COPENHAGEN, Denmark (AP) -- Danish health officials fear more than 4,000 people may be infected with salmonella and are checking everything from refrigerators to credit card receipts to find the source of what may be the worst outbreak in 15 years.

Kare Moelbak of the Ministry of Health said 330 cases have been confirmed and about a quarter of those people have been hospitalized. No deaths have been reported.

He said officials at the government's center for prevention and control of infectious diseases say the real number probably exceeds 4,000 people.

Moelbak said he suspects the source is some sort of Danish food product distributed only in Denmark, since neighboring countries have not reported an outbreak. They believe it probably is meat, but they do not know which product.

"Food control units are out to visit patients and see what they have in their refrigerators. We have even had access to electronic files to get an overview of what people have bought using their credit cards," he said.

A serious salmonella outbreak in the United States has hit more than 800 people, and health authorities there continue to search for the source of contamination.

Moelbak said a few cases were reported in Denmark as early as February, but that the number has risen alarmingly over the past six weeks. There are no signs of it slowing down; about 30 new cases are confirmed per day.

"We fear it will continue at this pace," said Moelbak. He said it was difficult to predict the course of such outbreaks, but that there was no indication it would stop soon.

Salmonella is seldom fatal among healthy adults, but can cause diarrhea, nausea, vomiting, abdominal cramps, fever and headaches.

The outbreak was of Salmonella typhimurium U292, a fairly rare type of the roughly 2,500 kinds of salmonella.

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