Epidemiological study of shigellosis in an urban area of Argentina

Introduction: Shigellosis represents one of the main causes of bloody diarrhoea in South America. This study aimed to establish the incidence of shigellosis in an urban zone of Buenos Aires, Argentina, by examining the type of Shigella and living conditions associated with this infection. Methodology: Between January 2009 and December 2010 we analyzed shigellosis in children admitted to the public health service with bloody diarrhoea from La Plata, the capital of Buenos Aires, Argentina. A total of 372 children under 15 years old with Shigella present in their stool samples were admitted to the study. Variables studied were patient age, type of Shigella, family economic status, and access to sewage services and safe drinking water. Results: Shigella flexneri was found to be present in 66.8% of the cases. Incidence was 187 cases/year/100,000 children under 15 years old. Cases were mainly observed during the summer (38.5%) in the population of under 5 years old (69.1% of all cases). The risk of shigellosis increased 12 times in those children who lacked safe drinking water and this risk increased 1.5 times in the population without sewage services. Fewer cases of shigellosis were noted in downtown areas, while hot spots were identified in the suburbs. Treating one case of shigellosis has a local cost of US $976 while assuring safe drinking water and sewage services for one family costs US $634. Conclusion: Incidence of shigellosis in urban areas is associated with quality of water and sewage services. Policies aimed at providing education and improving public utilities networks can help to reduce the incidence of shigellosis.


Introduction
Shigellosis is a disease that causes an acute intestinal infection.Shigella bacteria multiply within colonic epithelial cells and cause inflammation, mucosal ulceration, and bleeding.Symptoms associated with this pathogen vary from mild watery diarrhoea to bacillary dysentery characterized by severe abdominal pain, cramps, fever and stools containing blood and mucus.This disease is mainly transmitted by fecal-oral contact.
Worldwide, the incidence of shigellosis is estimated to be 164.7 million cases per year, of which 163.2 million were in developing countries, where 1.1 million deaths occurred [1].More than half of all these deaths attributable to shigellosis involved children younger than 5 years.The incidence in developing countries could be up to 20 times greater than those in developed countries; however incidence varies within the same region [2], or even inside each country [3,4].
The genus Shigella comprises four species or serogroups.The most common in developing countries is Shigella sonnei, which is responsible for 60% to 80% of cases reported worldwide.In South America, however, the most common species is Shigella flexneri, particularly of serotype 2, followed by S. sonnei [4][5][6][7].With regard to Paraguay [8] and Chile [9], S. flexneri is also more prevalent than S. sonnei.
Studies undertaken in Argentina showed that this type of infectious diarrhoea is found mainly among residents of semi-rural areas and low-income settlements in the outskirts of the cities [10].
This epidemiological study aimed to assess the incidence and characteristics of acute diarrhoea caused by Shigella spp. in an urban area of Argentina.

Study type
This was a descriptive-analytical study of cases of shigellosis and factors associated with them.

Data collection tool
We analyzed databases from the major hospitals in the region serving the pediatric population, as well as data from the State Department of Bacteriology which diagnose and study this disease.

Period of study
Cases of shigellosis in children under 15 years of age for the period of 1 January 2009 through 31 December 2010 were recorded.Also, a comparative group, assessed from historical cases recorded in 2001, 2003, 2005 and 2007, was used to compare the actual incidence of shigellosis.

Unit of analysis
Patients were residents of La Plata under fifteen years of age who suffered from acute diarrhoea during the study period, from which Shigella was isolated when patients were admitted to the study at the Public Health assistance.

Case definition
A case was considered as a unit of analysis when an individual, who was a resident of the city of La Plata, under fifteen years of age, submitted three or more watery stools in a 24-hour period, bloody stools (WHO 1996), and showed any of the following symptoms: fever, abdominal pain, bearing down and/or rectal tenesmus.

Health data
Both the safe drinking water supply and sewage disposal system were analyzed, and the progression of these networks in the past ten years was compared with the 2010 map.

Demographic data
The population of La Plata is located mostly downtown (Urban Square, population (P) 201000 [7] with 26622 young residents (YR) under 15 years of age).However, La Plata's suburban areas are located in the same geographical region, sharing the same environment, and therefore included in the study.These suburban areas were Abasto (P 7577; 2167 YR); A. Segui

Statistical analysis
This study was conducted using the calculation of means and standard deviations for continuous variables and proportions for categorical variables.The comparison between two categorical groups was performed using  2 .The software used was SPSS version 15 for Windows (IBM, Chicago, USA).A p value < 0.05 was considered as significant.

