Despite being the oldest antibiotic, there are still many uses for Penicillin today. It remains effective against many different types of bacteria even amongst the growth of antibiotic resistance. To continue to conserve its effectiveness we need to use it cautiously just like any antibiotic. It is also available in a few different veterinary formulations labeled for use in horses, two of which are used most often.
In the hospital we are able to use an intravenous or IV formulation of penicillin that is paired with potassium or as some people call it KPen. Although safe when given appropriately it is not often used outside of a hospital or clinic setting since it must be given directly into the vein and needs to be given every 6 hours, causing one treatment to fall in the middle of the night. It also must be given at a slow rate as giving any solution with a potassium base too fast can cause muscle cramping and in extreme cases can stop the heart. It is also almost always paired with a second IV antibiotic in order to provide coverage to a wider range of bacteria, as many times we are treating multiple types of bacteria at once or due to the critical nature of the case, are unable to wait a week for test results to tell us exactly what bacteria we are dealing with so we need to ensure we cover for the most common kinds of bacteria, which Penicillin alone is not able to do.
The second common formulation is procaine penicillin G. This is the thick white solution available at local retailers designed to go intramuscular or IM. While the ability to give procaine penicillin G directly in the muscle is much more ideal for on the farm use it has fallen out of favor with equine veterinarians and would be considered below the standard of care if used alone to treat most diseases. As it is available over the counter, this drug is often used without consulting a veterinarian and very often in my experience used at THE WRONG DOSE! When looking at the recommended dose on the bottle it may be off by almost a factor of 10 per day!!! The problem comes with the way drugs are approved and marketed in the US. As medicine changes the drugs are not necessarily relabeled with up to date doses or recommendations for use as changing the label claims is extremely expensive due to all the red tape, paperwork and research that would need to be done. This is less of a problem for veterinarians as it is legal for them to use drugs off label in certain situations and they are able to stay up to date with more recent dosing recommendations and change based on current research. But the average horse owner has no access to this information or correct dosing instructions without talking to their veterinarian.
Further problems also arise with the side effects of procaine penicillin G. Adverse reactions are not uncommon, even when it is attempted to be given correctly. Anaphylaxis or an allergic reaction is more common with this medication than others, but a more serious problem is accidental injection into a vein instead of in the muscle. When a large amount of procaine enters a vein or other vessel horses may have seizures, aberrant abnormal behavior, and may even die. In one study of 11 horses with adverse reactions 5 horses died (Aust Vet J. Nielsen IL et al. Adverse reaction to procaine penicillin G in horses. 1988 Jun;65(6):181-5.). This may sound easily preventable by pulling back on the plunger before giving the medicine, but when a horse has had many injections over several days it is easier to do as their muscles become swollen and irritated and blood vessels in the area may be dilated or enlarged. As a precaution horses given procaine penicillin G should be observed for a half hour after injection and if any abnormal behavior is observed the medication should be discontinued and a veterinarian should be called. Unfortunately it is often not possible to save a horse that is seizuring following a procaine reaction, and human safety should always be observed first- a seizuring or aberrantly behaving horse is extremely dangerous.
Many of the problems with Penicillin that I have seen would have been preventable if a veterinarian had been involved from the beginning. Although it might be tempting to try and treat that snotty nose or wound by yourself, a phone call to your local equine vet is never misplaced.
Have more questions? Call Great Lakes Equine at 920-779-4444 today.