leucopaenia in older foals is a sign of severe illness. Leucocytosis is
associated with infection (older foals), stress or exogenous steroids.
Lymphopaenia (<1x109/l) at birth can be associated with EHV (and EAV),
prematurity and sepsis.
Thrombocytes – similar to adult levels. Thrombocytopaenia can be
associated with immune-mediated disease – anti-platelet antibodies via
colostrum or drug related.
Biochemistry
Proteins -Immunoglobulin G (IgG)
As there is no transplacental transfer of IgG before birth, foals are born
essentially agammaglobulinaemic. During the last few months of gestation,
mares concentrate IgG, IgG(T), IgA and IgM in their colostrum. It is these
essential immunoglobulins that will confer immunity until the foals own
immune system is mature. The colostrum also contains complement and
lactoferrin, proteins essential for immune function.
Specialist enterocytes absorb the immunoglobulins by pinocytosis. These
cells have a lifespan of a maximum of twenty-four hours. Maximum
absorption occurs within eight hours of life. To obtain adequate transfer of
immunity, the foal must ingest around 1 litre of good quality colostrum within
the first six hours of life. Maternal IgG has a half-life of 20-23 days. Serum
IgG concentrationsa are at their lowest at 1-2 months of age.
Foals’ intestines are capable of absorbing IgG for their first 12 hours of life.
Providing the mare makes colostrum of good quality in terms of IgG
concentration, she does not ‘run milk’ before foaling and the foal sucks
sufficient colostrum within the first 12 hours, the foal acquires good circulating
IgG levels and therefore adequate passive immunity.
In the author’s opinion, foals should have their serum IgG levels checked as a
routine preventive medicine policy. Serum samples collected from foals after
12 hours of age should have IgG levels of more than 4g/l and ideally more
than 8g/l. Levels of less than 4g/l indicate failure of transfer of colostral
immunity and levels of 4-8 g/l indicate partial failure.
Foals with levels below 4g/l are considered at risk for neonatal infections and
should ideally be transfused with commercial hypermmune plasma and their
serum IgG levels re-checked 24 hours later to make sure that IgG levels have
risen to acceptable levels.
None of the currently available ‘stable-side’ foal serum IgG tests are reliably
accurate at the 0-4g/l end of the scale and they are all affected by the
presence of inflammatory proteins, especially fibrinogen.
IgG can be measured in colostrum immediately after parturition semiquantitatively, using a refractometer. If readings suggest an IgG level of less
than 45 g/l, the foal should be considered for donor colostrum
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
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