Passive immunity, an important aspect of equine immunology, occurs when 'preformed' antibodies are made in one animal and passed onto another. For example a cat passes on her own antibodies to her kitten via colostrum (first milk after parturition) and milk. Antibodies can be transferred to other individuals via colostrum, milk, crossing of the placenta, vaccination, and plasma transfusions.
Neonates require maternal antibodies because:
1. They have poorly developed immune systems and are immunocompromised at birth.
Lymphocytes in neonates: Increased numbers of mostly mature and functional T and B lymphocytes are found in the blood. However the ability to mount an immune response to certain antigens is absent. This may be due to:
- Immaturity of some cells - T helper cells, B cells and Antigen presenting cells.
- Lack of expression of genes encoding receptors for each antigen.
- The presence of maternal antibody which binds antigen and removes it thus preventing the neonate from developing active immunity.
Antibodies in neonates:
2. Neonatal mucosal surfaces are particularly vulnerable
3. Colostral and milk antibodies protect neonates from infections
Passive transfer via placenta
|SPECIES||PLACENTA TYPE||IgG TRANSFER?|
Due to the placentas of ruminants and horses having 5 tissue layers between the maternal and fetal circulation there is no transfer of antibodies across the placenta.
Passive transfer via colostrum
Colostrum is the pre-milk fluid usually thick and yellow in colour that is secreted from the mammary glands for only a short time after birth by most mammals.
- There is species variation with the composition of colostrum.
- It contains:
- Immune factors - immunoglobulins, lactoferrins, protein-rich polypeptides, leukocytes, cytokines, trypsin and protease inhibitors.
- Growth factors: EgF, IGF-I, IGF-II, FyF, PDGF, TgF A and B and growth hormone (GH) - which aid in rebuilding damaged body systems and stimulating the bodies metabolism to burn fat for energy instead of the bodies own muscle tissue.
- Vitamins and minerals.
- IgG is most abudant in colostrum, followed by IgA and IgM
- During the first 30 hours of life, immunoglobulins are absorbed through the intact jejunum and pass to the lacteals, thoracic duct and into the systemic circulation.
- The first diagram (entitled Transfer of IgG across intestine via FcRn) shows IgG being present in the intestine of a neonate, from the ingestion of colostrum. Fc receptors (FcRn) are present on the surface of the intestinal epithelial cell in neonates for the first 30 hours. The IgG binds to the receptors and is transported through the cell by pinocytosis and released into the lacteals unchanged.
- The capacity for immunoglobulin absorption progressively declines from birth due to the FcRn only being present for a limited period of time. After 6 hours a third of the FcRn are non functional.
- It is essential that all animals with epitheliochorial placentas (ruminants, horses, pigs) receive adequate colostrum intake within the first 4 hours (optimal absorption period) of life because no transfer of immunoglobulins via the placenta can occur.
- The second diagram (entitled Passive Transfer) shows the relationship between passive immunity from the mother and the neonatal production of antibodies.
- Colostrum also provides the neonate with a vital energy source to generate heat and enzymes to aid digestion of colostrum products.
Milk is the primary source of nutrition for young before they are able to digest other food. IgA is most abundant, followed by IgG and IgM.
- Milk has approximately 1/100th of the antibody concentration of colostrum.
- Between colostrum and milk there is a period where a substance similar to them both is secreted called transition milk. By approximately 5-7 days milk is being secreted.
- The capacity for immunoglobulin absorption from the gastrointestinal tract is completely absent 30 hours post partum so the main role of milk (apart from a nutritional source) is to protect the intestinal lining.
- IgA remain in the intestine and attach to the intestinal villi protecting the intestinal lining against enteric pathogens.
Failure of passive transfer (FPT)
Inadequate absorption of .
It occurs because of 4 main reasons:
- The neonate did not receive any colostrum.
- The neonate did not receive enough colostrum.
- The neonate did not absorb enough colostrum.
- The quality of the colostrum was inadequate.
Colostrum quality (amount of IgG) depends on:
- Stress - Queens under increased stress produce colostrum of reduced quality.
- Immunostatus of the Queen - Queens exposed to specific diseases mount an immune response and produce antibodies for that particular disease, these antibodies are then transferred to the neonate via colostrum. Dams may also be vaccinated which, if done during the colostrum production period (last 2 months of gestation), will also provide the neonate with some protection via passive transfer.
- Milk yield - increased yield dilutes the amount of immunoglobulins in the colostrum.
Colostrum quantity depends on:
- Amount the neonate suckles - a strong neonate and good mothering increases the amount the neonate suckles. Udder conformation also influences the accessibilty of the teats to the neonate. Dystocia may reduce mothering and produce a weak calf potentially causes an increased time to the first suck and/or reducing the amount the neonate consumes.
- Premature births - if born early the Queen has a shorter period of time for concentration of antibodies from the blood to the colostrum. It is more likely that the neonate will also be weaker and smaller, causing the amount of colostrum sucked to be reduced.
Failure of absorption from the intestines:
- If the neonate is provided with colostrum 30 hours post partum the FcRn receptors will be completely absent and thus no IgG will be absorbed.
As a rough guide 8-10% of the neonate's body weight of good quality colostrum should be given over the first 12 hour period of a neonate's life to prevent FPT. This can be done effectively by hand feeding using bottles or with oesophageal feeders.
Testing colostrum quality
Colostrum quality can not be determined by appearance. Colour and consistency only indicate the fat components of the colostrum, and a measure of the IgG either in the colostrum or in the blood of the neonate is required to determine the immunological quality.
Storage: Colostrum can be stored, however this must be done correctly to prevent reduction in immunological status and to prevent outbreaks of disease. Disease outbreaks may occur from feeding colostrum as it is a very good medium for bacterial growth. The best way to store colostrum is to freeze it immediately after milking in a small, clean, secure container. It can be frozen for a year with minimal reduction in quality. It should be thawed thoroughly before giving it the the neonate by leaving it at room temperature, or by gently warming it in water (no hotter than 38 C). Thawing colostrum at high temperatures or by using a microwave can irreversibly denature the immunoglobulin (proteins) and thus reduce the colostrum's immunological status. Colostrum can be refrigerated for approximately 5-7 days. However, this must be done immediately after milking, and it must be discarded after this period to prevent colostrum with bacterial overgrowth being fed to the neonates.
1. A.H. Andrewa Bovine Medicine - Diseases and Husbandry of Cattle Blackwell Publishing 2004 2nd Edition
2. Koterba, Drummound and Kosch Equine Clinical Neonatology Williams and Wilkins 1990
3. P. Lydyard, A. Whelan and M.W. Fanger Immunology Garland Science 2nd Edition 2004