MULTIPLE ANALYSIS OF A FOODBORNE OUTBREAK CAUSED BY INFANT FORMULA CONTAMINATED BY AN ATYPICAL SALMONELLA VIRCHOW STRAIN

An outbreak of food poisoning involving most autonomous Spanish commun ities was detected in the first half of 1994. The causative food was i nfant formula milk contaminated by lactose-fermenting Salmonella virch ow. It was not possible to isolate the causative strain from the manuf acturer's facilities. During the same period of time, there was a sign ificant increase in lactose-non-fermenting Salmonella virchow strains compared with the same period in previous years. Simultaneously, lacto se-non-fermenting strains were recovered from clinical samples from ch ildren and from some milk samples that were involved in the outbreak. Therefore, it was speculated that the outbreak might be more extensive than initially thought. The following epidemiological markers were us ed for typing the Salmonella virchow strains involved in the outbreak: (i) phage typing; (ii) ribotyping, using a set of 20 different endonu cleases; and (iii) pulsed-field gel electrophoresis, using three diffe rent endonucleases. The most useful markers for this serotype were pha ge typing and pulsed-field gel electrophoresis, since ribotyping was n ot able to distinguish all strains tested. The results obtained reveal ed that the outbreak was caused by at least two strains: one presentin g phage type 4-4a and pulsed-field patterns Al or A2 and L+ or L-, and another presenting phage type 2 and pulsed-field patterns A1 or A2 an d L+ or L-. The results indicate that the outbreak was more extensive than initially thought and that the Virchow serotype is very clonal in Spain.