Penicillin Allergy vs Procaine Reaction
Penicillin is an old, but still very useful, antibiotic. It is effective for treating many kinds of infections and is relatively cheap and safe. However, allergic reactions to penicillin, though rare, are reported in both people and animals, including horses. Signs of a reaction may include hives, sweating, weakness and collapse shortly after treatment.
What many people may not realize is that a horse that has a reaction to a penicillin injection is not necessarily allergic to the penicillin. Most of these animals actually react to procaine, which is a component of the white penicillin that is given by injection into the muscle. If procaine reaches the blood supply too quickly (e.g. if the injection accidently goes directly into a vein instead of the muscle), it can cause a very sudden, dramatic reaction including signs of hyperexcitability, manic behaviour and rearing over. Horses that have procaine reactions typically recover quickly, but they can injure themselves when falling.
It is important to differentiate penicillin allergy from procaine reaction. If a horse has a penicillin allergy, it should never be treated with penicillin (of any kind) ever again. Diagnosing a horse with a penicillin allergy is therefore not something that should be done lightly or unnecessarily, because penicillin is a very useful drug. A small percentage of individuals with penicillin allergy are also allergic to the related antibiotic class, cephalosporins (including the common drug ceftiofur (Excenel, Naxcel)). Only a small percentage of horses are allergic to both classes, so we don't automatically ban the use of both drug classes in a horse with a penicillin allergy. However, it does warrant caution when a cephalosporin is used in the animal for the first time, and a veterinarian should be present in case there is a reaction. If a horse has had a severe allergic reaction to penicillin, cephalosporins should probably be avoided anyway, to be on the safe side.
If you are going to use penicillin, you should ensure that you know how to inject it properly. Improper injection technique increases the risk of a procaine reaction.
White penicillin must NEVER be injected intravenously.
While penicillin is cheap and easy to get, but as with any antibiotic, it should only be used on the recommendation of your veterinarian.
Photo: Penicillium notatum, the mold from which penicillin was originally produced
(photo source: Dept of Biology, University of North Carolina at Charlotte)
