Public Health Significance of Diarrheal Illnesses
Continuing surveillance and longitudinal studies allow tracking of current levels and trends in diarrhea incidence and mortality and provide the basis for future projections and for evaluations of different control strategies.
Morbidity
Comparisons over time of the global burden of diarrheal diseases have revealed secular trends and demonstrated the impact of public health interventions (Bern and others 1992; Kosek, Bern, and Guerrant 2003; Snyder and Merson 1982). The long-term consequences of diarrhea are only now being systematically assessed and are not reflected in earlier studies.
Reviews in 1992 (Bern and others) and 2003 (Kosek, Bern, and Guerrant) are similar in many ways—for example, assessing morbidity at least twice weekly—but differ significantly in the use of different sources for data on children under five and in the inclusion of studies differing in design and data collection protocols (and only the later study includes data from China). Remarkably, the estimated median incidence of diarrheal disease in children under five in developing countries has not changed much since the early 1990s (figure 19.1): 3.2 episodes per child per year in 2003 (Parashar and others 2003) compared with 3.5 episodes per child per year in 1993 (Jamison and others 1993). However, many fewer surveys were available for the most recent review (31 in 20 countries) compared with the 1993 consensus (276 in 60 countries), reflecting diminished support for the systematic collection of incidence data. Incidence rates in Sub-Saharan Africa and Latin America are clearly greater than in Asia or the Western Pacific, while subject to greater data limitations from individual countries. Incidence continues to show a peak in infants age 6 to 11 months, dropping steadily thereafter.
[Figure
19.1]
The seemingly lower estimates of diarrheal incidence before 1980 (Snyder and Merson 1982) are likely due to methodological differences. These estimates are not precise or directly comparable; the trends are most relevant. The persistently high rates of diarrhea throughout the 1990s despite intensive efforts at control, particularly among children age 6 to 24 months, is of particular concern. Early childhood diarrhea during periods of critical postnatal development may have long-term effects on linear growth and on physical and cognitive functions.
Data on the incidence of shigellosis, the principal cause of bloody diarrhea in developing countries, are even more limited. Kotloff and others' (1999) review of studies on Shigella infection estimates that more than 113 million episodes occur every year in children under five in developing countries, or 0.2 episodes of bloody diarrhea per year caused by Shigella species.
Mortality
Bern and others (1992); Kosek, Bern, and Guerrant (2003); and Snyder and Merson (1982) also estimate diarrheal mortality using data from longitudinal studies with active surveillance in place (figure 19.2). The estimate before 1980 was 4.6 million deaths per year. This estimate dropped to 3.3 million per year between 1980 and 1990 and to 2.6 million per year between 1990 and 2000. Two other studies (Parashar and others 2003; Boschi-Pinto and Tomaskovic forthcoming) report even lower figures for 1990-2000: 2.1 million and 1.6 million deaths per year, respectively. Methodological variations (inclusion of studies with different designs and data collection methods and inclusion of data from China, different sources for estimating the number of children under five, and different strategies for calculating mortality for this age group) may account for some of the striking differences. However, the end of the 20th century witnessed significant reductions in diarrheal deaths in children under five.
[Figure
19.2]
This steady decline in diarrheal mortality, despite the lack of significant changes in incidence, is most likely due to modern case management (introduced since the 1980s) and to the improved nutrition of infants and children. Major recommendations include the following:
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counseling mothers to begin suitable home-prepared rehydration fluids immediately on the onset of diarrhea
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treating mild to moderate dehydration early with oral rehydration solution (ORS), reserving intravenous electrolytes for severe dehydration
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continuing breastfeeding and complementary foods during diarrhea and increasing intake afterward
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limiting antibiotic use to cases of bloody diarrhea or dysentery and avoiding antidiarrheal and antimotility drugs
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advising mothers to increase fluids and continue feeding during future episodes.
Victora and others' (2000) review provides evidence that this strategy, and especially oral rehydration therapy (ORT), has influenced the outcome of dehydrating diarrhea. Data from 99 national surveys carried out in the mid 1990s and compiled by the United Nations Children's Fund (UNICEF) increasingly show that diarrhea patients are appropriately managed in most parts of the world,with overall use rates of ORS or recommended home fluids reaching 49 percent. Country case studies in Brazil, the Arab Republic of Egypt, Mexico, and the Philippines showed a dramatic reduction of diarrhea mortality as ORT use rates increased from close to zero in the early 1980s to 35 percent in Brazil, 50 percent in Egypt, 81 percent in Mexico, and 33 percent in the Philippines in the early 1990s. Hospital admissions for diarrhea also plummeted (Victora and others 2000). As mortality attributable to acute dehydration decreased, the proportionate mortality associated with persistent diarrhea increased. Data from Brazil and Egypt suggest that even relatively low ORT use rates can positively affect mortality,because ORT use tends to be much higher for severe illness (Victora and others 2000).
Worldwide mortality caused by Shigella infection is estimated to be 600,000 deaths per year among children under five, or a quarter to a third of all diarrhea-related mortality in this age group (Kotloff and others 1999). Because mortality caused by bloody diarrhea is not tracked separately, it is difficult to assess the impact of standard case management recommendations, and disease-specific trends cannot be tracked. In the past few years, however, data from the International Centre for Diarrheal Disease Research, in Bangladesh, have shown a marked decrease in the rate of hospitalization caused by Shigella, especially S. dysenteriae type 1, the most severe form of shigellosis. Some investigators have suggested that this decrease may be because Shigella infections are now in the low part of a 10-year cycle (Legros 2004). The observed change could also be explained by better case management with more efficacious antimicrobials. More comprehensive, syndrome-specific surveillance data will be required if rational control priorities are to be set, because the options for dehydrating and bloody diarrheal diseases differ substantially.
Despite national data that indicate a significant decline in mortality (Baltazar, Nadera, and Victora 2002; Miller and Hirschhorn 1995), diarrheal diseases remain among the five top preventable killers of children under five in developing countries and among the top two in many.
Long-Term Consequences
The long-term consequences of diarrheal diseases remain poorly studied, and analyses of global trends have not considered them. Niehaus and others (2002) recently evaluated the long-term consequences of acute diarrheal disease on psychomotor and cognitive development in young children. Following a cohort of 47 children in a poor urban community in northeastern Brazil, they correlated the number of diarrheal episodes in the first two years of life with measures of cognitive function obtained four to seven years later. They found a significant inverse correlation (average decrease of 5.6 percent) between episodes of early diarrheal disease and overall intellectual capacity and concentration, even when controlling for maternal education or helminth infection, which are known to be independent predictors of malnutrition and cognitive defects. Test scores were also 25 to 65 percent lower in children with an earlier history of persistent diarrhea.
Recent evidence suggests that genetic factors may also be involved in the developmental response to repeated diarrhea (Oria and others 2005). Better and more sensitive assessment tools are needed to define the relationships between diarrheal diseases and developmental disorders and to calculate individual and societal costs and the cost-effectiveness of interventions. In addition, early childhood malnutrition resulting from any cause reduces physical fitness and work productivity in adults (Dobbing 1990).
