In a nation still struggling to insure humans against illness, insurers say fewer than two million Americans have policies for their pets - maybe 1 percent of the potential market. In England and Sweden, 25 percent or more of pet owners have policies.

But marketing to veterinarians and animal drugmakers who sell expensive new therapies has boosted sales and prospects for Newtown Square-based Petplan and others in the small-but- growing ranks of U.S. pet insurers.

Bosses and backers of Petplan, founded by English expats Chris and Natasha Ashton when they were Wharton students coping with high vet bills for her cat, thought it a measure of fiscal validation recently when rival Trupanion, the Seattle company that owns American Pet Insurance Co., filed to raise $89 million in an initial public stock offering.

If the partial-share sale goes through as planned, it would imply a corporate value of more than $400 million, and a potentially lucrative investor market for Petplan and other industry pioneers.

Trupanion, which underwrites its own policies, is a bigger business - $84 million in sales last year vs. $53 million for Petplan, which markets policies from global insurer Allianz.

But Petplan backer Vernon Hill, the Moorestown-based banking and real estate magnate who helped the Ashtons with their business plan and became an early investor, says Petplan's model makes it more likely to become the first U.S. pet insurer to pass $1 billion in yearly sales.

Petplan "owns the customers and runs the program, with insurance policies from a big company. So they don't have to worry about the level of state regulations, or state capital constraints," Hill said.

"Trupanion, which is an insurance company, has never made a nickel," Hill added. "Petplan is a pet company that happens to sell insurance; it's been profitable for the last four years. That's fundamentally different."

Trupanion's plans and its bold predictions of rapid sales growth and higher profits are laid out in recent filings with the Securities and Exchange Commission.

It's always "Take Your Dog to Work Day" at Petplan, which employs 120 at its modern headquarters up the hill from SAP AG.

"We're the only tenant allowed to do this" by landlord Equus Capital Partners, says Chris Ashton, a former British military commando. He and Natasha had a tough time finding local properties where dogs were allowed when they decided to move from smaller quarters near Philadelphia International Airport last year.

"We love dogs," Equus principal Dan DiLella said. He said it had been a pleasure doing business with the Ashtons.

"On the West Coast this is common - you can't rent a lot of properties without allowing pets," Natasha Ashton says. "But it's only starting to creep to the East."

It can be done: On a visit to the skylit space recently, with large and small animal companions parked at workers' desks or chowing down at an office pet lounge, I smelled no whiff of dog stink, heard no annoying barks.