By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - U.S. health officials received thousands of complaints earlier this year about pets killed by contaminated pet food, but veterinarians said on Thursday they had been able to confirm just 224 deaths.

A major recall was begun last March after ingredients imported from China were found to have contaminated some pet food. Canadian manufacturer Menu Foods Income Fund was hardest hit, recalling 60 million packages of pet food.

The U.S. Food and Drug Administration said it received 17,000 complaints of related pet deaths, although it had confirmed just 16.

A survey posted on the Internet, and widely publicized by the American Veterinary Medical Association, attracted just 500 responses and of those, only 348 cases of pet sickness met the criteria for kidney failure caused by the contamination, the team at Michigan State University found.

"It is easy to think that every death or every sickness is occurring because of the pet food problem," Wilson Rumbeiha, who worked on the study, said in a telephone interview.

But when strict criteria were applied, it appears that far fewer deaths could be blamed on the pet food.

The deaths from contaminated pet food may have been caused by mixing two compounds -- melamine and cyanuric acid, Rumbeiha said.

"Separately, those two compounds are pretty harmless," Rumbeiha said.

"But when combined, they form crystals which can block the kidneys. And, unfortunately, these crystals don't dissolve easily. They go away slowly, if at all, so there is the potential for chronic toxicity."

The Canadian-based Menu Foods makes pet foods sold under labels such as Iams, Eukanuba, President's Choice as well as store brands sold at Wal-Mart, major grocery chains and pet stores.

Rumbeiha and colleagues designed a questionnaire to determine how many animals had become ill or died from eating bad food.

They found 348 cases that met the criteria for what he called kidney disease caused by the compounds -- 236 cats and 112 dogs.

"Two-thirds of the animals affected were cats, but proportionally, more dogs died from it than cats," Rumbeiha said in a statement.

Almost all the cases were in the United States, with the other 2 percent from Canada.

The first analysis covers responses received through June -- more vets submitted cases through October, and Rumbeiha said his team is now analyzing those. He said vets in all 50 U.S. states and from across Canada had filled out the survey.

"What this does is provide a snapshot of this disease. It helps us to better characterize the disease. The good news is we are not seeing any new cases," Rumbeiha said.

His team worked with the American Association of Veterinary Laboratory Diagnosticians and pathologists from the University of Guelph in Ontario and Colorado State University to do the survey, presented at a meeting of the American Association of Veterinary Laboratory Diagnosticians.

(Editing by Will Dunham and Eric Beech)