Canada's food watchdog said Friday it has intercepted one shipment of corn gluten imported from China that tested positive for melamine and cyanuric acid.

Melamine, a chemical used to make plastics and fertilizers, was found in more than 100 brands of contaminated pet food that were recalled from the marketplace in Canada and the United States in mid-March. The manufacturers had used tainted wheat flour imported from China.

The Canadian Food Inspection Agency has tested 32 samples of wheat, rice, soy and corn gluten and protein concentrates coming into the country from China. The federal agency is also testing imports that entered the country over the past year.

"Shipments are being tested for both melamine and cyanuric acid since, although neither substance is believed to be particularly toxic by itself, their potency appears to be increased when they are present together," the federal agency said in a statement.

One in every five samples is also screened for other substances.

China's food and drug safety record has come under scrutiny in recent months, with investigators suggesting that Chinese companies are using potentially harmful ingredients in their products.

The U.S. Food and Drug Administration has fielded about 17,000 consumer calls about contaminated pet food and related pet illnesses since March.

On Thursday, U.S. health officials began checking shipments of toothpaste from China after thousands of tubes of imported toothpaste were withdrawn from the marketplace in other countries.

Health authorities in the Dominican Republic on Tuesday issued a recall of the Chinese-made Excel and Mr. Cool toothpastes after tests showed the products contained diethylene glycol, a chemical used in antifreeze and brake fluid. The toothpaste was also sold in Panama and Australia.