Transmitted via the fecal-oral route, shigellosis is a common bacterial disease in the United States with approximately 500,000 cases diagnosed each year.1,2 Although most cases are self-limiting, symptoms most often include watery diarrhea, cramps, fever, and nausea and may require hospitalization.2 One of the most common methods of contracting shigellosis is through ingesting food or water contaminated with Shigella bacteria. Transmission can also occur sexually by practices that foster fecal-oral exposure, including direct oral-anal contact.2,3 Outbreaks of shigellosis among men who have sex with men (MSM) have been well documented in the United States, Canada, and Europe, and phylogenetic analyses have revealed high rates of antibiotic resistance.4–6 Although there have been anecdotal reports of increasing incidence of shigellosis among MSM in Massachusetts, there is a paucity of scientific literature that documents such occurrences. We sought to use a geographic information system (GIS) to identify the geographic distribution of shigellosis cases in Massachusetts by sex from 2005 to 2015. We also explored the geographic distribution for MSM-related cases, using age and sex as a proxy for MSM activity based on previous research.
MATERIALS AND METHODS
Per Massachusetts state regulations, health care providers and clinical laboratories are required to report cases of shigellosis and cultured evidence of Shigella infection to the Massachusetts Department of Public Health (MDPH).7 We acquired shigellosis surveillance data for individuals diagnosed from January 1, 2005, to October 13, 2015, aggregated at the municipality level and stratified by age and sex from MDPH. Information at the time of diagnosis was used to determine age, sex, and residence. In accordance with MDPH data suppression standards, municipalities with fewer than 5 cases were suppressed to maintain patient confidentiality.
Because sexual orientation or sexual preference was not a part of the standard data collection methods for shigellosis reporting at the time of this study, we used men between the ages of 20 and 49 years as a proxy measure for MSM activity. We used this age group because from 1970 to 1983, a dramatic increase of Shigella flexneri was seen among men aged 20 to 49 years in the United States.8 This led public health officials to postulate that transmission likely occurred sexually among MSM, because this increase was not seen among women and it decreased among young children.8
To determine a standardized rate of shigellosis, population data for Massachusetts were obtained from the 2010 US Census Bureau, stratified by sex and age. Shigellosis rates were calculated by dividing the number of reported shigellosis cases within the municipality by the municipality's total population of men or women 20 to 49 years of age. A factor of 100,000 was then multiplied to determine the standardized rate of shigellosis per 100,000 persons. An independent-sample t test was then conducted to determine statistically significant differences between both sexes. Following the Centers for Disease Control and Prevention historic methods to indirectly estimate rates of disease among MSM, we calculated male-female rate ratios (MFRRs) for each municipality comparing the rate among men aged 20 to 49 years with the rate among women of the same age range.9 Thematic maps were then generated, stratifying data by quartiles. Data cleaning and analyses were performed using SAS, version 9.4 (SAS Institute Inc, Cary, NC). Mapping was conducted using ArcGIS, version 10.3.1 (Esri, Redlands, CA).
From January 1, 2005, to October 13, 2015, 124 (35.3%) of 351 municipalities reported at least 1 case of shigellosis among men aged 20 to 49 years, and 99 (28.2%) of 351 municipalities reported at least 1 case of shigellosis among women aged 20 to 49 years. In descriptive analyses, men aged 20 to 49 years had statistically significant higher counts of shigellosis across the 11-year period (n = 453) when compared with women of the same age range (n = 320; P < 0.05).
When assessing shigellosis across Massachusetts' municipalities, we identified larger case counts in Boston (n = 124), Cambridge (n = 18), and Worcester (n = 15) among men aged 20 to 49 years (Fig. 1A), whereas higher case counts were found in Boston (n = 63), Lynn (n = 18), and Lowell (n = 15) among women 20 to 49 years (Fig. 1B). Rates of shigellosis were highest in Provincetown (1,726.8), Chelsea (141.7), and Boston (77.1) per 100,000 men aged 20 to 49 years (Fig. 1C), whereas highest rates per 100,000 women aged 20 to 49 years were identified in Chelsea (145.0), Lynn (91.6), and Lawrence (74.6; Fig. 1D). Although the highest rate of shigellosis was seen in Provincetown among men aged 20 to 49 years, no cases were reported among women of the same age range. Lastly, we found the highest MFRRs in Boston (MFRR, 2.1), Worcester (MFRR, 1.7), and Cambridge (MFRR, 1.4).
In this study, we present the first picture of the spatial distribution of shigellosis in Massachusetts. Notably, we found that Provincetown—a town with a relatively small population (n = 2942), a large MSM community, and the highest rate of new HIV diagnoses in the Commonwealth—has the highest shigellosis rate per 100,000 men aged 20 to 49 years in the state.10,11 We also identified municipalities, including Boston, with more than 1.0 MFRRs.
Given that there are no physiologic reasons why men are at increased risk for contracting shigellosis, our findings suggest that differential behavior is a reason for increased infections. Our findings are important because intensified tracking of infectious diseases like shigellosis within MSM communities may predict or parallel increased transmission of other sexually transmitted infections, including HIV. This is critical because incidence of syphilis and of gonorrhea have significantly increased over the past several years among MSM in Massachusetts and in the town of Provicetown.11 In addition, given the broad differential diagnosis for persons presenting with acute gastrointestinal syndromes, clinicians in regions with high shigellosis rates or high densities of MSM populations should be educated about proper diagnostic and treatment modalities for shigellosis. Prompt identification and treatment can prevent outbreaks and lessen health care utilization.
Our analysis has several limitations. As discussed previously, although there are no documented sex-specific risk factors for acquiring shigellosis, our proxy measure based on previous research assumes that shigellosis cases among men aged 20 to 49 years contracted the disease through MSM activity. Although we found that men aged 20 to 49 years had significantly higher case counts compared with women of the same age, further case information regarding sexual orientation or preference is needed to validate this assumption. Most municipalities (men, n = 107; women, n = 87) had suppressed data, because they had fewer than 5 shigellosis cases. This limits our ability to present a complete picture of the spatial distribution of shigellosis but still allows us to observe locations with considerable disease burden and risk for future transmission. In addition, because most cases of shigellosis are self-limiting and do not require medical attention, cases reported to MDPH may be underestimated.
To our knowledge, our study is the first of its kind to use GIS to investigate the geographic distribution of shigellosis using age and sex as a proxy for MSM activity in Massachusetts. Improved surveillance data collection methods are needed to better monitor shigellosis infection patterns among MSM. Moreover, future research using geospatial analyses (e.g., hotspot cluster analysis) is needed because this approach can help inform public health policy decisions and improve targeting of future public health interventions.
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