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Pets itch when they have fleas. But your pet will be especially miserable if she happens to be allergic to the tiny pests.

So how do you tell if your pet is itching from fleas or itching from allergies?

"In the past we have often told clients that one flea bite is enough to cause a reaction if your pet is allergic," says Christine L. Cain, DVM, assistant professor at the University of Pennsylvania School of Veterinary Medicine. "In reality, that probably isn’t the case,” she says. It often takes more than one. Plus, fleas will bite treated pets every now and again. Prevention doesn’t stop 100% of those pesky pests.

So what happens if your pet is allergic to fleas? Look for itchy spots, red bumps, or hair loss on the back half of her body, especially around the base of the tail, the inner thigh, groin, and stomach. Cats with flea allergies may also be itchy around their necks. Allergic cats can get crusty little bumps called "miliary dermatitis" all over their bodies.

Fleas usually avoid an animal’s feet and head, so if those areas are what’s bothering your furry friend, it’s probably due to something else, Cain says.

If your pet is allergic to fleas, you’ll probably notice her gnawing at irritated areas. Look out for the "lick, chew, rub, or roll," says William Miller, VMD, medical director of the Companion Animal Hospital at Cornell University’s College of Veterinary Medicine.

This makes matters worse -- leading to hair loss, raw skin, scabs, even infections. "Affected pets feel bad, look horrible, and often develop a severe staph infection, which could even kill the animal if it becomes very deep and widespread," Miller says.

If your pet gets a bacterial or yeast infection from her flea allergy, she might need to take antibiotics or an antifungal medication. And if she’s especially miserable, she might also need an anti-itch remedy such as an antihistamine or steroids.

Like people, pets can be allergic to mold spores, pollen, and dust mites. Animals with those allergies often struggle with fleas, too. What they are actually reacting to is the flea’s saliva, which contains all sorts of irritating things that can stir up the immune system. Cats tend to be a little less allergic in general than dogs. Certain dog breeds, such as terriers, Labradors, and golden retrievers, seem to be especially allergy prone, Cain says.