Melons contaminated with deadly listeriosis bacteria were exported to at least nine countries from Australia, where an outbreak has killed seven people and caused one miscarriage, the World Health Organization said on Monday.

The rockmelons, or cantaloupes, were sent to Hong Kong, Japan, Kuwait, Malaysia, Oman, Qatar, Singapore, the United Arab Emirates and Bahrain, and may also have gone to the Seychelles, a WHO statement said.

Between January 17 and April 6, Australia reported 19 confirmed and one probable case of listeriosis, all of whom were hospitalised. Seven died.

The Listeria monocytogenes bacterium has a potentially long incubation period, usually one or two weeks but possibly up to 90 days, so more cases may be reported, the WHO said.

"Cases in the affected countries may still be identified," it said.

The Australian melon producer, which the WHO did not name, recalled the fruit on Feb. 27.

On March 2 Australian authorities discovered the firm's melons had been exported, and they sent detailed notifications through the International Food Safety Authorities Network to the countries concerned.

"It is believed that the cause of the outbreak was a combination of environmental conditions and weather contaminating the surface of the fruit, with low levels of the bacteria persisting after the washing process," the WHO said.

"The grower continues to work closely with the relevant authorities and has returned to supply rockmelons (during the week starting 2 April) after testing cleared the property."

Listeriosis can come in a mild form that causes diarrhoea and fever in healthy people within a few days.

But it also has a severe form that can cause septicemia and meningitis among more high-risk people, such as pregnant women, infants, old people, and people having treatment for cancer, AIDS or organ transplants.

The severe form has a 20-30 percent mortality rate.

As well as unwashed fruit, high risk foods include dairy products made of unpasteurized milk, soft cheeses, deli meat products, ice creams, raw seafood, crustaceans and shellfish.

A separate listeriosis outbreak in South Africa killed at least 180 people earlier this year, sparking a class-action lawsuit against South African food producer Tiger Brands.