I got this from Equisearch.com
The horsefly-sized insects known as warbles lay eggs on their hosts' legs. Once the warble larvae hatch, they burrow into the skin and migrate through the tissues. When they reach a likely spot-usually the horse's back-they form anthill shaped lumps on the skin surface. Each larva's apartment has a tiny breathing hole that oozes pink fluid. Getting rid of warbles is tricky, since crushing one in the skin can set off a possibly fatal allergic reaction. A veterinarian must carefully widen the breathing holes and then draw the larvae out. Ivermectin kills warble larvae before they can migrate.
Sounds like a nasty bugger