When I left my dog Harper with a pet sitter recently while I was at a conference, I didn’t have too many concerns. Harper is a cavalier King Charles spaniel, a breed that’s known for being friendly and outgoing. A cavalier’s motto is usually “Love the one you’re with.”

But when I called to find out how things were going, I received the surprising news that Harper was barking nonstop when she was left alone. She was fine if the pet sitter was there, but even pet sitters have to leave the house sometimes, and Harper was not pleased about being crated in her absence.

Dogs who break housetraining, chew destructively — especially at doors and windows — or bark or howl in distress when left alone aren’t necessarily being bad, according to the upcoming book “From Fearful to Fear Free” (scheduled for publication in April 2018). They may be suffering from separation anxiety.

“Besides being noisy or destructive, dogs with separation anxiety may drool excessively, pace, lick themselves incessantly, or refuse to eat or drink,” write co-authors Dr. Marty Becker, Dr. Lisa Radosta, Dr. Wailani Sung and Mikkel Becker.

Dogs can learn from an early age how to be comfortable when left alone.

We start by leaving our dogs crated for short periods, gradually increasing the amount of time we’re gone. They always get a treat when we leave so that our departure is a positive experience, and returns are low-key to encourage the dog to remain calm.

Although she still looks and acts like a puppy, Harper is 10 years old. Was her change in behavior due to advancing age, I wondered? Veterinary behaviorist Dr. Sung says that’s not necessarily the cause, noting that only a small percentage of the cases of separation anxiety she sees involve older animals. But separation anxiety can occur at any age and may be related to changes in the dog’s life.

“When the family or owner schedule gets disrupted, the animals have more difficulty adjusting, and sometimes they become gradually distressed over time,” she said.

For pets with mild cases of separation anxiety, Dr. Sung has some advice:

“Maintain the same schedule and routine,” she says. “Provide both physical and mental exercise through walks and food-dispensing puzzle toys.”

If your dog shows signs of severe anxiety, check with your vet to see if medicine or behavior modification under the guidance of an experienced trainer or a veterinary behaviorist can help.