First, you don't do injections intradermally--that would be into the layers of the skin itself. What she described is subcutaneously---in between the skin and the muscle but that isn't always an easy task in horses because they have less loose skin. So, right off the bat I would call back and talk directly to the vet to get an appropriate dose and directions for giving the injection. This is an off-label use of this product, meaning that it hasn't been studied for safety or efficacy in horses. There have been severe adverse reactions to giving ivomec injections in horses.
Licensed Veterinary Technician