Andy lost his appetite. Then came the vomiting, the unquenchable thirst, the constant need to urinate. Over several days last year, the spunky 4-year-old West Highland white terrier grew lethargic and lost more than 10 percent of his weight.

“It got bad,” said Andy’s owner, Alfredo Gude, a retiree in Cape Coral, Fla. “I knew that he was in trouble.”

Gude and his wife rushed Andy to their veterinarian, who referred him to a clinic 15 miles away. Doctors there sent a urine sample to a specialized metabolic lab at the University of Pennsylvania. Days later, test results confirmed the diagnosis: Fanconi syndrome, a rare, often fatal illness that affects the kidneys. The suspected cause: chicken jerky pet treats manufactured in China.

The incident is part of a troubling mystery lasting more than seven years, with reports of at least 600 dogs dying and thousands of others sickened. It has outraged unsuspecting pet owners, confounded the Food and Drug Administration and put the pet food industry’s manufacturing practices under a microscope.

Since 2007, when the FDA warned of an apparent link between jerky treats and sick and dying dogs, agency officials have spent countless hours trying to nail down what might be behind the illnesses, with little success.

The agency continues to seek leads from veterinarians and pet owners, sorting through the 1,500 reports that have come in since a public appeal for information in the fall. Now, companies that voluntarily pulled some treats from the market last year are bringing reformulated versions back to store shelves, even though the government has been unable to figure out why pets continue to get sick.

Bernadette Dunham, director of the FDA’s Center for Veterinary Medicine, has called it “one of the most elusive and mysterious outbreaks we’ve encountered,” a sentiment echoed by others at the agency.

“We are frustrated,” said Martine Hartogensis, who oversees the FDA’s ongoing investigation. “It’s been a long, winding, twisting road . . . [But] we haven’t given up.”

The FDA says it has tested more than 1,200 jerky treats in recent years, looking for salmonella, mold, pesticides, toxic metals, outlawed antibiotics, nephrotoxins and other contaminants. Federal officials have inspected factories in China that manufacture chicken jerky products for U.S. companies and sought input from academics, state and university research labs, foreign governments and the pet food industry. The agency even made its own jerky treats to try to duplicate the commercial process.

Still, it hasn’t figured out the cause.

FDA investigators say that the illnesses overwhelmingly affect dogs but that some cats have been made ill. The majority of complaints involve chicken jerky but also include treats in which chicken or duck jerky is wrapped around dried fruit, sweet potatoes or yams. Officials said there’s no clear pattern based on breed or geography — pets have been sickened in every state, as well as in countries such as Australia — and the problems don’t seem specific to any particular brand or manufacturer.

Until the agency is able to zero in on a specific toxin, officials say, it can do little more than warn people about the potential dangers of jerky treats. “Unless a contaminant is detected and we have evidence that a product is adulterated,” the agency said in one update, “we are limited in what regulatory actions we can take.”

The long-running investigation has paralleled a striking increase in the amount of pet food China exports to the United States. That volume increased from barely 1 million pounds in 2003 to an estimated 86 million pounds by 2011, according to the FDA.

Pet treats, including the jerky treats at the heart of the current investigation, have made up a fast-growing sliver of the pet food market. Part of the reason many U.S. companies have looked to China to produce chicken jerky treats, industry officials say, is that unlike in America, people in China overwhelmingly prefer dark meat. That leaves a larger supply of the white meat used in pet treats available for exporting.

Given the continuing problems, the FDA’s efforts simply aren’t good enough, say some pet owners.

“It’s maddening that it has gone on this long,” said Susan Thixton, who runs the Web site TruthAboutPetFood.com, which has repeatedly demanded that the agency do more. “If this were humans dying, and they couldn’t figure out a cause for seven years, members of Congress would be screaming at them.”

The home page of her site displays a clock tracking how long jerky treats from China have been killing and sickening pets. It asks: “When will FDA make this clock stop?” As of Friday, the count stood at 2,643 days.

“My job is to point out that they aren’t doing their job,” Thixton said. “I have a lot of respect for what they have to accomplish. They have huge responsibilities, but this is one of them.”

Angry pet owners also have heaped criticism on U.S. companies that continue to manufacture jerky treats with ingredients from China. The backlash includes everything from skepticism over the industry’s assurances that the treats have never posed health risks to lawsuits alleging harm.

Early last year, two industry giants, Nestlé Purina and Del Monte, voluntarily pulled several popular chicken jerky treats made in China off the market after New York state agriculture officials found trace amounts of antibiotics.

The brands included Nestlé Purina’s Waggin’ Train and Canyon Creek Ranch jerky treats as well as Milo’s Kitchen treats produced by Del Monte’s pet food division (now known as Big Heart Pet Brands).

The companies insisted that the treats had never been unsafe for pets and that the withdrawals were precautionary.

Complaints related to jerky treats dropped sharply in the months that followed. But the FDA agreed with company officials that the reduction probably reflected the smaller number of products on store shelves rather than any problems involving the antibiotics.

A spokeswoman for Big Heart Pet Brands said that this month, the company is introducing reformulated Milo’s Kitchen treats, manufactured in the United States with domestically sourced ingredients.

After revamping its manufacturing process and overhauling its supply chain, Nestlé Purina also has relaunched several kinds of Waggin’ Train treats, including two varieties that are made in the United States. The company still produces its Chicken Jerky Tenders in China but now gets its chicken there from a single U.S.-owned supplier, which oversees the process “from egg all the way through to treat,” said Bill Cooper, Nestlé Purina’s vice president of manufacturing, who declined to name the supplier. He said the company now routinely tests for 40 types of antibiotics and detailed its new quality controls in meetings with FDA officials.

“We have added to the strict protocols we already had in place,” Cooper said, noting that the company received very few complaints even before last year’s voluntary withdrawals. “We certainly believe that we have the highest level of quality.”

Nina Leigh Krueger, head of the Waggin’ Train brand, said most retailers and customers have welcomed the treats back. “Thousands of consumers have been calling and asking us for Waggin’ Train treats to be back on the market,” she said.

Terry Safranek is not one of them.

“It’s a kick in the gut to see them back on the shelf,” said Safranek, whose 9-year-old fox terrier, Sampson, who had eaten jerky treats, died of kidney failure in 2012. Since then, Safranek has become a plaintiff in a class-action lawsuit against Nestlé Purina and retailers including Target and Wal-Mart. She helped create Animal Parents Against Pet Treats and Food Made in China, a group that has petitioned the FDA to do better in alerting people about the potential dangers of jerky treats produced in Chinese factories.

For now, on Florida’s west coast, Andy the terrier has returned to normal after months of treatments — about $3,500 worth — to restore his kidney function. “We feel very lucky,” said Gude, who has taken the advice of many vets around the country to steer clear of pet jerky treats altogether. “It could have gone another way.”

At the FDA, a small cadre of investigators, their offices littered with jerky treat packages, continues to chase leads from the latest batch of reports, hoping to finally decipher a mystery seven years in the making.

“They want to solve it more than anything,” said Hartogensis, the FDA official overseeing the effort. “I’m confident we’ll get there. It’s a really complicated issue, but we’ve got a lot of great people working on it. And I think we’re getting closer.”