Outbreak of Uncommon O4 Non-Agglutinating <i>Salmonella</i> Typhimurium Linked to Minced Pork, Saxony-Anhalt, Germany, January to April 2013

<div><p>Introduction</p><p>In January 2013, the National Reference Centre for <i>Salmonella</i> (NRC) detected a salmonellosis cluster in Saxony-Anhalt, Germany, caused by uncommon O4 non-agglutinating, monophasic <i>Salmonella</i> (<i>S</i>.) Typhimurium DT193. Circulating predominant monophasic <i>S</i>. Typhimurium DT193 clones typically display resistance phenotype ASSuT. We investigated common exposures to control the outbreak, and conducted microbiological investigations to assess the strains’ phenotype.</p><p>Methods</p><p>We conducted a case-control study defining cases as persons living or working in Saxony-Anhalt diagnosed with the O4 non-agglutinating strain between January and March 2013. We selected two controls contemporarily reported with norovirus infection, frequency-matched on residence and age group, per case. We interviewed regarding food consumption, especially pork and its place of purchase. We calculated odds ratios (ORs) with 95% confidence intervals (95% CI) using logistic regression. The NRC investigated human and food isolates by PCR, SDS-PAGE, MLST, PFGE, MLVA and susceptibility testing.</p><p>Results</p><p>Altogether, 68 O4 non-agglutinating human isolates were confirmed between January and April 2013. Of those, 61 were assigned to the outbreak (median age 57 years, 44% female); 83% cases ≥ 60 years were hospitalized. Eating raw minced pork from butcheries within 3 days was associated with disease (31 cases, 28 controls; OR adjusted for sex: 3.6; 95% CI: 1.0-13). Phage type DT193 and MLST ST34 were assigned, and isolates’ lipopolysaccharide (LPS) matched control strains. Isolates linked to Saxony-Anhalt exhibited PFGE type 5. ASSuT- and ACSSuT phenotype proportions were 34 and 39% respectively; 54% were resistant to chloramphenicol. Three pork isolates matched the outbreak strain.</p><p>Discussion</p><p>Raw minced pork was the most likely infection vehicle in this first reported outbreak caused by O4 non-agglutinating, mostly chloramphenicol-resistant <i>S</i>. Typhimurium DT193. High hospitalization proportions demand awareness on the risk of consumption of raw pork among elderly. LPS analysis indicated O4 expression; therefore, testing with antisera from different lots is recommendable in unexpected agglutination reactions.</p></div

Silver-stained SDS-gel of LPS.

<p>Lanes: 1) protein ladder, 2)– 5) smooth isolates of outbreak strain, 6) rough isolate of outbreak strain, 7) biphasic <i>S</i>. Typhimurium control strain NTCC12023, 8) monophasic <i>S</i>. Typhimurium control strain 06–01900.</p