articleSymptoms and sources of Yersinia enterocolitica-infection : a case-control study

Background: Yersinia enterocolitica (YE) is the causative agent of yersiniosis. YE encompass strains of diverse pathogenicity: YE biotypes 1B and 2-5 are considered pathogenic, whereas biotype 1A is in general considered nonvirulent. Also YE-like species, which can sometimes be misidentified as YE, are considered nonvirulent. Methods: In order to study differences in clinical picture caused by different YE types and their possible sources a casecontrol study was conducted in 2006. In this case-control study, 295 case-patients with YE or YE-like finding and their 758 controls responded to the questionnaire about symptoms and possible sources of infection. Results: Strains of pathogenic YE bio/serotypes 3-4/O:3 or 2/O:9 were found in 18%, YE biotype 1A in 65% and YE -like strains of 17% of the patients. Patients infected with the strains of pathogenic YE bio/serotypes were younger and had fever more often than those with BT 1A who suffered more from vomiting. Symptoms of reactive arthritis were reported by 10% of pathogenic YE infections, 3% of YE BT 1A, and 0.3% of the controls. Eating or tasting raw or medium done pork was a significant risk factor for pathogenic YE bio/serotype infection (OR 6.6; 95% CI 1.7-24.9) as well as eating in a canteen (OR 3.5; 95% CI 1.6-7.9). Imported fruits and berries were associated with increased risk of YE BT 1A finding. Conclusions: The symptoms of the patients with YE BT 1A differed from yersiniosis caused by the classic pathogenic YE bio/serotypes. In addition, the patients with YE BT 1A had more protracted gastrointestinal disorders and unspecific complaints. Small children were overrepresented in classic pathogenic bio/serotypes while in BT 1A or YE-like species were not found among children younger than two years. This suggests the lacking virulence of the BT 1A strains. We can not, however, rule out the possibility that some strains of genetically heterogeneous group of BT 1A may cause an illness. Background Yersinia enterocolitica (YE) is a zoonotic bacterial species that causes food-transmitted infections. The most common clinical manifestation of a YE infection is self-limited gastroenteritis, but extraintestinal manifestations and postinfectious sequelae such as reactive arthritis occur as well [1]. YE infections are usually sporadic, although outbreaks have been reported [2-5]. Small household infection clusters may be more common than recognized [6]. Pigs are a known reservoir of YE strains [1] and raw pork products serve as an important vehicle for infection [7]. For example in Norway, undercooked pork, sausage products and untreated water have been associated with YE infections [8]. In the United States, contaminated tofu [9] and pasteurized milk [2] were found to be vehicles for YE in outbreaks. YE species is divided into six biotypes (1A, 1B, 2, 3, 4, 5), which include several serotypes. The species encompasses both pathogenic strains including bio/serotypes 4/ O:3 and 2/O:9 and strains which are classically considered non-pathogenic, namely strains of biotype (BT) 1A or non-biotypable strains. Closely related YE-like species, Y. aldovae, Y. bercovieri, Y. frederiksenii, Y. intermedia, Y. kristensenii, Y. mollaretii, Y. rohdei, Y. alecksiciae, and Y. massiliensis, can often be misidentified as YE [10]. YElike species are also considered non-pathogenic[1]. However, several studies that suggest potential pathogenicity of BT 1A have been published [11-15]. In Finland, YE findings are notifiable and the National Institute for Health and Welfare (THL), formerly the * Correspondence: leila.sihvonen@thl.fi 2 Bacteriology Unit, National Institute for Health and Welfare (THL), Helsinki, Finland Full list of author information is available at the end of the article Huovinen et al. BMC Infectious Diseases 2010, 10:122 http://www.biomedcentral.com/1471-2334/10/122 Page 2 of 9 National Public Health Institute (KTL), maintains the National Infectious Disease Register (NIDR) to which clinical microbiology laboratories report both cultureand antibody-confirmed yersinia findings. However, laboratories are not required to provide the bio/serotype information of the YE strains. In Finland, diarrhoeic stool samples are routinely cultured for presence of Yersinia strains in addition to Salmonella, Campylobacter and Shigella. Approximately 500 YE cases are reported yearly. Most of the YE infections are sporadic and only a few verified outbreaks have occurred in Finland. The most commonly notified pathogenic YE bio/serotypes are 4/O:3 and 2/O:9. However, the majority of all the notified findings consist of BT 1A YE and YE-like strains [10]. Correct identification of the strains that cause clinical symptoms is important for collecting reliable statistics to benefit decision-making in health care and food handling sectors. Therefore, we previously reported a laboratory study where methods to identify all YE isolates accurately where described [10]. The current case-control study focuses on symptoms and risk factors of the YE infections. The aims are to determine the clinical features caused by different YE bio/serotypes, especially to compare the traditionally pathogenic YE bio/serotypes to YE BT 1A and to identify the sources of infection in Finland. Methods Study design All Yersinia strains (excluding Y. pseudotuberculosis) isolated from 41,848 stool specimens during January 1st 2006 through December 31st 2006 in 10 clinical microbiology laboratories were collected. Annually these 10 out of 22 laboratories report approximately two-thirds of all Finnish Yersinia cases. Laboratories were selected from different geographical locations to represent the whole Finland. The isolates were forwarded to the Bacteriology Unit (TABA; formerly Enteric Bacteria Laboratory), THL for further investigations. Identification of the Yersinia strains were described in detail earlier [10]. The amount of yersinia in stool samples was estimated semi-quantitatively by detecting growth on four streaking areas of CIN primary culture plates. If colonies that morphologically resembled Yersinia strains were detected only on the 1st streaking area or only after cold-enrichment, the amount of Yersinia bacteria in stool of a patient was estimated to be low. If there were colonies on the 2nd, 3rd or 4th streaking area, the amount was estimated to be high. Bio/serotypes 3-4/O:3 or 2/O:9 are here called pathogenic YE types. Two nonbiotypable YE strains were analysed together with BT 1A strains and they are all here collectively called BT 1A. During the study period, 462 YE and YE-like strains isolated from routinely submitted stool samples were found. Of those isolates, 29 were excluded from the present study, because another yersinia isolate from the same patient had already been included in the study. In addition, 27 isolates were excluded because also another microbe from the same patient was reported to NIDR (Campylobacter in 10 patients, Salmonella in 7, virus in 4, Shigella and Cryptosporidium in one patient each) or there was another YE finding within one week to half a year previously (four patients). Thus, 406 patients were accepted into this study. In a case-control study, for each case patient four agematched (same month and year of birth, widened if controls not found), gender-matched and geographicallymatched (same municipality of residence) controls were selected from the Finnish population register every two weeks. A detailed questionnaire was sent to each case immediately after their yersinia isolate was received by TASU. For the cases, questions on exposures covered the time period of two weeks before the onset of the symptoms (or if person had no symptoms, to the time period two weeks before the stool sample was given). For the controls, the time period covered two weeks before answering the questionnaire. A reminder with a new questionnaire form was sent two weeks later to the case or the control if there was no response. The questionnaire included questions related to symptoms and sequelae of yersinia infection as well as to specific exposures including consumption of different food items and travelling abroad. Questionnaires were identical for the cases and controls, except the questions concerning the yersinia infection. Instead, controls were asked if they had had diarrhoea or abdominal cramps with fever during the previous month or if they had ever had yersinia infection. Joint symptoms were asked identically from cases and controls. A probable reactive arthritis in a case or control was defined as swelling with gleam/redness in a joint and the onset of joint symptoms reported with the accuracy of 1-3 days. The study was approved by the Ethics Committee of THL. Statistical analysis Comparisons of age, gender and symptoms between patients with different bio/serotypes were made using either two-sample t-tests (age and duration of symptoms) or chi square test (others). Source analyses were done using univariate conditional logistic regressions, i.e. between cases and controls, separately for pathogenic YE and YE BT 1A. For each source, missing values (due to missing choice, "unknown" or "do not remember") were omitted from the analysis. Cases were included in the source analyses if the onset of symptoms was reported with the accuracy of 1-3 days and the time from the onset of symptoms to the stool sample was not longer than six weeks. Controls were included in the analyses if their case Huovinen et al. BMC Infectious Diseases 2010, 10:122 http://www.biomedcentral.com/1471-2334/10/122 Page 3 of 9 was included. All analyses were made using Stata version 9.2. Results Defining the patients to the study Of the 406 patients, pathogenic YE were found in stool samples of 74 (18%), BT 1A strains of 263 (65%) and YE like strains of 69 (17%) subjects. YE-like strains included Y. bercovieri, Y. frederiksenii, Y. intermedia, Y. kristensenii, Y. mollaretii and Y. rohdei (table 1). Patients with pathogenic YE ty