3. Parasites. Fleas, ticks, mange, and mites are just a few of the many parasites that can cause hair loss and/or bald patches. According to Fetch WebMD, Flea bites are behind most dog allergies&For a particularly allergic dog, it may take as little as one flea bite to start an allergic reaction that lingers for days. Some clues that your pooch may have a parasite infestation would include scratching, biting, knawing at his skin, redness, and inflammation.