Escherichia coli O157:H7 outbreak linked to raw milk cheese in Quebec, Canada: use of exact probability calculation and casecase study approaches to foodborne outbreak investigation.

The analytical studies used to investigate foodborne outbreak are mostly case-control or retrospective cohort studies. However, these studies can be complex to perform and susceptible to biases. This article addresses basic principles of epidemiology, probability, and the use of case-case design to identify the source of an Escherichia coli O157:H7 outbreak linked to raw milk cheese consumption in Quebec, Canada; a small number of cases with the same pulsed-field gel electrophoresis (PFGE) profile were involved. Between 4 December 2008 and 15 January 2009, a cumulative total of 16 E. coli O157:H7 cases with the same PFGE profile were reported to Quebec public health authorities. Among the first six cases reported, three had consumed raw milk cheese from the same producer (cheese A). Raw milk cheese is consumed by about 2 % of the Quebec population. By using the exact probability calculation, it was found that a significantly higher proportion of E. coli O157:H7 cases (with the specific PFGE profile) than expected had consumed cheese A (P < 0.001). These computations were updated during the course of the investigation to include subsequent cases and gave the same results. A case-case study corroborated this result. This article considers alternative statistical and epidemiological approaches to investigate a foodborne outbreak-in particular with an exact probability calculation and case-case comparisons. This approach could offer a fast and inexpensive alternative to regular case-control studies to target public health actions, particularly during a foodborne outbreak.

Regional profiling for determination of genotype diversity of mastitis-specific Staphylococcus aureus lineage in Canada by use of clumping factor A, pulsed-field gel electrophoresis, and spa typing.

One of the major concerns in global public health and the dairy industry is the emergence of host-specific virulent Staphylococcus aureus strains. The high degree of stability of the species genome renders detection of genetic microvariations difficult. Thus, approaches for the rapid tracking of specialized lineages are urgently needed. We used clumping factor A (clfA) to profile 87 bovine mastitis isolates from four regions in Canada and compared the results to those obtained by pulsed-field gel electrophoresis (PFGE) and spa typing. Twenty-five pulsotypes were obtained by PFGE with an index of discrimination of 0.91. These were assigned to six PFGE lineage groups A to F and seven spa types, including two novel ones. Group A had 48.3% of the isolates and group D had 43.7% of the isolates, while only 8% of the isolates were variable. The results of antimicrobial susceptibility testing indicated that all isolates were sensitive to methicillin and the non-beta-lactam antibiotics, while three isolates were resistant to penicillin and one isolate was resistant to tetracycline. All isolates had the clfA gene and belonged to 20 clfA repeat types with an index of discrimination of 0.90. The dominant clfA types, types X, Q, C, and Z, formed 82% and 43% of PFGE groups A and D, respectively, and had copy numbers that varied only within a narrow range of between 46 and 52 copies, implying clonal selection. The rest were variable and region specific. Furthermore, the dominant groups contained subpopulations in different regions across Canada. Sequence information confirmed the relatedness obtained by the use of clfA repeat copy numbers and other methods and further revealed the occurrence of full-repeat deletions and conserved host-specific codon-triplet position biases at 18-bp units. Thus, concordant with the results of PFGE and spa typing, clfA typing proved useful for revealing the clonal nature of the mastitis isolate lineage and for the rapid profiling of subpopulations with comparable discriminatory powers.

Ceftiofur resistance in Salmonella enterica serovar Heidelberg from chicken meat and humans, Canada.

The Canadian Integrated Program for Antimicrobial Resistance Surveillance describes a strong correlation (r = 0.9, p<0.0001) between ceftiofur-resistant Salmonella enterica serovar Heidelberg isolated from retail chicken and incidence of ceftiofur-resistant Salmonella serovar Heidelberg infections in humans across Canada. In Quebec, changes of ceftiofur resistance in chicken Salmonella Heidelberg and Escherichia coli isolates appear related to changing levels of ceftiofur use in hatcheries during the study period, from highest to lowest levels before and after a voluntary withdrawal, to increasing levels after reintroduction of use (62% to 7% to 20%, and 34% to 6% to 19%, respectively). These events provide evidence that ceftiofur use in chickens results in extended-spectrum cephalosporin resistance in bacteria from chicken and humans. To ensure the continued effectiveness of extended-spectrum cephalosporins for treating serious infections in humans, multidisciplinary efforts are needed to scrutinize and, where appropriate, limit use of ceftiofur in chicken production in Canada.

Panton-Valentine leukocidin in pediatric community-acquired Staphylococcus aureus infections.

PURPOSE: Panton-Valentine Leukocidin (PVL) is an exotoxin produced by strains of Staphylococcus aureus (SA). Its importance as a virulence factor is controversial. We aim to further characterize the role of PVL in pediatric community-acquired SA infections. METHODS: In a cohort study conducted from July to November 2006, we prospectively collected all strains of SA isolated at the Montreal Children's Hospital causing community-acquired infections in children aged 18 years or younger. The strains were analyzed for the presence of the PVL encoding genes by PCR and were phage typed. Strains resistant to methicillin or pvl+ were analyzed by pulsed-field gel electrophoresis. A medical chart review blinded to patient pvl status was performed to retrieve demographic and clinical data. Data were analyzed by logistic regression. RESULTS: We identified 74 pediatric community-acquired SA infections. Nineteen strains (25.7%) were positive for the pvl genes. Four isolates (5.4%) were resistant to methicillin and three of these were pvl+. No predominant clone was identified by phage typing or pulsed field gel electrophoresis. Pvl+ and pvl- infections were statistically similar for patient age, hospital admission, length of hospital stay, invasive disease, intravenous antibiotics and outcomes. Pvl+ strains were more likely to cause abscesses (OR 20.79; 95% CI 4.93 - 87.58), less likely to cause superficial skin infections (OR 0.18; 95% CI 0.05 - 0.64) and less likely to be resistant to erythromycin (OR 0.048; 95% CI 0.004 - 0.52). CONCLUSIONS: In a clonally heterogeneous population of pediatric community-acquired SA infections, pvl+ strains were associated with abscess formation and erythromycin susceptibility, but not invasive disease.

Sporadic infections of Salmonella Paratyphi B, var. Java associated with fish tanks.

OBJECTIVE: Identify a source of contamination to explain the increase in the number of cases of S. Paratyphi B var. Java infection. METHOD: A descriptive study was conducted to gather more information about cases and potential risk factors for infection. Between January 2000 and June 2003, we identified and interviewed by questionnaire 53 people with S. Paratyphi B infection. Due to the suspected link with fish tanks, an environmental investigation was conducted for each case at patients' homes as well as at pet stores and wholesalers. Various samples were taken from fish tanks. The fish were purchased at different pet shops. The pulsed field gel electrophoresis method was used to compare the human and aquarium strains. RESULTS: Of those infected, 60% had bought or had contact with an aquarium or tropical fishes before becoming ill. More than 50% of the samples taken from fish tanks in homes, pet shops, and wholesaler operations contained a number of serotypes of Salmonella in addition to Salmonella Paratyphi B var. Java. There were similarities between the PFGE patterns of human strains and aquarium samples. DISCUSSION: Contact with fish tanks could be a risk factor for developing a Salmonella infection and the public may be unaware of this risk. It was recommended that information be produced and distributed to pet shops and their customers to inform them of safety precautions in order to reduce the risk of contamination from bacterial pathogens that may be present in aquarium water.