Twelve people have been hospitalised with listeria infections, nine of them having become ill after eating deadly Quargel cheese produced by Styrian firm Prolactal GmbH.

The Austrian Agency for Health and Food Safety (AGES) also reported today that a 57-year-old man became the seventh person to die from eating the tainted cheese. The previously known deaths -- four Austrians and two Germans --occurred in 2009.

AGES said that all infections occurred before Prolactal's tainted cheese was taken off supermarket shelves on January 23.

Health authorities have struggled to link the listeriosis deaths to Prolactal's cheese, because the cases occurred only sporadically and the disease has a long incubation period.

A Prolactal spokesman said,

"A comprehensive investigation that will determine the cause of the contamination is our highest priority."

The firm said it had received more than 500 calls as of Wednesday last week on the hotline it set up for concerned consumers on 0800-201080.

The relatives of the six people who died are planning to sue Prolactal.

According to a Eurosurveillance report earlier in Feb., approximately 16 tons of Quargel per week are produced by the Austrian manufacturer. Fifty-three per cent of the product is exported to the German market and small amounts to the Czech Republic, Poland and Slovakia. This cheese is made of curdled milk, which ripens after addition of starter cultures for one day at 28°C, and after being sprayed with Brevibacterium linens for another two days at 14°C. The shelf life after packing and marketing is two months.