Your foal probably doesn't need protection now.
If you vaccinated your mare a month before her foaling date, and if your foal nursed in his first 6 hours of life, his first meal was antibody-rich colostrum (first milk) from his mom. The maternal antibodies he took in will circulate in his bloodstream to help protect him against infection for 6 months or longer. If you vaccinate while these maternal antibodies are still in full force, his immune system will simply ignore the vaccine-he doesn't need any more protection than nature has already given him.
Early shots can cause problems later.
If you vaccinate your foal when his maternal antibodies are still working hard, his immune system will "get to know" the antigen (the substance in the vaccine that's supposed to trigger production of antibodies-disease-fighting material). If that happens, his system may ignore the antigen forever, so he'll never build antibodies in response to the vaccine. This problem is most likely with the influenza virus (a highly contagious respiratory disease)-so it's especially important to wait before giving your foal that vaccine.
Here's a vaccination schedule that will minimize risks and maximize protection. Besides influenza, you'll vaccinate your foal for tetanus (an infectious disease caused by the bacillus toxin, Clostridium tetani), sleeping sickness (or encephalomyelitis, a viral disease affecting the brain and nervous system), and equine viral rhinopneumonitis (or EVR, a highly contagious viral upper-respiratory infection).
Also, check with your vet about optional vaccinations that may be recommended in your area, such as botulism (intoxication caused by bacteria in forage), rabies (a viral infection of the central nervous system), strangles (also known as distemper, a bacterial lymph-gland infection), and Potomac horse fever (also known as equine monocytic erlichiosis, an often fatal bacterial disease).
(Note: Ask your vet how you should modify this schedule if your foal failed to nurse during his first 6 hours, or if your mare didn't get prefoaling shots. Intranasal vaccinations, those given through the nose, don't count-they produce only local immunity, not circulating antibodies your mare can pass to her foal.)
Dr. Crabbe is an Oregon-based equine practitioner.
This article first appeared in the May, 2001 issue of Horse & Rider magazine.