The burden of shigellosis is greatest in resource-poor countries. Although this diarrheal disease has been thought to cause considerable morbidity and mortality in excess of 1,000,000 deaths globally per year, little recent data are available to guide intervention strategies in Asia. We conducted a prospective, population-based study in six Asian countries to gain a better understanding of the current disease burden, clinical manifestations, and microbiology of shigellosis in Asia.
Over 600,000 persons of all ages residing in Bangladesh, China, Pakistan, Indonesia, Vietnam, and Thailand were included in the surveillance. Shigella was isolated from 2,927 (5%) of 56,958 diarrhoea episodes detected between 2000 and 2004. The overall incidence of treated shigellosis was 2.1 episodes per 1,000 residents per year in all ages and 13.2/1,000/y in children under 60 months old. Shigellosis incidence increased after age 40 years. S. flexneri was the most frequently isolated Shigella species (1,976/2,927 [68%]) in all sites except in Thailand, where S. sonnei was most frequently detected (124/146 [85%]). S. flexneri serotypes were highly heterogeneous in their distribution from site to site, and even from year to year. PCR detected ipaH, the gene encoding invasion plasmid antigen H in 33% of a sample of culture-negative stool specimens. The majority of S. flexneri isolates in each site were resistant to amoxicillin and cotrimoxazole. Ciprofloxacin-resistant S. flexneri isolates were identified in China (18/305 [6%]), Pakistan (8/242 [3%]), and Vietnam (5/282 [2%]).
Shigella appears to be more ubiquitous in Asian impoverished populations than previously thought, and antibiotic-resistant strains of different species and serotypes have emerged. Focusing on prevention of shigellosis could exert an immediate benefit first by substantially reducing the overall diarrhoea burden in the region and second by preventing the spread of panresistant Shigella strains. The heterogeneous distribution of Shigella species and serotypes suggest that multivalent or cross-protective Shigella vaccines will be needed to prevent shigellosis in Asia.
A prospective, population-based study in six Asian countries showed that Shigella appears to be more ubiquitous in Asian impoverished populations than previously thought, and antibiotic-resistant strains have emerged.
Infections that cause diarrhea are a major public health problem in developing countries and other places where resources are scarce, particularly in young children. Although deaths from diarrhea have decreased considerably in recent decades, diarrheal illnesses continue to cause some 2.5 million deaths each year. Shigella, a group of rod-shaped bacteria closely related to those that normally live in the human intestine, is known to cause severe diarrhea in both developed and developing countries, but the global impact of Shigella infection (shigellosis) has not been well characterized. Shigella exists in more than 40 different varieties, an increasing number of cases have been found to be resistant to available antibiotics, and no vaccine is licensed except one oral vaccine in China.
The best information available on the impact of shigellosis has been based on historical estimates, which are subject to inaccuracy. More recent studies suggest that the older reports may have underestimated the impact of shigellosis. The authors of this study wanted to obtain more accurate, current estimates of the impact of shigellosis in developing countries.
In addition, immunity to one type of Shigella does not necessarily provide protection against other types. Therefore, in order to develop an effective vaccine, researchers would need to know which types of Shigella are causing illness in affected parts of the world. Accordingly, the authors of this study also wanted to investigate the specific types of Shigella (called “serotypes” because they can be distinguished using serum from immune individuals) involved in cases of diarrhea.
The researchers set up surveillance projects for diarrhea in six developing countries throughout Asia: at three rural or semirural sites (in China, Vietnam, and Thailand) and three urban slum sites (in Bangladesh, Pakistan, and Indonesia). They conducted information campaigns in each area to encourage residents to visit a participating clinic if they or their children developed diarrhea. Patients presenting with diarrhea were enrolled in the study and their medical findings were documented on standardized report forms. Stool or rectal swab specimens were obtained (with patient consent) and sent to laboratories to test for Shigella. When Shigella was identified, the bacteria was serotyped and tested for resistance to antibiotics. Because standard culture methods do not always detect Shigella when it is present, as a double-check, the researchers also tested some of the specimens for a type of DNA (called the ipaH gene) that serves as a molecular “footprint” of Shigella. Patients received treatment according to national guidelines.
The study involved approximately 600,000 participants over 1–3 years, and detected approximately 60,000 cases of diarrhea. Shigella was found in 5% of diarrhea episodes, meaning that two new cases of shigellosis occurred per 1,000 people (of all ages) per year. Rates were higher in children and in people over age 40. Among children less than 5 years old, there were 13 new cases per 1,000 children per year. Rates of shigellosis were higher in the Bangladesh site than in the China, Pakistan, and Indonesia sites, which in turn had higher rates than the Vietnam and Thailand sites.
In contrast to prior studies, no deaths were detected following episodes of shigellosis, and less than one-third of cases of shigellosis were associated with bloody diarrhea (dysentery).
The distribution of serotypes was found to differ from one site to another and within a given site over time. A high percentage of Shigella detected at all sites were resistant to two or more antibiotics. Testing for the ipaH gene was able to identify Shigella in half of patients with bloody diarrhea whose routine stool cultures did not reveal Shigella.
This study found that shigellosis occurs in these Asian sites at a rate approximately 100 times higher than in industrialized countries. The finding that shigellosis frequently occurs in the absence of bloody stool means that government data collections using dysentery as part of the case definition can be expected to miss the majority of shigellosis cases. Also, the increased rate of shigellosis above age 40 shows that older people share significantly in the burden (and most likely the transmission) of shigellosis.
The generally benign clinical course of Shigella-associated diarrhea calls into question the priority that this disease should receive in global vaccine development efforts, especially given the technological challenges posed by the complex and variable distribution of serotypes. Nonetheless, the emergence of multidrug-resistant strains clearly remains a threat, and raises the perennial issue of improved sanitation, rather than new antibiotics, as a long-term solution to the plethora of water-borne illnesses that disproportionately affect developing countries.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0030353.
Wikipedia entry on Shigella (note: Wikipedia is a free Internet encyclopedia that anyone can edit)