basically a blood incompatibility in horses, just like humans get when a mom and baby have different gene types. The big difference is that a human will need treatment during pregnancy, where foals need help just after birth. So this is not just a Friesian thing, it's a horse thing. But it's very much talked about on the breeding pages and Friesian chat groups so it's important that I post it here.
Simple summary -
The foal gets it's immunity from the first 24 hours of milk from mom, called colostrum. If the mom is producing enzymes that "fight" the blood type of the foal it's carrying, then when the foal drinks the colostrum, the colostrum will "fight the foal". It's FATAL so take it very seriously and don't take your eyes off the foal. That is about as basic as it comes for an explanation.
It's a red blood cell disorder where the mares antibodies to attack the foals red bloodcells, causing oxygen deprivation, decreased immunity and ultimately organ failure.
The quick fix is early identification that the foal is getting weaker after ingesting the colostrum. Then remove them from that situation. Either muzzle the foal or put them in a foal holding box, or anything you can to keep that foal from nursing for the first 36 (48 to be safe) hours after birth. In the meantime feed the foal colostrum from another mare, powder, and consider a plasma transfusion. For that 36-48 hours that you are keeping the foal off the dam, milk her like crazy, get that colostrum out of her and make sure she's running white milk before the foal is back on the dam. At 48 hours the foals gut will stop absorbing the colostrum.
Symptoms to be on the watch for in the first 3 days
decreased suckling, weakness, increased heart rate, increased respiration, white or yellowing gums, lethargy and weakness, decreased flight response (a baby should have some fight even when imprinted)
IF you didn't catch it quickly you will likely need a plasma transfusion for the foal. IV fluids and serious vet care to make sure that the foals organs are not damaged. Even if you caught it early it's always important to have the vet check them.
Before putting the foal back with the mare you will want to get a Jaundice Foal Agglutination (JFA) test done to make sure that the foals blood and the Mare's colostrum/milk are combatable.
The foal shown below has jaundice from being compromised from NI. At this point the foal has been in full vet care, it's 5 days old in this photo.
UC Davis Blood Test for NI UK blood test for NI (we are not affiliated with any testing facility this is just to help because so few places do blood typing now that DNA has replaced blood type for parent verification)
In summary you can test for NI, it's not breed specific, it's easy to treat if you know you have a positive mare, and it's fatal if you don't catch it in time and give the foal proper treatment.
more posts on NI
NI posting 2