ARCHIVED - Home or away? Investigation of Salmonella Enteritidis PFGE pattern SENXAI.0003 and SENBNI.0003, phage type 8, in the Maritimes, 2005

 

Canada Communicable Disease Report

15 October 2006

Volume 32
Number 20

A Currie, MHSc (1), H Akwar, DVM, PhD (2), W MacDonald, MD (2), Andrea Saunders, RN, MSc (1,3), M Baikie, BScPhm, MD, DOHS, DTM&H,MSc, FRCPC (3), L Sweet, MD (4), L Landry, MSc (5),W Demczuk, BSc (6), L Panaro, MDCM, MHSc, FRCPC (1)

  1. Canadian Field Epidemiology Program, Public Health Agency of Canada, Ontario

  2. Department of Health and Wellness, Fredericton, New Brunswick

  3. Nova Scotia Department of Health, Halifax, Nova Scotia

  4. Department of Health and Social Services, Charlottetown, Prince Edward Island

  5. Foodborne, Waterborne, Zoonotic Infections Division, Public Health Agency of Canada, Guelph, Ontario

  6. National Microbiology Laboratory, Public Health Agency of Canada, Winnipeg, Manitoba

Introduction

From data obtained through the National Enteric Surveillance Program (NESP), the Department of Health and Wellness in New Brunswick (the Department) identified six isolates of Salmonella Enteritidis in May and June 2005. These were thought to be sporadic cases until pulse field gel electrophoresis (PFGE) and phage type (PT) results linked the organisms (PFGE patterns SENXAI.0003 and SENBNI.0003, PT 8). Upon review of provincial surveillance data, the Department identified an additional two isolates of S. Enteritidis with matching PFGE patterns and PT, one in September 2004 and one in March 2005. These PFGE patterns and PT had been observed previously, independently and in combination, but September 2004 was the first time the Department had identified S. Enteritidis yielding all three typing outcomes. The identification of six isolates with matching PFGE profiles by two enzymes and a matching phage type over a 2-month period in the province of New Brunswick was unusual and suggested a common source.

The investigative team reviewed surveillance data in Canada and from the United States through NESP, PulseNet, PulseNet Canada, and the Canadian Integrated Program for Antimicrobial Resistance Surveillance. Nova Scotia and Prince Edward Island had identified six isolates of S. Enteritidis SNEXAI.0003 and SENBNI.0003, PT 8, in February and March 2005. Colleagues in the United States reported a cluster of cases due to S. Enteritidis SENXAI.0003 and SENBNI.0003 (PT not available) in New Hampshire in June 2005. The cases were patrons who had eaten at one restaurant during a 7-day period in late May, but investigators were not able to identify a source. According to the available data, no other provinces or states had identified matching isolates. A continuous common source for the infections in the three Canadian provinces was suspected, and a joint investigation was initiated on 13 July, 2005.

This article summarizes the epidemiologic investigation that disproved the continuous common source theory. The investigation highlighted the importance of understanding the limitations of national surveillance data and laboratory typing methods for common serovars of Salmonella, and the need to interpret laboratory data in conjunction with epidemiologic data.

Methods

The investigative team defined confirmed cases as all persons residing in or visiting New Brunswick, Nova Scotia, or Prince Edward Island who with laboratory-confirmed S. Enteritidis (SENXAI.0003, SENBNI.0003, PT 8) since 1 August, 2004.

The Department notified health regions in New Brunswick to encourage case finding. Health regions were requested to collect stool samples from all individuals reporting symptoms compatible with salmonellosis and to forward all Salmonella isolates as per the routine protocol to the New Brunswick Region Two Enteric Reference Laboratory for confirmation of serotype and PFGE analysis1-6 and the National Microbiology Laboratory (NML) for phage typing7-9. The Department, NML, the Foodborne, Waterborne and Zoonotic Infections Division, and provincial public health authorities in Nova Scotia and Prince Edward Island reviewed surveillance data for isolates with matching PFGE patterns and PT.

