When you receive injections, your doctor first wipes your skin with alcohol to prevent bacteria from entering the wound made by the hypodermic needle. When delivering an injection to your horse, your veterinarian skips this step.
Why? Because the density of your horse's coat means that the application of alcohol to an injection site does little, if anything, to reduce the risk of infection. If your horse is filthy, your veterinarian may ask you to wash and dry the target area, but chances are she'll just wipe the top of the medication bottle with alcohol and leave the horse alone.
Fortunately, infections at injection sites are not common in horses. Occasionally, a horse may develop a little localized swelling or even a small pus-filled abscess after receiving a shot. These typically heal with little treatment. In rare cases, a horse may develop a severe, life-threatening infection caused by clostridial0 bacteria. This is characterized by a large swelling that is hot and painful to the touch. This risk is the reason veterinarians prefer to administer injections to the side of the neck where any infections would be easier to drain and treat.
Veterinarians do, however, disinfect the skin before making an injection near a horse's joints. The immune system cannot easily reach the fluids and tissues inside a joint, and any infections that take hold there can become catastrophic. Before injecting a joint, your veterinarian will scrub the skin with a topical disinfectant and may shave the coat to further reduce the chances of contamination.