Not much, some argue.
Most of the cans and pouches of recalled food contain variations of beef, chicken or lamb chunks, with gravy included. A couple boast rice. A couple include vegetables. But other than packaging and price, they have a lot in common.
"If you look at the labels, they're all very similar," said Jean Hofve, a Boulder, Colo., veterinarian who studies the pet food industry.
"It's like a cake. One might have buttercream frosting. You might rather have that particular cake. But it still has the same basic ingredients."
Alka Chandna, a senior researcher for the People for the Ethical Treatment of Animals, was more blunt.
"It's the magic of marketing," she said. "It's total consumer betrayal."
The Pet Food Institute, which represents pet food manufacturers, says that's not true. Different pet food brands have different formulas - some of which are closely guarded - said Duane Ekedahl, the institute's president. A company such as Menu Foods produces whatever formula its customers ask for.
Quinton Rogers, professor emeritus of animal nutrition from the University of California Davis Veterinary School of Medicine, agrees that one brand can be better than another, even if both come from a single manufacturer.
One company could request more of a certain ingredient, making the product a higher-quality buy. But a higher price doesn't necessarily guarantee a better product.
"There could be a zero difference," he said, between brands that are priced differently. Marketing, he noted, is a big part of the industry.
While pet food makers are under some government regulation - ingredients must be listed, for instance - the oversight is limited. Government inspectors rarely visit plants, because their primary focus is on food made for people.
While some criticize the industry for using meat and poultry byproducts - the remains of animals that can't be fed to humans, which can include, say chicken feet - Rogers says the use of those ingredients is "not a big deal."
If a dog were in the wild, it would eat the entire carcass of its prey, for instance. The manufacturing of food products made from animals - including foods intended for human consumption - is never a pretty picture, he noted.
Some say the pet food industry was a time bomb waiting to go off and the recall didn't shock them.
A recall two years ago involved dry food made by Diamond Pet Foods. More than 75 dogs died because of liver failure due to a fungus found on the product.
This time as many as 60 million cans and pouches of pet food were recalled, but not before at least 16 pets died of kidney failure caused by rat poison that somehow became mixed into the food.
The public is mostly clueless about the pet food industry and its practices, asserts Jan Rasmusen, a San Diego animal activist and author of "Scared Poopless: The Straight Scoop on Dog Care."
Some pet food includes the remains of road-kill and diseased and dying animals, Rasmusen said. Pet food makers also depend on carbohydrate fillers - such as wheat and corn - which are hard to digest and don't add much protein. Preservatives and additives are part of the mix, too.
"It's shocking what goes into the food," she said. "Switching brands is like tossing hand grenades. You simply hope that the next one doesn't explode."
The Pet Institute says all the ingredients that go into pet food are government approved for that purpose. It also argues that there's no reason to veer from popular mass-produced brands. Only 1 percent of the pet food supply was recalled, it notes.
Last year, U.S. pet owners spent an estimated $15.4 billion on pet food, the bulk of it for the estimated 90 million cats and 74 million dogs owned by U.S. households, according to the American Pet Products Manufacturers Association.
No. 1 on pet owners' minds is their pet's health, according to a survey released in February by the association. Store shelves are being lined with weight-management foods, sports drinks, energy treats, treatments and supplements, as well as an increase in organic and all-natural foods and treats.
Rasmusen is part of a growing number of pet owners who feed their pets raw or slightly cooked food. It's a myth, she said, that so-called "people food" is bad for pets.
That's how Americans fed their pets generations ago - before the advent of a commercial pet food industry.
It's too early to say if consumers will dramatically change their buying habits. But the recall apparently is having some effect.
Lucy Postins - co-owner of The Honest Kitchen, a San Diego company that produces raw, dehydrated dog food made from a mix of human-food-grade ingredients that include chicken, potatoes, apples and zucchini - said the recall has left many pet owners feeling "duped."
"I think people have lost confidence," said Postins. "They thought they were feeding their pets something good because it was a premium brand with a pretty label, but everything is made in the same facilities and with the same ingredients as things sold at a fraction of the price."
Her online business sales have tripled this week, she said.
Marilyn Reese had nothing to worry about when the recall occurred. She belongs to a co-op that provides her with raw meat for her two German shepherds and two foster dogs. The Lakeside, Calif., woman made the change three years ago because "I heard too many horror stories about pet food."
About 16 pet owners belong to the co-op, which gets meat from wholesalers in nearby Orange County.
"It makes sense," she said. "You don't see wolves out in the forest grinding wheat to put in their food."
But that's hardly practical for many pet owners. Consumer advocates advise pet owners to read the labels to get a handle on what they're buying.
Just because a label may have the word "beef" on it doesn't necessarily mean it's loaded with beef. Other ingredients could supersede it.
The list of ingredients is key. The one leading the list is the most prominent. Experts suggest looking for brands with meats at the top of the list.
Correspondent Penni Crabtree contributed to this report.
