The easiest way to manage horses is to assume that they are trying to die from the moment they are born, and then make a plan to stop them.
Eat the wrong hay? Their gut stops working, they colic and die. Eat too much fresh grass? They get laminitis and their hooves can literally fall off. Too much calcium? They’ll build a stone in their gut and colic and die. Teeth don’t wear correctly? They won’t be able to eat right, then colic and die. Get their hooves trimmed incorrectly? Maybe they get laminitis and the foot rotates enough that the bone comes out through the bottom of the sole. They can even develop balls of fat on a stalk inside their abdomen which can then strangle their intestines leading to — you guessed it — colic and death.
But even before they can live long enough to get colic or laminitis, they have to survive the first few days after birth. And that’s where colostrum comes in.
(Photo credit: Elina Sazonova (Pexels))
The epitheliochorial placenta
The placenta is the uterine connection where nutrients and oxygen are transferred to the fetus, and byproducts such as carbon dioxide and ammonia are taken away. If you think about it, not only is mom eating for two, she’s also peeing for two. (Luckily the fetus isn’t eating much other than a bit of amniotic fluid, so there’s no need to poop for two because the placenta really isn’t set up for that.)
The basic idea of the placenta is that there are three layers of cells on the fetal side that match up to three layers on the maternal side. The maternal blood stays in the mother, the fetal blood stays in the fetus, things diffuse in both directions, and the fetus has everything it needs to grow and not poison itself.
In a human placenta (technically a hemochorial placenta), the three maternal layers are mostly gone, so maternal blood is making direct contact with the fetal layers. Because of that, maternal antibodies can cross over to the fetus during gestation, and when the human baby is born it is already protected from a lot of diseases.
Horses, however, retain all three maternal layers, leading to an epitheliochorial placenta. Antibodies can’t cross from the mother to the fetus. So when a foal is born, it has no protection at all from the bacteria in its environment. Eventually it will start making its own antibodies, but that takes a week or two to really get up to the point where it can fight off infection. And a foal is really not in a sterile environment when it is born.
The importance of colostrum
So if a foal can’t get antibodies during gestation, how does it avoid infection during the first week or two? The answer is that it receives the maternal antibodies during its first meal of colostrum, the first milk a mare produces, which is super high in antibodies.
For about the first twenty-four hours, the foal’s gut has special cells to absorb antibodies, so the antibodies in the colostrum pass into the foal’s blood stream, and then it’s ready to fight off infection. This is known as passive transfer.
Failure of passive transfer
Imagine all the things that can go wrong. What if the mare doesn’t produce colostrum with enough antibodies? What if the foal takes too long to figure out how to stand up and nurse, and the specialized intestinal cells are no longer around to absorb the antibodies? What if the mare dies and there’s no equine colostrum available? What if the horse gave birth unobserved and you’re just not sure if the foal got enough colostrum?
The good news is that it’s pretty easy to check the antibody levels in the foal with a simple blood test. If the levels are too low, the fix is a transfusion of plasma from an adult horse, which puts the adult horse’s antibodies straight into the foal’s bloodstream.
Over the next couple of weeks the maternal antibodies in the foal will gradually get used or destroyed, but at the same time the foal is starting to produce its own, so it is never left unprotected. With a little luck it will grow up and live a nice long life during which it tries new and creative ways to kill itself.
Want to know more?
Let’s face it — all I really wanted to know about reproduction in vet school was how to stop it, so don’t take my word for any of this!
Different types of placentation in mammals: http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/structure.html
More info about maternal antibodies: https://pubmed.ncbi.nlm.nih.gov/26507553/
And a good overview of the basics: Large Animal Internal Medicine, Bradford P. Smith, 3rd edition.
During Fun Facts Friday, I talk about something I think is interesting. Do you have questions? Suggestions for a future post? Add a comment below!
