life (maximum 6–8 hours). The minimum protective level of IgG in foal’s
blood at 24 hours is probably between 4 and 6 g/L. Partial FPT is
described as serum IgG levels of < 2–4 g/L and total FPT as < 2 g/L.
Up to 25% of foals are thought to suffer from FPT, the incidence
varying between stud farms.35,36 Foals that are abnormal at birth (for
any reason) have a dramatically increased incidence of FPT. The
function of neutrophil leukocytes is reduced in foals prior to ingestion of
colostrum so there are also cellular implications for failure of passive
transfer. Testing at 12 hours makes intestinal absorption an ineffective
therapeutic option. Current recommendations include administration of
1 litre of plasma for every 2g/litre of immunoglobulins required to reach
8g/litre. In addition to the bioactive proteins in plasma, it contributes
towards oncotic pressure.
Usually by the time FPT is recognized (i.e. at the 24-hour blood
sample stage, see p. 391), administration of colostrum is too late with
regard to sufficient IgG absorption. Local effects may be valuable,
however, in controlling some enteric infections. The only current
therapy is to administer plasma.40,41 Almost all complete FPT foals
will need more than 2 liters of plasma intravenously.
GCT tests, while a bit crude, are probably the most common tool for
screening for failure of passive transfer. Idexx makes a useful IgG
SNAP test which is more accurate, but pricey. Many studs will keep a
bank of frozen plasma for foals requiring plasma tranfusions for failure
of passive transfer. Failing that, one can draw blood from a donor horse
on the stud, with double blood collection bags, wait for it to sediment
out and separate the plasma. This process takes a few hours. Time is
often in desperately short supply in the Thoroughbred breeding season
and one may be tempted to cut corners by administering whole blood.
The red cells are not necessary, but the immunglobulins and other
immune factors are all included and will become available to the foal.
Administration of whole blood to a foal with failure of passive transfer is
not advisable. Cross-matching of blood donors and recipients is
advisable prior to transfusion, but is not always possible in the
ambulatory practice setting. For certain conditions, such as neonatal
isoerythrolysis, whole blood or packed cell administration may be
indicated, and may need to be performed without the benefit of crossmatching or washed red cells, in emergency circumstances.
The case in point is a 2 day old Thoroughbred colt with FPT which was
given a blood transfusion, instead of plasma. Extreme time limitations
precluded the attending veterinarian from separating out the plasma
and red cells before administration. The foal tolerated the transfusion
well, and there was no transfusion reaction. The follow up SNAP test
revealed a satisfactory IgG level 24 hours post transfusion, and the foal
appeared clinically normal. 7 days later, however, the foal presented
collapsed and severely icteric. It was pyrexic, tachycardic, tachypnoeic
and in a moribund state, which progressed to coma and seizures. What
happened?
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
74