WASHINGTON – The pet food linked to the deaths of 16 animals has shown no signs of contamination, the manufacturer says, and the company cannot explain why the cats and dog developed acute kidney failure and died.

In an interview with The Associated Press, Paul Henderson, the chief executive and president of Menu Foods, said Wednesday the company was looking at a single ingredient. He wouldn’t identify it, but the Food and Drug Administration has said the investigation was focusing on wheat gluten.

“Our hypothesis is that it is that ingredient that in fact represents the highest probability as to the cause,” Henderson said. “But we have been unable to prove that through scientific information.”

The animal deaths have led to the recall of 60 million cans and pouches of pet food sold throughout North America under 95 brand names.

“This is a puzzling and troubling experience for everybody within this organization,” Henderson said in an interview from company headquarters in Streetsville, Ontario. “It is extremely disheartening that we haven’t been able to find the causative agent.”

There have been tests on 10 cats that died performed by an outside company for Menu Foods. The results show only that the cats died of acute kidney failure.

There have been widespread reports of kidney failure, some fatal, in pets that have eaten the recalled brands.

Menu Foods last week recalled “cuts and gravy” style dog and cat food. The recall has sparked concern among pet owners across North America. It includes food sold under store brands carried by Wal-Mart, Kroger, Safeway and other large retailers, as well as private labels like Iams, Nutro and Eukanuba. Call centers hired by the company received 47,000 calls over the weekend, Henderson said.

“Our hearts go out to all of the pet owners across Canada, the United States and Mexico for any losses they experience and certainly for the worry this incident may be causing,” Henderson said.

The FDA has sent investigators to New Jersey and Kansas plants operated by Menu Foods.

Wheat gluten is a source of protein and was used to thicken the gravy in the pet food, sold in both cans and pouches, according to the FDA.

A complete list of the recalled products along with product codes, descriptions and production dates was available from the Menu Foods Web site, http://www.menufoods.com/recall . The company also designated two phone numbers that pet owners could call for information – (866) 463-6738 and (866) 895-2708.

Menu Foods is majority owned by Menu Foods Income Fund of Streetsville. The company also makes foods for zoo cats, but those products are unaffected by the recall.

Henderson said the company delayed announcing the recall until it could confirm that the animals had eaten its product before dying. Two earlier complaints from consumers whose cats had died involved animals that lived outside or had access to a garage, which left open the possibility they had been poisoned by something other than contaminated food, he said.

Wheat gluten itself would not cause kidney failure, but the common ingredient could have been contaminated by heavy metals or mold toxins, according to the FDA.

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On the Net:

FDA pet food recall information: http://www.fda.gov/oc/opacom/hottopics/petfood.html