Sharon L. Giovinazzo, president and chief executive of World Services for the Blind, was recently walking through an airport with her trained service dog Watson when a “pocket pooch” growled and then bit him, she said.

The owner apologized and said the dog was her service animal. Ms. Giovinazzo, an Army veteran who lost her sight to multiple sclerosis in 2001, was not having it.

“‘Yeah, yeah. Sure, sure, lady,’” she recalled telling the owner. “‘Then your animal should be secured and trained not to do that.’”

Ms. Giovinazzo said the dog was an untrained pet masquerading as a service animal in what advocates for people with disabilities said had become a growing problem in the last few years.