Research seminar 10.11.2009: Infection routes of the bacterium Yersinia pseudotuberculosis – practical viewpoints on investigating outbreak


<p> <em>Yersinia pseudotuberculosis</em> repeatedly since the beginning of 1997 has caused outbreaks in Finland. From 1997 to 2008, ten outbreaks caused by <em>Y.pseudotuberculosis</em> were registered, mostly related with school meals. In widespread outbreaks, in excess of 400 persons fell ill during one single outbreak, the majority then being school or day-care children. From 1997 to 2001, <em>Y. pseudotuberculosis</em> caused outbreaks via iceberg lettuce, and later through domestic carrots stored over the winter.</p>

Researcher Saija Hallanvuo, Microbiology Research Unit
Evira/Helsinki Viikki, Auditorium C111 Kalevi
Tue 10.11.2009, 3:00 – 4:00 pm

Y. pseudotuberculosis as cause of food poisoning
Y. pseudotuberculosis causes stomach disease, the symptoms being fever, intensive stomach pain and infrequently diarrhoea. This stomach pain can be similar to that of an inflammation of the appendix and triggers unnecessary surgery. Complications such as reactive arthritis and skin-symptomatic erythema nodosum may occur. Y. pseudotuberculosis is common in the environment and has its origin in many animals, particularly rodents, deer, hares and birds. Infections often are transmitted via contaminated food or water. Y. pseudotuberculosis is able to reproduce while cold-stored, and for carrot-borne epidemics, long storage periods starting in autumn and ending even as late as the next summer most probably have made possible the reproduction of the bacteria to levels that give rise to food poisonings.

Modern food distribution chains challenge outbreaks investigations
Current food distribution chains present challenges to investigations of food-borne outbreaks as these comprise of often minor, geographically distributed disease clusters, which are difficult to recognize as belonging to the same outbreak. Hence, co-operation between authorities and laboratories is of primary importance for the identification and investigation of outbreaks. Since 1997, the molecular biological genotyping method PFGE has been applied to trace the infection routes of Y. pseudotuberculosis bacteria. This has proved an efficient tool for laboratory-based monitoring of outbreaks, by which one reliably can identify that geographically scattered disease clusters are parts of the same outbreak. Only through research combining epidemiology and PFGE typing was possible to decide with sufficient reliability that iceberg lettuce and carrots are food that transmit outbreaks.

Different research methods versus outbreaks investigations
Cultivation methods based on cold enrichment traditionally have been used to define the presence of Y. pseudotuberculosis bacteria in samples collected while investigating an outbreak. Cold enrichment perfectly is applicable to analyzing, e.g., environmental samples collected in carrot storages, but because of long cold-enrichment periods it may take up to three weeks before achieving research results. An alternative appearing in later years is a PCR method based on duplicating individual genes for a bacterium, by which one in significantly shorter time (about 48h) can define appreciably smaller numbers of Y. pseudotuberculosis bacteria (<10 cells/25g sample) than using cultivation methods. In food that transmits outbreaks, the number of Y. pseudotuberculosis bacteria has usually been minute, and only PCR methods have brought the possibility to detect bacteria in food samples. With regard to outbreaks investigations, isolating bacteria with cultivation methods still is required, as without an isolated bacterium strain and its genotyping, it would not be possible completely to explain an outbreak.

Bacteria lairs in food premises that handle carrots
In carrot-borne outbreaks Y. pseudotuberculosis bacteria has been found in carrot storage rooms by using PCR as well as cultivation methods and it has been possible to isolate the cause for subsequent typing. The bacteria have been tough to prevail on surfaces in storage rooms; this still has been shown several months after an outbreak. Mechanical washing has been of prime importance to evict these bacteria, as without washing the surfaces, even large volumes of disinfectants have not affected the bacteria. In plant premises for handling carrots, Y. pseudotuberculosis has been found by using each method or the PCR method only, e.g., at conveyors associated with the washing phase. At the cleanest side of processing (e.g., sorting and packaging), so far no bacteria have been found on surfaces, however, using the PCR method, they have been found as isolated occurrences on carrots that have passed the process.

Research to monitor Y. pseudotuberculosis bacteria
Research on Y. pseudotuberculosis bacteria (by former TavastLab, currently Kokemäenjoen vesistön vesiensuojeluyhdistys ry, Hämeenlinna Laboratory) routinely has been done for some time on samples of fresh vegetables and grated food. So far bacteria have been found only in food samples associated with epidemics, so finds in food should be taken seriously. Evira in spring and summer 2009 last issued instructions on handling carrots. According to Evira’s instruction 10501/1, food premises handling domestic carrots should monitor the presence of Y. pseudotuberculosis bacteria in the production environment by taking samples targeted at the spring season (from January onwards until the carrots from the previous harvest are being used). In addition, Evira has issued recommendations to carrot farmers, carrot storages and food premises handling carrots on handling procedures that reduce the presence of Y. pseudotuberculosis bacteria on carrots.

The lecture is in Finnish.

Additional information:
Researcher Saija Hallanvuo, Microbiology Research Unit
tel. +358 2077 24135, saija.hallanvuo


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