Thanks for all your hard work! This is very exciting. Do you feel that the mortality gap relative diarrhea is something in need of explanation? It increase my confidence in the results of the meta-analysis if there was some indication where it was coming from. Or would something like waterborne parasites, malarial mosquito larvae in the water tanks be enough to cover it?
This is a big question, and one we’d very much like to have a better explanation for! It’s important to note that our estimates are uncertain, so the gap between the mortality benefit of water quality interventions and mortality attributed to diarrhea is not necessarily as large as it appears. But we do find it plausible that a larger-than-expected mortality reduction would exist, for a few reasons:
Global Burden of Disease (GBD) estimates of diarrhea deaths in children under five assume that each death has just one cause. However, deaths are often the result of several contributing causes, and in some situations removing any single one of those causes would mean averting a death. So we think it’s likely that just extrapolating diarrhea mortality from diarrhea morbidity underestimates the total mortality benefit of avoiding diarrhea.
On a related note, there is observational evidence that repeated bouts of diarrhea may be a significant cause of malnutrition, and evidence that malnutrition increases risk of dying from infectious diseases, such as malaria and measles. Frequent diarrhea generally seems to lead to undernourishment and worse health, leaving people, especially children, vulnerable to other illnesses such as respiratory infections.
There is evidence from studies of historical water quality improvement projects that improving a population’s drinking water leads to mortality benefits beyond what would be expected from a reduction in waterborne diseases alone, such as reductions in death from respiratory infections; this is known as the Mills-Reincke phenomenon. The effect on child mortality observed in these studies is consistent with the estimate we arrived at in our own meta-analysis. More recent observational evidence points to a link between diarrheal infection and increased risk of respiratory illness.
For more details and citations for the above, see this section of our water quality intervention report (particularly beginning with “We are uncertain about our central estimate of the mortality impact of chlorination…”).
In the course of our investigation, we also spoke to several researchers who found it plausible that water quality improvements would have a larger-than-expected mortality benefit. These researchers offered their own insight into why water treatment might also reduce non–diarrheal illness, or why the reduction in diarrhea mortality might be larger than what one would calculate indirectly from diarrhea morbidity. See our notes from these conversations here, here, and here.
In sum, we still have uncertainty about what explains the gap between mortality reduction estimates, but the impact of averting waterborne diseases on death from non-waterborne diseases probably plays a role. We think that further empirical evidence of the size of the mortality effect, or a better understanding of the mechanisms that link water quality to mortality, could help to substantially reduce our uncertainty.
Thanks for all your hard work! This is very exciting. Do you feel that the mortality gap relative diarrhea is something in need of explanation? It increase my confidence in the results of the meta-analysis if there was some indication where it was coming from. Or would something like waterborne parasites, malarial mosquito larvae in the water tanks be enough to cover it?
Hi, Joel,
This is a big question, and one we’d very much like to have a better explanation for! It’s important to note that our estimates are uncertain, so the gap between the mortality benefit of water quality interventions and mortality attributed to diarrhea is not necessarily as large as it appears. But we do find it plausible that a larger-than-expected mortality reduction would exist, for a few reasons:
Global Burden of Disease (GBD) estimates of diarrhea deaths in children under five assume that each death has just one cause. However, deaths are often the result of several contributing causes, and in some situations removing any single one of those causes would mean averting a death. So we think it’s likely that just extrapolating diarrhea mortality from diarrhea morbidity underestimates the total mortality benefit of avoiding diarrhea.
On a related note, there is observational evidence that repeated bouts of diarrhea may be a significant cause of malnutrition, and evidence that malnutrition increases risk of dying from infectious diseases, such as malaria and measles. Frequent diarrhea generally seems to lead to undernourishment and worse health, leaving people, especially children, vulnerable to other illnesses such as respiratory infections.
There is evidence from studies of historical water quality improvement projects that improving a population’s drinking water leads to mortality benefits beyond what would be expected from a reduction in waterborne diseases alone, such as reductions in death from respiratory infections; this is known as the Mills-Reincke phenomenon. The effect on child mortality observed in these studies is consistent with the estimate we arrived at in our own meta-analysis. More recent observational evidence points to a link between diarrheal infection and increased risk of respiratory illness.
For more details and citations for the above, see this section of our water quality intervention report (particularly beginning with “We are uncertain about our central estimate of the mortality impact of chlorination…”).
In the course of our investigation, we also spoke to several researchers who found it plausible that water quality improvements would have a larger-than-expected mortality benefit. These researchers offered their own insight into why water treatment might also reduce non–diarrheal illness, or why the reduction in diarrhea mortality might be larger than what one would calculate indirectly from diarrhea morbidity. See our notes from these conversations here, here, and here.
In sum, we still have uncertainty about what explains the gap between mortality reduction estimates, but the impact of averting waterborne diseases on death from non-waterborne diseases probably plays a role. We think that further empirical evidence of the size of the mortality effect, or a better understanding of the mechanisms that link water quality to mortality, could help to substantially reduce our uncertainty.
Thanks you very much for taking the time to reply!