of a 50-kg foal by 200–300 mg/dL; therefore, 2–4 L of plasma may be necessary to achieve a final serum IgG concentration of >800 mg/dL in a foal with complete FPT (initial IgG <200 mg/dL).
Frozen plasma should be thawed and warmed slowly to room temperature using warm water, and subsequently administered slowly through an aseptically placed jugular catheter, using an appropriate blood filter set. Initially, plasma is administered at 0.5 mL/kg over 10–20 min (~20–30 mL to an average foal) while monitoring for transfusion reactions. Clinical signs of transfusion reactions may include muscle fasciculation, increased heart or respiratory rates, fever, respiratory distress, laryngeal swelling, abdominal pain, pale mucous membranes, or collapse. If no adverse reactions are observed during the initial slow infusion, the remainder of the transfusion may be administered at rates up to 40 mL/kg over 60–90 min (eg, 2 L throughout 60 min for a 50-kg foal).
Although serum IgG is measured less commonly in ruminants, concentrations >1,600 mg/dL are ideal. Serum total protein may also be used as a rough estimate of colostral transfer in ruminants and should exceed 5–5.5 g/dL. An IgG concentration >1,000 mg/dL is considered adequate in neonatal camelids. Administration of camelid plasma (either IV or IP) is an acceptable treatment of FPT in llamas and alpacas, although IP administration should be limited to vigorous neonates, to minimize the risk of secondary complications.