You may make sure that your horses are up to date with their tetanus vaccinations; however, you may need to take extra steps to protect pregnant mares who are about to foal. For example, you may be told that your mare should have a tetanus booster towards the end of her pregnancy.
Why is this a good idea?
Why Give a Pregnant Mare a Booster Shot?
If you give a mare a tetanus booster a few weeks before she gives birth, then the vaccine and its antibodies will still be in her body after she foals. The milk her foal then feeds from will contain tetanus antibodies.
So, as the foal feeds, it will get some automatic protection from tetanus from its mother's milk. This protection will last for the first few weeks of its life when it may be at its most vulnerable.
If your horses are kept on grass, then this protection could be vital. Young foals may be more likely to have accidents. A simple cut or scrape could introduce tetanus into their system. While tetanus is a serious disease for any horse, foals may not be strong enough to recover from it.
Why Not Vaccinate the Foal?
Unless there is a pressing reason to vaccinate a foal, vets generally don't give shots too early. Newborn foals are vulnerable. It takes them a while to build their immune systems; they may not be developed enough for a vaccine to work as it should.
For example, animal vaccinations work by teaching animals' immune systems how to protect against diseases. After a shot, an older horse's immune system will produce antibodies against the disease based on the vaccination's contents. So, if the horse then contracts tetanus, its body will identify it as a problem and find the antibodies to deal with it. This may not work with younger animals whose immune systems aren't fully developed.
Juvenile systems may not be able to work out when a virus or disease is dangerous. They may not develop the antibodies they need and know when to use them. Plus, a foal will pick up traces of its mother's existing tetanus antibodies from milk, even if the mare hasn't had a booster. These maternal antibodies may prevent a foal's vaccination from working.
Therefore, it may make more sense to give your mare a tetanus booster before it foals. To find out when you should do this, contact your vet.