Injections of ivermectin are not recommended in horses because of instances of abscess with Clostridial infection and massive tissue destruction or death.  It's not common but has always been enough to stop me from doing it.

Oral ivermectin is very well absorbed.  It also has a wide safety margin so you can ramp up the dose if unsure of the horse's weight.  If symptoms return, consider getting skin scrapings to make sure the larvae are back.  Biting insects could actually be the problem.

Eleanor in PA
www.drkellon.com
EC Co-owner
Feb 2001