Results
During the study period, 372 cases of diarrhoea associated with shigellosis were recorded in the paediatric population.Incidence was 187 cases/year/100,000 children < 15 years old (Table 1).
Table 2 shows the prevalence of diarrhoea associated with Shigella according to age: 69.1% of the cases were detected in children under five years old, 22.8% in children between five and nine years old, and 8.1% in children over ten and under fifteen years old.Differences in infections between these three age groupings were statistically significant: under five years old compared to five to nine year olds, p = 0.04; and five to nine years olds compared to the oldest group p = 0.01.A peak in cases was observed during the months of January and February, when summer falls in the southern hemisphere.During those months, local temperatures reach up to 40ºC (104ºF) and average rainfall is 115 mm each month.
Shigella spp.identified in study patients were Shigella flexneri in 66.8% of cases, S. sonnei in 25.5% and S. boydii in the remaining 7.7%.No significant differences in these proportions of Shigella spp.were found, either with regard to the geographical areas studied or with regard to the seasonal fluctuations (data not shown).
Although the highest population density is concentrated in the downtown area, the cases were found mostly in the suburban areas (Table 1).Even though the downtown area and the suburbs have similar architectural characteristics, there are noticeable differences between these two areas with regard to access to adequate sewage disposal and safe drinking water.In the downtown area, 100% of the households have both services, while in the suburbs, access to sewage disposal and drinking water is reduced to 40%-60% of the households.Statistical analysis shows that children under 15 years of age have a 54% additional risk of shigellosis if they live in a household without a sewer (p = 0.0064) (Table 3).Further analysis shows that if this risk factor were addressed, and sewers were built in those areas which are lacking them, the probability of children under 15 years of age being infected by shigellosis would be reduced by nearly 35% (etiological fraction among the exposed cases), while the prevalence of the disease would be reduced by 26% for the target population < 15 years (table 3).
When comparing the differences observed among the geographical areas with regard to cases diagnosed with shigellosis, we found a statistically significant difference between the downtown area and the suburbs (Table 4).
When we analyzed the relationship between access to safe drinking water and the incidence of shigellosis, we found that children under 15 years of age living in areas without safe drinking water have a twelve times higher risk of developing the disease, which is a highly statistically significant difference (p = < 0.00001).(Table 5).More detailed statistical analysis shows that the provision of safe drinking water in critical areas would reduce the incidence of shigellosis by more than 92% among population younger than 15 years old, while for the general population the prevalence of the disease would be reduced in 70.9% (Table 5).

Discussion
Bacillary dysentery is one of the most communicable diseases among bacterial acute bloody diarrhoeas [12,13].Shigellosis is transmitted by fewer than two hundred viable Shigella cells [14].The low dose of microorganisms required for disease explains the frequency of interpersonal transmission as well as the fact that dysentery is a major health problem in crowded populations or in institutional care.Control of endemic cases or of a community outbreak of shigellosis is difficult because of the ease of its transmission among young children.
The present study clearly shows that the highest incidence of the disease is found in those geographical areas which lack sewers or safe drinking water.A  correlation was found between shigellosis and the absence of safe drinking water in households in urban areas, to the point that if such households had a safe drinking water supply, the incidence of shigellosis would be reduced in 92% of children under 15 years of age, while the incidence of shigellosis in the general population would be reduced by 71%.Each standard case of bloody diarrhoea admitted to a hospital requires a monetary investment of US $976 [15].This includes professional consultations, hospital supplies, diagnostics and hospital care.The installation cost of a sewer is US 245 per 100 feet, while the cost of a safe drinking water supply is US 323 per 100 feet.[16].Thus public policies targeting direct access to safe drinking water for the inhabitants of the suburbs are not only desirable from a health status point of view, but also cheaper than the cost of treating the disease.
Individual household strategies such as backyard perforations, at a cost of USD 389 (197 feet deep) per household, [15] is an individual solution that still does not avoid the risk of contamination [17].Instead, there should be an investment in public services that provide an adequate sewage disposal and safe drinking water to control infectious diarrhoea in urban areas.

Table 1 .
Study population and number of cases of shigellosis by location (according to National Census 2001) * average of the years 2009 and 2010 (From a total number of 372 cases, in 26 of them, location was not able to be confirmed)

Table 2 .
Case prevalence according to age groups

Table 3 .
Prevalence ratio for households with and without sewers

Table 4 .
Shigellosis risk by comparing downtown to each suburb NS : Non significant risk ; PR Prevalence Ratio for risk factor; NE: Non Evaluable

Table 5 .
Analysis of shigellosis according to availability of safe drinking water *Prevalence Ratio for risk factor ** Etiological Fraction among exposed ***Population Etiological Fraction