I posted on fly-bite dermatitis two years ago, as part of a longer post. What brought it back to mind was this dog. Since he has one ear up and erect, while the other is down and floppy, we have one dog who shows the typical sore places for both styles of ear.
The mosquito causes a lot of problems for us, primarily heartworms in our area (malaria and other diseases in other countries). Their bites cause an allergic reaction that itches and swells, but it doesn't make a raw, sore place.
The mosquito mouth parts resemble a hypodermic needle, which pretty much leaves a self-sealing puncture. We may have problems with the irritating saliva (or parasites)that are left behind, but the wound doesn't stay open and bleed and get infected.
Flies don't have the needle-like proboscis of the mosquito. When they
bite, it's more like a knife and a sponge. They make a little cut, then
sop up the blood that runs out. Get a few hundred tiny cuts, and
you've got a big sore on the dog's ear. This will generally be on the part of the dog's ear that is the highest elevation of the pinna (the floppy part).
Repelling the flies is the prevention and the cure. When the weather warms up, start putting insect repellent on the ears every day. I like VIP ointment, as it it is not messy. You can just put a transparently thin coating on the hair.
If you have let things get away from you and there is already an open sore, the dog won't leave the insect repellent on. The insect repellent burns if you get it in the sore place. Slop fly-dope all over the sore ear, and watch the dog run away and rub it off just as fast as he can.
You need to put a first-aid cream with local anesthetic (like
Neosporin Pain Relief, or a generic equivalent) on the spots FIRST. This numbs the area, plus provides a physical barrier between the sore place and the insect repellent. Then cover the area
with insect repellent, like VIP ointment. Don't put the insecticide in
the sore places – it burns and they won't leave it on.
Bad spots will need to be treated twice daily.