During the investigation period, 51.9% of SE cases in Ontario were TR. This finding was surprising to us as unpublished Reportable Disease data by Vrbova et. al., indicated that the proportion of TR cases for salmonellosis was approximately 20%. This finding was consistent, however, with a 2010 publication of findings from a sub-population of Ontario. Of the SE cases identified in the Region of Waterloo, 48.7% were classified as TR for the period June 2005 - May 2009 [11
]. In our study, the TR SE cases occurred predominately in the winter months when more Ontario residents travel to “sun” destinations. Previously, similar findings pertaining to TR cases and seasonality were found in the province of British Columbia [12
]. Separating TR and DA cases in analyses of SE surveillance data facilitates identifying relevant trends and potential clusters for investigation.
The number of SE cases in Ontario was 1,035 in 2010 and the estimated population was 13,227,800 [9
]. Thus, the rate of SE cases in Ontario in 2010 was 7.82 per 100,000 persons. The rates of SE per 100,000 travellers for the Caribbean and Mexico region (24.11), Antigua (74.57), Cuba (26.75), Dominican Republic (29.14), and Jamaica (92.23) were markedly higher than the overall rate for Ontario. In contrast, the SE rates for travellers to the USA (0.03) and Europe (0.63) were lower than the overall rate for Ontario. Regression analysis revealed some statistically significant findings. The odds of acquiring SE for travellers to the Caribbean were 37 times greater than for travellers to Europe. Further, the odds of acquiring SE in the Caribbean were significantly higher than the odds of acquiring SE for travellers to Asia and Mexico. Among travellers to the Caribbean and Mexico region specifically, there were five countries (i.e., Antigua, Jamaica, Barbados, Dominican Republic, and Cuba) for which the odds of acquiring SE were significantly higher than for Mexico. The degree to which the rates in the various travel destinations change from year to year is not known. Nonetheless, having a good understanding of the rates in the travellers to different parts of the world would assist those providing advice to prospective travellers to those regions with a higher risk of acquiring illness. Freedman et. al., found that only 30% of travellers to the Caribbean sought pre-travel medical advice which was less than the 55% of travellers seeking advice for travelling to other developing regions [21
]. This might indicate that travellers believe the health risks associated with the Caribbean are less than those associated with other travel destinations in the developing world.
The Caribbean countries and Mexico are well known winter travel destinations for Ontario residents, in part, because of the numerous “all-inclusive” resorts. For this reason, Mexico was grouped with the Caribbean a priori
. Our data showed that 88.9% of reported TR illnesses were acquired in this region. Further, our data revealed that 90.1% of all SE cases travelling to the Caribbean and Mexico region had stayed at a resort. Given the high percentage of SE cases that travelled to the Caribbean and who stayed at a resort, further investigation should be considered in regard to the role that these resorts have in being a source of illness. While our investigation only considered SE, further investigation should also be considered in regard to the role of the Caribbean and resorts for other enteric pathogens. A recent study of a Canadian community by Ravel et. al., reported that 25% of Campylobacter
, 13% of Giardia
, 66% of non-typhoidal Salmonella
, 44% of Shigella
, and 89% of Yersiniosis TR cases stayed at a resort [11
Identifying both the Salmonella
serotypes and PTs can be instrumental for identifying sources of infection for sporadic and outbreak cases [22
]. In our investigation, we also made use of the PT findings by making associations with the travel status of a case. In Ontario, prior to this investigation, there were three PTs that were thought to be most frequently DA (i.e., PT 8, 13 and 13a). Other PTs were thought to be most frequently TR (i.e., 1, 4, and 5b). There were other PTs, especially newly identified and infrequently identified PTs, for which the TR or DA association was not well understood (e.g., Atypical, 21c, 51). The investigation improved our knowledge of the association between travel and PTs. We learned that there were a higher percentage of TR PT 8 and PT 13a cases than previously thought, although these PTs remain predominantly DA, and that these TR cases occurred primarily in the winter consistent with the winter travel period. We also confirmed our understanding that PTs, 1, 4, and 5b were TR although we did learn that there was a small percentage of these PTs that were acquired in Ontario. Improvements in the quality of Reportable Disease data would assist with improving the capability to assess the TR status of the various PTs.
Our investigation also revealed that there were distinct associations between various PTs and the five most frequently visited countries, i.e., Antigua and Jamaica – PT 8 and 13, Cuba – PT 5b, Dominican Republic – PT 1, and Mexico PT 7a. It is not known whether these associations are consistent from year to year. Further investigation in regard to the PT trends over time from the various countries is warranted. These associations may assist with identifying the source of the illnesses, possibly in regard to the consumption of imported and domestically obtained food items.
The purpose of the original study was to identify the source of the increase of SE in Ontario using a hypothesis-generating stage to inform the subsequent case–control study. Thus, the original study design was not intended to consider detailed characteristics of TR cases. Further, for logistical reasons, in the transition between the hypothesis-generating stage and the case–control stage, case interviews were discontinued from December 1, 2010 to January 19, 2011. Certainly, having the data for this omitted period would have been useful for continuity.
The definition of a TR case was a person who travelled outside of Canada within the three days prior to onset of illness. Misclassification of TR cases may occur with this definition resulting in an over-estimation of TR illness. For example, cases with onset of illness soon after departure, or cases with illness onsets two or three days after return, may have acquired their illness in Ontario. If we reclassified the nine cases in this study who had a symptom onset date within one day of travel departure and the 44 cases with illness onsets two or three days following their return, the proportion of travel-related cases would decrease by less than 10% from 51.9% to 43.4%.
Data on illness was not collected on the traveller respondents in the Statistics Canada International Travel Survey. Therefore, it is possible that this control group could include some cases. However, using the highest rate we found in travellers, we would expect three cases at most to have been misclassified as controls, and having included them in our analysis would have biased our results toward the null. Another limitation was that controls for the various travel destinations were obtained from 2009 data, since 2010 and 2011 data were not available at the time of writing. While it is unlikely that large changes in travel patterns occurred between 2009 and 2010–11, such changes would impact both our rate calculations and our logistic regression results, especially for smaller destinations in the Caribbean.