The strain identified by the Veterinary and Food Board (VTA) and which has infected the majority people is termed ST87, which is a different strain from ST1247, the strain found multiple times over several months at fish processing plant M.V.Wool, leading to a VTA order for the company to cease its operations until it is clear of Listeria.

Of the 20 people who contracted Listeria infections, 12 were infected with ST87 and the type of bacteria infecting the remaining eight is unknown.

The VTA said the ST87 strains were passed on via food, though the source is not yet known.

"What kind of food [the ST87 Listeria strains] came though is currently being identified. We already have some data coming from the European Food Safety Agency, but we are not yet publishing it," said Elena Tomasova, deputy director general for health at the Health Board.

Tomasova noted that finding a foodstuff infected with LIsteria bacteria is difficult and the process is lengthy. Conclusions as to whether an illness comes from food consumed can be made by sequencing an entire genome. 

The latency rate for Listeria ranges from three to 70 days, meaning a person could first present with Listeriosis symptoms – fever, muscle aches and gastro-intestinal issues - as long as over two months following infection, with laboratory tests being the only way to determine potential sources, Tomasova said.

Discussing the issue on Thursday, the health board says it will be drawing up a food questionnaire to present to those falling sick with Listeriosis, as well as improving ways of identifying potentially infected food.

"This is perhaps the place to start, what these sick people have eaten and, above all, who they are and where they live, what they have eaten over the past three months and what they have come in contact with, where they have been, what eateries have they been to, etc." said Irja Lutsar, Professor of Medical Microbiology at the University of Tartu.

Lutsar added that consideration had been given to reducing the shelf life of foods not heat treated or preserved in brine or other substances.

Listeriosis tracked to M.V.Wool products (many of the positive samples found by the VTA were at its production facilities) was linked with some of its cold salmon and trout products, though ST1247 strains were even linked to heat-treated fish at least once.

Listeria bacteria can survive very low temperatures, though high temperatures can destroy them; those purchasing cold fish products such as smoked salmon or trout are advised to pre-heat them before eating; the very young, the elderly, and expecting mothers are particularly vulnerable to Listeriosis.

ST1247 has been linked with M.V.Wool products and the deaths of two people in Estonia and several more Europe-wide.

Other recent Listeria outbreaks reported in the media include at meat-packing company Oskari Lihatööstus, and at supermarket Rimi. In the first case, supermarket Selver cancelled its orders with the company as a supplier, though the VTA said Thursday that no new Listeria traces had been found following inspections at the company. Rimi itself closed its salad preparation kitchens and recalled all salad products from its shelves late last week after it detected Listeria traces on work surfaces of its own facilities, an example of self-regulation rather than awaiting VTA swoops and then fighting them in the courts as M.V.Wool has done.

The original Aktuaalne kaamera broadcast (in Estonian) is here.