One of the most important things that you, as a horse owner, can do for the health of your animal is to set up a regular vaccination and worming program. What you vaccinate for and how often you worm depends on what area of the country you live in and how your horse is used.
For instance, in the South, sleeping sickness is quite prevalent, so vaccinating for Eastern/Western Encephalomyelitis twice yearly is appropriate. In the Northeast and Midwest, Potomac horse fever is widespread so vaccination is recommended every six months.
Also included are yearly vaccines for flu, tetanus, rhinopneumonitis and rabies. Of course, if you are actively showing your horse, more frequent flu and rhino shots may be appropriate. If you have broodmares, then vaccinating against the rhino that causes abortion is recommended at 5, 7, and 9 months of gestation. Read the label on the vaccine carefully, the strain used to prevent abortion is different than that for the respiratory disease. Broodmares also should be given their yearly vaccination about thirty days prior to foaling date. Protection passes through the colostrum (first milk) to the foal in the form of antibodies.
Most allergic reactions to vaccines are caused by injecting into a vein rather than the muscle. Be sure to pull the plunger back after inserting the needle to make sure no blood is present. Also, use a 1 1/2" needle and bury it deep into the muscle. The closer the injection is to the skin, the more likely the horse is to get a site reaction (welts, redness, etc.). As for more severe reactions, they are quite uncommon, but if your horse is going to react adversely to the vaccine, it will happen in a very short period of time. Treatment in this case is usually a shot of epinephrine. Consult with your veterinarian if this occurs.
When choosing a vaccine, stick with a name you know. Good quality vaccines will have been purified. This process eliminates almost all foreign proteins, which are also a cause of allergic reactions. Also, those all-in-one shots are better than individual shots for each vaccine. The theory is, the less injection sites, the less chance for site reactions.
Whether you vaccinate your horse yourself, or have a veterinarian do it, be sure to keep accurate records. By knowing your horse is up to date on his shots, finding out his pasture buddy has Potomac horse fever won't be as alarming as it could be.
As for worming, there are also regional differences. In the South, horses are usually wormed every 30 or 60 days. In the Northeast and Midwest, quarterly worming is acceptable.
When using commercial wormers (i.e. paste), it is very hard to "over worm" your horse. They have a very wide range of safety. So trying for complete accuracy when it comes to how much wormer per pound of horse is not a real issue. One vet I spoke with recommends using an ivermectin wormer in the late fall land again at the beginning of summer to combat bots. During the other two quarters, use tetrahydropyrimidine (i.e. Strongid) and benzimidazole (i.e. Panacur).
Broodmares and foals are a different story. It's recommended that broodmares are wormed 30 days prior to foaling date, so that the colostrum will afford the foal some protection against parasites.
A new theory out is that the foal heat scours is not caused by the mare's estrus affecting her milk, but, rather, by a parasite. With this in mind, worm the foal at 5 to 7 days old. Also, worming the foal every three to four weeks until they are six months old is recommended.
Again, keep records of your worming program. Rotation of wormers is very important. Even if you only have one horse. It makes good business and health sense to always know exactly where you stand when it comes to disease and parasites.