In the veterinary world, the act of vaccinating is a subtle art

Russ Daly
Why would one route of vaccine administration be preferred over another? From the medication’s point of view, there are real differences.

When, as a student, my career exploration journey reached the point when I was hanging around with livestock veterinarians, I remember being impressed with the efficiency and speed of how they wielded their syringes and needles while working large groups of cattle. Looking back at those early experiences, I now know that certain aspects of those vaccinations and treatments were lost on me.

My untrained eye was witnessing a syringe with a needle being stuck into a calf, the trigger on the pistol-grip syringe being pulled, and the syringe removed and ready for the next calf. But besides “in the calf,” where exactly was that vaccine or medication going?

The vast majority of “parenteral” products (those given non-orally) in livestock are labeled to be given either intramuscularly (IM) or subcutaneously (SQ). What’s the difference between these administration routes? And does it make a difference for the animal being treated?

It’s more common nowadays to see vaccine and drug labels requiring the SQ administration route. This is when the medication is deposited in the space between the skin and the muscle. Intramuscular medications, on the other hand, are given with a needle that passes through the skin, the underlying fat and connective tissue, and into the muscle tissue itself.

The logistics therefore are different between SQ and IM injections. If SQ administration is required, a shorter needle should be used, to avoid depositing the medication into the deeper muscle. Additionally, the needle should go into the animal at more of an angle instead of straight on. Some people advocate “tenting” (pulling up the skin with the non-syringe hand) and injecting into that skin fold. Personally, I prefer the safer and more efficient method of choosing a shorter needle and an angled approach. Intramuscular injections are just the opposite, utilizing a longer needle and a straight-on approach into the animal.

Why would one route of administration be preferred over another? From the medication’s point of view, there are real differences. Drugs deposited under the skin are more slowly absorbed from the injection site, and therefore last longer in the animal’s system. This might be important for some treatments: long-acting antibiotics, for example.

Drugs injected IM, on the other hand, enter tissue (muscle) that has much more blood supply than the area under the skin. As a result, they are absorbed more rapidly into the animal’s system and have the potential to reach higher concentrations throughout the body.

Knowing this, how is a veterinarian or livestock producer to decide on what administration route is best for a specific medication? That’s the easy part. It’s right there on the product label. The drug or vaccine company has already determined the route for which the specific medication works best and produces the least amount of swelling and irritation for the animal. That instruction should be followed to the letter.

What happens when, due to a jumpy calf or operator inattention, vaccine intended for SQ use ends up in the muscle? Or vice-versa?

Vaccine and medication labels reflect how the manufacturers tested those products. If something’s labeled IM, the company knows how the medication works…IM. When the calf gets that drug SQ, in the mouth, or otherwise, they really don’t know how that affects the disease or ailment we’re trying to address. It might work, it might not: they haven’t tested the medication that way.

In the case of antibiotics, effectiveness is just one consideration. Antibiotics given differently than the label states may require longer slaughter withdrawal times than what’s indicated on the label. This also constitutes “extra label drug use,” which is subject to more regulation than a use that’s completely by the books.

Some medications will give you the choice between IM and SQ. In that case, always choose the SQ route. At slaughter time, in case something went wrong with the injection such as an infection or abscess, that problem can be dealt with more easily if it can be peeled away with the skin rather than trimmed away from the cut of meat.

There’s more to giving calves injections than just getting it “IC” (“in the calf”)! Take a close look at the product label the next time you get your syringe out. Making sure the right route of administration is used might be the difference between that treatment working or not.

Russ Daly, State Public Health Veterinarian

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University.