Los Angeles ? Allergies can cause misery for pets and humans alike. But allergies in animals are not always easy to diagnose and treat.

“I feel so bad because he can’t tell me what’s wrong,” said Angela Duyao of Gilroy, 80 miles south of San Francisco. Harley, her 2-year-old Shih Tzu, has food and seasonal allergies.

It took Duyao, an administrative assistant, 18 months and more than $1,000 to find out Harley was allergic to poultry and pollen.

All dogs and cats can get allergies, and the most common reaction is scratching, said veterinarian Donna Spector, an internal medicine specialist based in Deerfield, Ill.

“Allergies are a real head-banger. They are frustrating for vets, they’re frustrating for clients and the dogs and cats itch like crazy so we know it is frustrating for them. Allergies are very challenging to diagnose accurately because it’s a diagnosis of exclusion. It takes a lot of money and a lot of time. It takes a very dedicated owner,” she said.

There are four kinds of pet allergies: airborne (tree, grass and weed pollen; mold, mildew and dust mites), fleas, food and contact (like carpeting or detergent). The most common pet allergy comes from fleas.

People and pets can cause each other problems: People can be allergic to pet hair or dander, and pets can be allergic to products humans use.

Most pet allergies cause scratching. Some other symptoms include discoloration of hair between toes, rashes, open sores, watery eyes, ear infections, runny noses, vomiting and diarrhea, said Spector, a frequent guest on “The Ellen DeGeneres Show.” She is a consultant to the pet food company co-owned by DeGeneres called Halo, Purely For Pets.