Health officials in Australia have linked a rise in cases involving a deadly strain of drug-resistant bacteria to onions imported from the U.S., which they say can become contaminated with human and animal feces that cannot be removed by typical washing procedures, according to the Daily Telegraph. The so-called superbug bacteria, known as Clostridium difficile, or C. diff, is one of the most common hospital transmitted bacteria. It causes severe diarrhea and kills an estimated 29,000 people in the U.S. every year.

The bacteria, which first spiked in Australia in 2011, infects at least 150 patients in New South Wales hospitals every month and has been detected in 450 hospitals across the country, health officials said. “Looking at the nasty strain in patients we found [the genomes] were identical so there had to be a common source and the most likely source is food,” Thomas Riley, a professor of microbiology at the University of Western Australia, told the Daily Telegraph. “We overlayed imports of onions at the time for this particular bug and it was a perfect match.”

Spores of the bacteria are hard to detect on produce, which can become tainted with C. diff through contact with fecal matter. Imported onions do not get tested for C. diff before being sold in Australian supermarkets, according to ABC Brisbane. “For a start we should insist all our food imported has the same standards we insist on here and it needs to be checked at the border, it’s an area we need much more information,” Riley said.

Symptoms of C. diff include severe diarrhea, abdominal pain, fever and weight loss. The toxins released by the bacteria in the digestive tract often cause irreversible colon damage. In the U.S., C. diff infects nearly 500,000 Americans every year. U.S. lawmakers have called it a “national security risk.”

Health experts have blamed the over-prescription of antibiotics in hospitals as the cause of C. diff outbreaks. Antibiotics disrupt the balance of good and bad bacteria in the human body, which allows superbugs like C. diff to thrive.