About five years ago, Gail Friedman noticed her Parson Russell Terrier was constantly licking his paws and seemed super uncomfortable.

“The poor dog. I would put baby socks on his feet so that he wouldn’t lick them or bite at them,” said Friedman, of Oak Brook, Illinois. “I was constantly changing the socks, washing his feet a lot. Nothing worked.”

It turned out her canine companion, Mr. Friedman, had allergies.

It’s a common and tricky problem in pets — caused by various things such as pollen, dust, mold, chemicals and food — but veterinarians say there are several ways to ease their suffering.

Allergies happen when the immune system overreacts to a foreign substance. Cats and dogs react to many of the same things people do, as well as pests like fleas.

There are no definitive recent statistics on how many pets have allergies, but research suggests the problem is growing.

“I probably see allergic dogs and cats every single day, probably multiple times a day,” said Dr. Karen Woodard, medical director at Thrive Pet Healthcare-Elmhurst in Illinois.

About 90% of allergic pets react to environmental triggers, Woodard said, and the rest have food allergies only.

Dog breeds that are especially vulnerable include various types of terriers, boxers and bulldogs; in cats, it’s Persians, Siamese and Himalayans.

Pets can even be allergic to other animals — cats to dogs, dogs to cats and either to another species.

“It’s possible for them to be allergic to us, just like we are to them,” said Thrive’s Dr. Anthea Elliott Schick of Scottsdale, Arizona, immediate past president of the American College of Veterinary Dermatology.

Allergic cats and dogs aren’t as likely as humans to sneeze and cough. More often, they scratch and lick themselves, shake their heads and develop ear infections.

Woodard said her Yorkie mix, Teddy, had the classic signs — scratching around his shoulders and getting rashes and ear infections starting as a six-month-old puppy. She lived in the South at the time, and he tested positive for allergies to various trees and grasses there.

A common sign of allergies in her feline patients is “overdoing their grooming,” said Woodard, who’s on the Chicago Veterinary Medical Association’s board of directors. “Cats shouldn’t be pulling their hair out when they groom. So if you start seeing bald patches on your cat, even though the skin underneath it looks normal, that could be a sign of allergies.”