In this decade-long cohort study of children followed from birth in an urban Brazilian shantytown, we found that ProD accounts for significant morbidity and is associated with early weaning, low maternal schooling, and undernutrition. Furthermore, we showed that ProD in infancy is a strong indicator of children's future risk of PD. To our knowledge, this is the first study to examine ProD as a distinct class of diarrhea, thus our findings have several important public health implications that merit further consideration.
First, although ProD and PD comprise only a small portion of diarrheal episodes (12% and 5%, respectively), together they account for half of all days of diarrhea. In addition, once an acute episode of diarrhea progresses to ProD, the relative risk that the episode will evolve into PD is 6-fold higher. These two findings suggest that targeted interventions to halt the progression from AD to ProD would not only mitigate PD and its consequences, but have a substantial impact on overall diarrhea burdens as well.
Our second key finding is that infants with ProD have a 2.2-fold higher risk of developing PD in later childhood. In our analysis, ProD remains a significant risk factor for PD even after controlling for a number of confounders. ProD may therefore identify infants whose pathogens, nutritional status, diet, environment, or genes predispose them to PD. Alternatively, ProD may play a direct role in the evolution of PD via effects on growth, innate immunity, disruption of intestinal barrier function, or alteration of gut flora. Regardless of its nature, ProD clearly highlights infants at risk of both PD and growth faltering who therefore warrant increased vigilance and support in order to prevent a vicious cycle of diarrhea and malnutrition.
Third, we find that lack of a primary school education and early weaning from exclusive breastfeeding are significant maternal risk factors for infantile ProD. Several studies from developing countries indicate that children of uneducated mothers are at increased risk of diarrhea.16-18
Despite limited schooling, a number of educational campaigns have been shown to reduce the incidence of childhood diarrhea in these settings, e.g., handwashing interventions in Pakistan,19, 20
and exclusive breastfeeding campaigns in India.21
Our present study indicates that early weaning from exclusive breastfeeding was associated with earlier onset of ProD. This supports previous findings by our group and others that breastfeeding is highly protective against diarrhea and limits the duration of diarrheal illnesses.6
Thus, promotion of breastfeeding must remain an essential component of diarrheal control programs.
Fourth, we demonstrate that a first episode of ProD is associated with undernutrition both before and after the illness. Prior to ProD, children's mean HAZ was already significantly below the WHO median. Following ProD, we see further declines in both HAZ (indicating stunting, a marker of chronic undernutrition) and WAZ (indicating underweight, a marker of more acute malnutrition). In addition, we find baseline decrements in HAZ and WAZ prior to both ProD and PD versus AD episodes. This trend is not seen for low WHZ (an indicator of wasting), which was less common in our study due to the number of children who were both underweight and stunted. We interpret these results as further evidence of a “vicious cycle” of diarrhea and malnutrition in which diarrhea causes undernutrition and, in turn, poor nutritional status predisposes to further, lengthier episodes of diarrhea. Importantly, these acute effects of ProD on nutritional status likely have chronic implications for children's growth. Despite catch-up growth following diarrheal illnesses, our studies and a more recent multi-country analysis have shown that early childhood diarrhea remains highly predictive of stunting at age 2 years and beyond.22, 23
Finally, we found that Shigella
species were etiologies significantly associated with ProD episodes. Shigella
is well-recognized as a major cause of dysentery in developing countries and has consistently been shown to associate with growth faltering, as well as ProD and PD.24
Because of this impact and the worldwide emergence of antibiotic-resistant strains, WHO has targeted the development of a Shigella
vaccine as a high priority.25
is increasingly recognized as an important cause of severe diarrhea and undernutrition in tropical, developing regions; however, existing therapies are only partially effective.26
The antiparasitic drug nitazoxanide has demonstrated efficacy for childhood cryptosporidiosis,27
however it is has not been shown effective for cryptosporidiosis in undernourished children or patients with HIV/AIDS.28
In addition to the pressing need for effective vaccines and therapies for Shigella
, recent WHO reports highlight more immediate approaches to control childhood diarrhea in developing countries, which are of course relevant to ProD. These include: 1) improved outcomes through rotavirus and measles vaccination; promotion of early breastfeeding and Vitamin A supplementation; promotion of handwashing with soap; improved water supply and quality, including household water treatment and safe storage of household water; and community-wide sanitation, and 2) improved management of diarrhea by scaling up implementation of low-osmolarity ORS and zinc supplementation worldwide.2, 29
Of note, Ascaris was found more frequently in control stools than in ProD specimens, but not AD specimens. We speculate that Ascaris might have a mitigating effect on the duration of intestinal infections by altering intestinal immune responses to diarrheal pathogens. Alternatively, ProD may purge Ascaris ova from the digestive tract, making them more difficult to identify in stools.
The major strengths of our study include a large sample of children followed intensively from birth for diarrheal, nutritional, and microbiologic surveillance over multiple seasons in a highly endemic setting. Our study was limited by several factors. First, we recorded substantial improvements over time in the incidence of diarrhea and undernutrition in this community—changes not seen in nearby shantytowns.12
Second, loss to follow-up was common and previous analyses suggest that dropouts were more likely to be underweight.22
Therefore, our current results may represent a “best-case scenario”, which underestimates the true impact of ProD in communities where undernutrition and diarrhea morbidity remain endemic. Third, stools were collected only during the first four years of the study and only 34% of ProD episodes were sampled. Fourth, all stools obtained from children with ProD were examined for bacterial and parasitic pathogens; however only 26% of samples were tested for viral pathogens. Previous findings from our study site (in a different cohort of children) revealed that the use of antibiotics is very common, often inappropriate, and may be a risk factor for diarrhea.30, 31
Lack of adequate data on antibiotic use in the current study prevented us from defining the extent to which antibiotics or other drugs may influence the risk of ProD or PD. Lastly, our study represents results from only one community and may not be generalizable across multiple settings. Several of these limitations will be addressed by the recently launched Global Enterics Multi-Center Study and MAL-ED Network, multi-country investigations of the complex interrelationships of malnutrition and a broader range of enteric infections.32, 33
There is limited consensus between pediatric and adult subspecialties on definitions of acute, prolonged, persistent, and chronic episodes of diarrhea. In general, infectious disease and pediatric GI texts use the WHO cutoff of ≥2 weeks to delineate persistent from acute episodes.34,35
Some experts use “chronic” for illnesses lasting >30 days.36
Conversely, adult GI texts use chronic to describe episodes lasting >2 or >4 weeks.37, 38
Because our data indicate that ProD identifies children at increased risk of PD and the vicious cycle of diarrhea and malnutrition, we propose that the current WHO classification of acute diarrhea as <14 days be revised as follows: 1) acute diarrhea, <7 days and 2) prolonged episodes of acute diarrhea, ≥7 and <14 days.
We conclude that ProD: 1) accounts for a substantial portion of diarrhea burdens in this tropical, developing setting; 2) robustly predicts persistent diarrhea; and 3) associates with undernutrition, growth faltering, early weaning, and low maternal educational status. Further studies are needed to assess the role of ProD in the pathogenesis of persistent diarrhea, as well as the recognition, prevention and treatment of ProD in resource-limited settings, with the overall goal of preventing persistent diarrhea, as well as long-term growth and neurodevelopmental deficits in at-risk children.