The Department posted an alert on the Canadian Integrated Outbreak Surveillance Centre (CIOSC) Public Health Alerts system to notify public health authorities across Canada and request patient information for matching isolates. The New Brunswick Region Two Enteric Reference Laboratory posted information about the investigation on PulseNet Canada and requested other provincial microbiologists to review and monitor surveillance data for matching isolates.

Public health inspectors interviewed confirmed cases in New Brunswick in accordance with the routine protocol for Salmonella notifications. Questions addressed demographic, clinical, and diagnostic information; high-risk occupations; illness among family, friends, and co-workers; environmental risk factors (e.g. travel, animals, water); restaurant dining; special events; and a 5-day food history or general food history when recall was limited.

Routine interview forms for confirmed salmonellosis cases in New Brunswick were reviewed to summarize risk factor information and to generate a detailed hypothesis-generating questionnaire addressing known risk factors for infection10-30. Where possible, the investigative team conducted hypothesis-generating interviews with cases whose symptoms had begun since 1 May, 2005, and obtained exposure histories for patients with clinical isolates matching those reported by other provinces.

Results

The investigation identified a total of 18 confirmed cases in the Maritimes between September 2004 and July 2005: nine cases in New Brunswick, five cases in Nova Scotia, and four cases in Prince Edward Island. Cases ranged in age from 7 to 69 years (mean age 28 years). Eight of 15 cases (53%) were female. One case in New Brunswick reported symptom onset on 30 August, 2004. For the remaining cases, symptoms began from 19 February to 30 June, 2005 (Figure 1). Investigators obtained symptom information for New Brunswick cases only. All nine cases experienced diarrhea and abdominal cramps. Seven of nine cases (78%) had nausea, six of nine cases (67%) fever, and two (22%) reported vomiting. Six of nine cases (67%) observed blood in their stool. Four cases (44%) were hospitalized. All cases recovered.

Figure 1. Cases of Salmonella Enteritidis SENXAI.0003, SENBNI.0003, PT 8, by date of symptom onset, New Brunswick, Nova Scotia, and Prince Edward Island, August 2004 to July 2005 (n = 17)

Figure 1. Cases of Salmonella Enteritidis SENXAI.0003, SENBNI.0003, PT 8, by date of symptom onset, New Brunswick, Nova Scotia, and Prince Edward Island, August 2004 to July 2005 (n = 17)

Note: One confirmed case from Nova Scotia was asymptomatic.

Public health staff conducted routine interviews with all cases with the exception of those in Prince Edward Island. Investigators conducted hypothesis-generating interviews with seven of the nine New Brunswick cases but were unable to contact cases in the other provinces. On the basis of the interviews, Salmonella exposures likely occurred in three different countries (Figure 2).

Cases exposed in the Czech Republic

Three cases from New Brunswick aged 19 to 20 years were in the Czech Republic 3 days before their symptoms began. They traveled together from 12 to 25 May, 2005, with 13 other Canadian youths. All denied eating a common food purchased in Canada and taken with them on the trip. Two of the cases became ill on 17 May, and the third became ill during the return flight on 25 May. The cases reported that others they had traveled with or visited were also ill throughout the trip. Provincial authorities reported to the investigative team that the same strain of S. Enteritidis had been isolated from youths returning to Nova Scotia from the Czech Republic in 2002.

Cases likely exposed in Jamaica

Three of five cases from Nova Scotia, three of four cases from Prince Edward Island, and two cases from New Brunswick reported travel to Jamaica prior to symptom onset. The cases from Nova Scotia, aged 27, 28 and 30 years, traveled to Jamaica from 14 to 21 February, 2005, and became ill before returning to Canada (19 to 21 February). The cases from Prince Edward Island, aged 36, 41 and 50 years, traveled together to Jamaica from 21 to 28 February, 2005. Two cases reported symptom onset on 27 February, and one became ill on 1 March. Two cases from New Brunswick, aged 29 and 32 years, returned from different resorts in Jamaica 5 and 8 days before symptom onset respectively. One traveled in August 2004 and the other in April 2005. One of these cases also reported routinely thawing chicken in hot water at home.

