Climatic Drivers of Diarrheal Disease in Thailand: The Role of Helminth Co-Infection

Abstract

Background: Diarrheal disease is the second leading cause of death in children under five worldwide and a major global contributor to disability. Climatic factors likely play a role in determining patterns of disease. However, there is considerable disagreement about the direction, strength, and nature of the relationships between climate and diarrheal disease. In addition, co-infection with helminths is likely in developing countries where diarrhea morbidity is high. Helminths modulate the host immune system, which may influence responses to other infections. However, the effect of helminth infection on susceptibility to diarrheal disease has not been evaluated. This study aimed to explore the impact of climatic variation in driving diarrheal disease in Thailand. The possible indirect effect of climate through helminth prevalence was concurrently assessed. Methods: Province-level variation in diarrhea morbidity in Thailand was evaluated. A strategy of multiple linear regression model selection was employed to identify the models that best explained spatial patterns of diarrheal disease. The prevalence of A. lumbricoides and T. trichiura, in conjunction with climatic indicators were evaluated as covariates. The interaction between climatic predictors and improved sanitation/water source was additionally assessed. Results: The lowest extreme maximum daily temperature in the year (EMXT min) and annual mean minimum temperature (MMNT mean) were significantly associated with higher and lower diarrhea morbidity, respectively. Improved water source was negatively associated with diarrheal disease across all models generated. A significant interaction was observed for improved water source and the number of days with maximum temperature equal to or greater than 90ºF. Helminth prevalence and precipitation measures were not associated with diarrhea morbidity. Conclusions: The multiple relationships between temperature and diarrheal disease may reflect the differential susceptibilities of diarrheal pathogens to climatic variation. Understanding these associations is increasingly important in the face of climate change. Improving water sources may mitigate the potentially negative effects of changing weather patterns. Interventions to improve hygiene infrastructure may additionally lower the incidence of all-cause diarrhea. This study also underscores the need for finer scale data in developing regions. Further investigation into the interaction between helminth infection and diarrheal disease is warranted, given the high prevalence of these co-endemic infections worldwide.