Bess, an arthritis sufferer, is put through her paces on an underwater treadmill

Eleven-year-old Bess waits patiently to be immersed in water just a little cooler than her body temperature.

Her hydrotherapy session will last only 17 minutes but it does wonders for this arthritis sufferer. Bess is a cat.

To ease her joint pain, the Maine Coon, a touch overweight at 21 pounds (10 kilograms), is taken every week to the Friendship Hospital for Animals, a veterinary clinic on a pleasant Washington street.

Beyond traditional veterinary treatment, the clinic offers forms of care once reserved for humans, such as acupuncture and therapeutic ultrasound.

Pets are now increasingly seen as genuine family members, said Christine Klippen, one of the 63 veterinarians who are available at the clinic every day of the year.

And this is particularly true for millennials, she said, who see themselves as moms and dads to their "fur babies." Given the higher incomes in the US capital, they