Diarrhea as a risk factor for acute lower respiratory tract infections among young children in low income settings

Background Diarrhea and acute lower respiratory tract infections (ALRI) are leading causes of morbidity and mortality among children under 5 years of age. We sought to quantify the correlation of diarrhea and respiratory infections within an individual child and to determine if infection with one illness increases the risk of infection with the other during the same time period. Methods We quantified the likelihood of an ALRI and a diarrhea episode occurring during the same week compared to the likelihood of each occurring independently in two cohorts of children under 3 years of age using a bivariate probit regression model. We also quantified the likelihood of an ALRI episode conditioned on a child's diarrhea history and the likelihood of a diarrhea episode conditioned on a child's ALRI history using Cox Proportional Hazard models. Results In Indian and Nepali children, diarrhea and ALRI occurred simultaneously more than chance alone. Incidence of ALRI increased in both cohorts as the number of days with diarrhea in the prior 28 days increased; the greatest incident rate ratio was reported among children with 20 or more days of diarrhea (1.02, 95% confidence interval (CI) 1.01 - 1.03 in Nepal and 1.07, 95% CI 1.05 - 1.09 in South India). Incidence of diarrhea was affected differently by ALRI prevalence depending on the season. Conclusions Diarrhea may be a direct risk factor for ALRI among children under 3 years of age. The risk of comorbidity increases as disease severity increases, providing additional rationale for prompt community case-management of both diarrhea and pneumonia.