Listeriosisis mainly affect human foetuses, neonates and persons with an underlying condition resulting in weak immune systems.
A study of three companies that produce farmed salmon in different parts of Norway identified 15 types of Listeria monocytogenes. A total of nine of the 15 types were of a genetic variant that scientists have also found in patients with listeriosis, the illness that can be triggered by L. monocytogenes.
“This background in not sufficient for us to claim that fish are the sources of the cases of listeriosis in our study. But on the other hand, we cannot ignore this possibility. Salmon are one of several potential sources of L. monocytogenes,” says NIFES scientist Bjørn Tore Lunestad.
Listeria bacteria have long been known to present a challenge in food preparation, including seafood production. The bacteria occasionally appear on premises where food is prepared, for example on equipment that is worn or difficult to keep clean. Listeria are extremely resistant, but heat treatment kills them off.
Mr Lunestad emphasises that listeriosis often appears as isolated cases, and much more seldom in the form of outbreaks in which two or more people are affected by the same bacteria from a single source. In the course of the past ten years, between seven and 50 cases have been diagnosed in Norway. In nearly all cases of food-borne listeriosis, we have been unable to identify which item of food was responsible.
In Norway, three outbreaks whose sources are known have been registered, and none of them were linked to seafood.
In 2007, 21 people were diagnosed with listeriosis, where the bacteria could be traced back to cheese produced in a dairy farm. In 2005, three cases were confirmed in Ålesund, and the source was probably processed meat and the machinery used to slice it. In 1992 processed meat was the source of an outbreak in Trøndelag County, in which eight cases of listeriosis were registered.
The salmon study is a collaborative effort involving NIFES and the Norwegian Institute of Public Health. The research team isolated bacteria from production companies and compared them with bacteria collected from listeriosis patients. Nine of the 15 strains identified were identical to bacteria taken from these patients. This means that we cannot ignore salmon as one of several potential sources of listeriosis in humans, even though the study does not prove the existence of such a connection.
The study has been published in Epidemiology & Infection.
