Exposure of Listeria monocytogenes within an epidemic caused by butter in Finland

Data on the levels of bacteria and the amounts of food consumed in food-bor ne outbreaks provides an excellent opportunity to study the effects of expo sure to Listeria monocytogenes. Between June 1998 and April 1999, an outbre ak caused by L. monocytogenes serotype 3a in butter occurred in Finland. Th e majority of the cases were immunocompromised and hospitalized at the Hels inki University Central Hospital (HUCH), where 7-g butter packages produced by a dairy plant were used as the only butter brand. The butter had also b een sold to 10 other central hospitals as well as to the retail market. Bas ed on the data on hospital stay, butter consumption and the qualitative and quantitative analyses of L. monocytogenes in butter, the attack rates and exposure were estimated. Incubation studies on the naturally contaminated s mall butter packages showed that the levels found in the packages at the ti me of detection of the outbreak could reliably be used for these estimation s. However, the levels of L. monocytogenes in 500-g packages increased. The attack rate among HUCH patients varied from 70 to 117 cases per 1000 patie nts at risk, depending on which estimate of the contamination level of butt er (100-60%) was used. The highest single dose (7.7 x 10(4) CFU in one meal ) could have been sufficient to cause the listeriosis cases at HUCH. Howeve r, this data also supports another hypothesis, according to which these lis teriosis cases were caused by a prolonged daily consumption of contaminated butter during the hospital stay. The estimated daily dose, based on the ho spital kitchen data or the highest detected level in a wholesale sample (11 ,000 CFU/g), would have varied from 1.4 x 10(1) to 2.2 x 10(3) CFU/day or f rom 2.2 x 10(4) to 3.1 x 10(5) CFU/day, respectively. The choice of the hyp othesis has a crucial impact on the interpretation of this data for the dos e-response estimations as well as for the discussion on Food Safety Objecti ves. Due to the susceptibility of hospital patients, special care must be t aken in order to avoid even low levels of L. monocytogenes in food served. (C) 2001 Elsevier Science B.V. All rights reserved.