NEW YORK (NYTIMES) - A salmonella outbreak linked to onions has expanded to 43 states and Canada, prompting a recall from a producer in California and various grocery chains, health officials said.

As of Sunday, there had been 640 reported salmonella cases, including 85 hospitalisations, tied to the outbreak, the Food and Drug Administration said.

Many of the reported cases were in the western US states of Utah (90), Oregon (85), California (76) and Montana (52).

There have also been 41 cases reported in Illinois.

The Public Health Agency of Canada reported 239 cases with 29 hospitalisations as of last Friday.

No deaths have been reported in Canada or the US.

The FDA identified the likely source of the outbreak as red onions from Thomson International, a produce supplier in Bakersfield, California.

Last week, the producer recalled red, yellow, white and sweet onions shipped since May 1 because of the risk of contamination.

The FDA has started an investigation at the company.

The onions were distributed to wholesalers, restaurants and retail stores across the country and in Canada, Thomson said last week.

The producer declined to comment on Sunday.

The Centres for Disease Control and Prevention has recommended that consumers throw away any onions or foods made with onions supplied by Thomson, or any related products that have been recalled.

Health experts say to throw away onions or products containing them if consumers are unsure of who supplied the onions.

The illness, named salmonellosis, can persist for four to seven days, the CDC said.

Children, older adults and people with weaker immune systems are most at risk of developing severe illness.

Symptoms include diarrhoea, fever and abdominal cramps.

Those who are severely ill may also experience a high fever, headaches, a rash or bloody stool.

It is likely that a substantial number of cases will be reported over the next few weeks because of the lag between when a person eats onions and gets sick, and when that person is tested and has an agency report the illness, said Prof Martin Wiedmann, a food safety professor at Cornell University.

He recommended not to consume the onions, even if they have been cooked, because cross contamination with utensils may have occurred.

"It's better to be safe than sorry," he said.

"Just don't use them."

The CDC estimates that salmonella causes about 1.35 million infections and 26,500 hospitalisations in the US each year.

Although most people recover without treatment, more than 400 deaths in the country annually are tied to acute salmonellosis.

Salmonella can be spread when the hands, surfaces and tools of food handlers are not clean, and when people eat raw or undercooked food, the FDA said. It can also be spread to people from animals.

Contamination can occur at many points during the harvesting and processing of produce - from the water used on the fields, from wildlife or even from an employee with salmonella at a factory, Prof Wiedmann said.

"No food ever will be zero risk," he said.