Intramuscular injections are commonly given to horses by veterinarians and animal owners, and it is not uncommon for complications to occur. The most common one is infection and abscessation at the injection site and in most cases, the infection is due to introducing bacteria in the tissue when the needle is advanced in the skin; this occurs with two methods. First, if the needle is not sterile or if there is a large amount of bacteria on the skin, the bacteria is taken into the wound on the outside of the needle. Secondly, when a needle pierces the skin, a process called coring occurs in which a tiny piece of skin is cut off by the needle and taken deep into the tissue. Along with the tiny piece of tissue, bacteria and tiny pieces of hair can be taken into the tissue as well.
It has been shown that the smaller size of the needle used, there is less potential for coring and thus less tissue is taken into the muscle. Also, clipping the hair actually increases the amount of hair taken into the tissues, and shaving the injection sites increases coring. A study was performed in Germany using 18 gauge needles and 22 gauge needles and results indicated that skin coring was increased in haired skin with the larger gauge needles. So when vaccinating your horse or giving an injection, using the smallest gauge needle possible should decrease the change of infection. Now, this is assuming that the hair coat is not soiled, in which case cleaning it first may be required. However, in general, clipping the hair from an injection site actually increases the chance of infection. If you are giving an injection to your horse, make sure you are using the smallest needle possible, clean the area but don’t clip the hair or shave the site, and use a sterile needle.