Dogs with fleas often bite compulsively. They chew up and down their limbs as if they're nibbling on a corncob because they are so itchy. Dogs also roll on their backs to scratch the itching skin that can't be reached with their paws. The skin may have red bites, rather like mosquito bites, or large raw, wet areas called hot spots. As the allergy worsens, the hot spots coalesce and become large open sores. Bacteria and yeast enter the skin and infect it creating oozing, pus-filled areas called pyodermatitis (Pyo means pus).
Cats with fleas may bite compulsively. They scratch at their head and neck because it itches. Flea-allergic cats can be so over-stimulated to groom that they actually barber their hair and develop bald areas. If this weren't enough, cats with flea allergies can develop miliary dermatitis, alopecia, eosinophilic plaque, and lip ulcers.
Flea bites occur along the back and hips, inner thighs, and the belly. These areas are difficult for your pet to reach to scratch and groom. With time, flea dermatitis can enlarge from its original area to become a generalized problem.
Find the fleas on your pet or use the presence of flea dirt to confirm your suspicion of fleas. Some prefer to confirm flea bite allergies by doing skin tests that show your pet's immune response to flea proteins.
If you part your pet's coat, you may find tiny copper-colored or blackish pepper flakes, about 1/8 inch long. Unless the flea infestation is severe, you may see nothing unusual because fleas are programmed to live in dark areas. They dash out of sight to hide as you part your pet's hair. Instead of finding fleas, you may find flea dirt, which looks like dark dust. When flea dirt is damp, it becomes red because it is made of blood. It's very difficult to see flea eggs because they are only 1/64th of an inch in size and because they fall off the pet and to the floor. If you have a microscope and want to look for flea eggs, search along moldings, in cracks between boards, or in leaf litter. Eggs aren't normally seen on the pet.
You might suspect fleas are causing your pet's skin problems, but can't find them. This frequently happens with cats because cats can groom so fastidiously after they are bitten that they destroy the flea. Instead of looking for fleas, look for flea dirt. To find flea dirt, brush your pet over a white surface, such as a white rubber bath mat or dish drainer. Lightly dampen the dirt that falls on the mat and look for tiny streaks of reddish brown. That reddish brown material is your pet's blood.