Figure 2. Cases of Salmonella Enteritidis SENXAI.0003, SENBNI.0003, PT 8, by location of exposure and date of onset, New Brunswick, Nova Scotia, and Prince Edward Island, August 2004 to July 2005 (n = 17)

Figure 2. Cases of Salmonella Enteritidis SENXAI.0003, SENBNI.0003, PT 8, by location of exposure and date of onset, New Brunswick, Nova Scotia, and Prince Edward Island, August 2004 to July 2005 (n = 17)

Note: One confirmed case exposed in Nova Scotia was asymptomatic.

Provincial authorities in Quebec and Saskatchewan responded to the CIOSC alert. Saskatchewan identified one matching isolate in June 2005, and Quebec identified 12 isolates since 1 January, 2005 (two in January, one in February, six in April, two in May, and one in June). Quebec provided exposure information for eight of the twelve cases. Six cases had traveled to countries including Jamaica and Mexico in the week before they became ill.

Cases exposed in the Maritimes

The seven remaining cases (aged 7 to 69 years; 71% male) were exposed in their respective Maritime provinces. Investigators re-interviewed three of the four cases in New Brunswick and the two cases in Nova Scotia and identified no food items in common. Investigators were unable to obtain exposure information for the case from Prince Edward Island.

Discussion

This investigation was initiated because it was unusual to identify six isolates of S. Enteritidis with matching PFGE and PT profiles within a 2-month period in New Brunswick. According to the national and provincial laboratory and epidemiologic surveillance data initially reported, isolates matching this profile were restricted to New Brunswick, Nova Scotia, and Prince Edward Island, and all but one case became ill between February and June 2005. The initial hypothesis was exposure to a continuous common source. However, support for this hypothesis diminished throughout the investigation.

Most of the Maritime cases acquired their illness while traveling in the Czech Republic or Jamaica. As the investigation proceeded, Nova Scotia and Quebec reported the same strain of Salmonella isolated from Canadian travelers to the Czech Republic in 2002 and from twelve Quebec residents in 2005, at least half of whom reported travel to countries including Jamaica and Mexico. As a first step in the investigation of a potential outbreak, dates of illness, incubation, and travel should be reviewed to distinguish travel-acquired illness from that acquired in Canada necessitating domestic investigation.

Public health professionals should be aware of the limitations of national surveillance data used to provide context in outbreak investigations. National molecular and phage typing databases are recent developments, they rely on voluntary provincial participation and are therefore incomplete. PFGE is conducted at varied frequency and on different organisms and isolates in each of the provinces. Results are not always accessible at the federal level. All Salmonella isolates from smaller provinces are forwarded to the NML for phage typing. However, Ontario, Quebec, British Columbia and Alberta only forward isolates cultured from the first to fifteenth day of each month. Complete national typing databases are required in the long-term. Full provincial participation is a prerequisite and should be encouraged. In the short-term, the limitations of national surveillance data should be understood, and provincial health authorities should continue to assist their colleagues through timely communication of provincial surveillance data not available at the national level.

It is also important for investigators to understand the limitations of typing methods and the need to interpret laboratory results in conjunction with epidemiologic data. S. Enteritidis is among the top three most frequent Salmonella serovars isolated from human sources in Canada, and PT 8 accounted for 11% of the 3,806 S.Enteritidis isolates phage typed from 2001 to 200431-33. Different clones of PT 8 isolates exist and can be transmitted to humans from different sources. However, the origin of these clones and their genetic makeup are similar34-36, which makes it difficult to differentiate the clones within isolates typed as PT 8 by DNA fingerprinting methods such as PFGE. As observed in this investigation, PT 8 isolates from different sources frequently yield similar PFGE patterns36-38. Typing methods with higher resolving power than PFGE and phage typing are required to provide good strain discrimination for identification and investigation of outbreaks due to S. Enteritidis38-40.While current molecular and phage typing methods aid outbreak detection by linking apparently unrelated cases, final confirmation of the existence of an outbreak relies on collaborative interpretation of laboratory and epidemiologic data together41.

Acknowledgements

The authors would like to thank Mark Allen, Nina Van Der Pluijm, and Kendra Long, Department of Health and Wellness, Fredericton, New Brunswick, for their contributions to the investigation.

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