25. passive immunity

what is hemochorial placentation? what species is it found in? how efficient is the antibody transfer?

• maternal blood in direct contact with chorionic epithelium

• primates, rabbits, rodents

• neonates have nearly equal IgG levels as the mother

what is endotheliochorial placentation? what species is it found in? how efficient is the antibody transfer?

• single layer of venous endothelium separates maternal blood from chorionic epithelium

• dogs, cats

• small amount of IgG transferred (~5-10% of maternal level)

what species have epitheliochorial placentation? how efficient is the antibody transfer?

• horses, swine

• no Ig transfer across placenta

what species have syndesmochorial placentation? how efficient is the antibody transfer?

• ruminants

• no Ig transfer across placenta

what triggers production of colostrum?

• hormonal shifts late in pregnancy activate FcReceptor nenonatal (FcRn) in mammary tissue → FcRn sequesters Ig into mammary gland from serum & concentrates

• memory T and B cells also migrate to colostrum

what are the components of colostrum?

• immunoglobulins

  • primarily IgG, with smaller amounts of IgM and IgA

• functional B and T lymphocytes

• cytokines, growth factors

• inhibitors of digestive enzymes

• nutritional components

how is colostrum absorbed?

• FcRn on mucosal surface of intestinal epithelium actively binds Ig to transport it

• endocytosis

• lack of tight junctions

• IgA can be actively moved back into the gut lumen in some species (bovine)

what is “gut closure”? when does it occur?

• FcRn receptors become exhausted or blocked

• tight junctions form between enterocytes

• most species: 24h or less

  • canine: absorption 50% less after 4h; closed by 12h

  • feline: 16h

why is maternal antibody crucial?

• risk of disease is greatest in neonates

• neonates are least able to clear infections

• passively derived immunity protects, but neonate is immunologically naive and completely susceptible once it is gone

what causes failure of passive transfer?

• inadequate colostrum

  • premature birth — none made

  • mammary gland leakage

  • low IgG levels in dam

  • singleton

• poor mothering

  • dystocia

  • rejection of offspring

• inadequate absorption

  • weak neonate

  • fed milk replacer before colostrum

what diagnostic tests are used to diagnose failure of passive transfer?

• refractometry

  • colostrum

  • neonatal serum

• single radial immuno diffusion test (SRID)

• foal snap — IDEXX

  • two control spots: 800 & 400 mg/dL

what should a foal’s serum Ab concentration be?

> 800 mg/dL

how is failure of passive transfer treated?

• in first 12-24h:

  • replace with compatible colostrum PO

  • if no colostrum available, can treat with high titer, compatible serum or plasma either PO or parenterally

• monoclonal antibody against CPV-2

• fresh frozen plasma (canine)

how does a nomograph work?

• dam titers determined

• 100% transfer assumed

• half-life degradation calculated

• conservative vaccination timing given

• follow-up titer test timing given

what are limitations of a nomograph?

• failure of passive transfer

  • dam doesn’t produce or leaks colostrum

  • dam has extremely large litter

  • extended or difficult whelping

  • maiden bitch

  • c-section

• don’t expect a nomograph will guarantee time frame that pups are protected from disease