Weather, geographic factors, and drinking-water sources all influence outbreaks of waterborne infectious diseases in the United States, Columbia University researchers report in PLOS Water, and their finding have important implications amid the heavier floods, more extreme droughts, and greater seasonal precipitation tied to climate change.
The researchers analyzed data from 516 hospitals in 25 US states on hospitalizations for 12 waterborne bacterial, parasitic, and viral infections, as well as weather and soil data, from 2000 to 2011 to determine the role of weather, drinking water sources, geographic area, and rural versus urban residence.
Precipitation, flooding, drought
The studied pathogens were Legionella, respiratory and intestinal Pseudomonas, nontuberculous mycobacteria, Salmonella, Campylobacter, Shigella, Escherichia coli, Cryptosporidium, Giardia, multiple species of amoeba and other protozoa, and norovirus.
"Waterborne pathogens can cause severe intestinal, respiratory, or systemic infections in vulnerable people," the study authors wrote. "Meteorology, hydroclimatology, and drinking water infrastructure influence the transmission of waterborne infectious diseases in the United States, but their roles are not well-understood and may vary by pathogen type or geographic region."
While biofilm-forming bacteria (eg, nontuberculosis mycobacteria, Pseudomonas, Legionella) are ubiquitous in environmental water, pathogens that cause gastrointestinal illness are often introduced into the environment through human or animal waste, the researchers noted.
Precipitation, flooding, and drought all influence the level and dispersion of waterborne pathogens. For example, floods stir up pathogens in sediment, soil, and water and overwhelm sanitation systems, resulting in the release of untreated sewage. Droughts, on the other hand, concentrate pathogens through low water levels.
Biofilm-forming bacteria behind 81% of admissions
Hospitalizations for waterborne illnesses during the 12-year study period totaled 57,335. Biofilm-forming bacteria accounted for nearly 81% of all hospitalizations, with 60% due to respiratory Pseudomonas infections. Other common causes of hospitalization were nontuberculous mycobacteria (9.6%), Salmonella (8.0%), and Legionella (4.1%).
Hospitalization rates for intestinal and biofilm-forming bacteria were significantly higher in areas in which groundwater rather than surface water was used for drinking and in those with privately owned community water systems. Hospitalization rates for cryptosporidiosis were nearly three times greater in groundwater than in surface water, while giardiasis rates were slightly higher in surface water.
Of the intestinal bacteria, Campylobacter and E coli hospitalizations were more often linked to groundwater, while those due to salmonellosis and shigellosis were similar between drinking-water categories.
Using a generalized linear mixed-model framework, the team found a 16% lower rate of hospitalizations for bacterial pathogens in urban versus rural areas. Notably, Campylobacter hospitalizations were 31% lower in urban areas, 27% lower when the illness was linked to surface drinking water, and 11% higher with a 1–standard-deviation (SD) rise in runoff.
Rural drinking-water sources more vulnerable
Legionnaires' disease, a severe type of pneumonia caused by inhalation of Legionella bacteria from soil or water, spiked 124% with a 1-SD increase in soil moisture. Legionnaires' was the only illness more common in urban rather than rural areas.
Rural communities typically use drinking water from private wells, which are vulnerable to inundation during floods, or groundwater sources, which are often undertreated relative to surface drinking water.
Admission rates for all pathogens were higher in small and rural hospitals, especially for parasitic diseases. Hospitalizations for parasitic diseases—primarily caused by Cryptosporidium—were 9% higher with a 1-SD climb in precipitation.
"Rural communities typically use drinking water from private wells, which are vulnerable to inundation during floods, or groundwater sources, which are often undertreated relative to surface drinking water," the researchers wrote. "This is of particular concern in agricultural regions; both increased pathogen concentrations in water and illnesses have been associated with wet conditions near farms."
Precipitation, runoff, and rural location were positively associated with hospitalizations for some intestinal bacterial and parasitic infections associated with livestock, especially in the Midwest, while hospitalizations for biofilm-forming bacterial infections were tied to soil moisture (a proxy for flooding), and hospitalization rates were higher in urban than rural areas.
"The ability to persist in the environment or evade water treatment measures varies by pathogen and may help explain why the effect of meteorological conditions is not uniform," the authors wrote.
Bacterial pathogens showed the most consistent seasonality, with hospitalizations peaking from July through September in all regions, with the highest spikes in the central and north-central Midwest.
More extreme weather, aging infrastructure
"Associations between hospitalization rates and meteorological conditions, location, and drinking water source varied among the specific pathogens; the pathogen-group level analyses masked several of these findings and were largely uninformative," the researchers wrote.
"Understanding the factors that give rise to these infections could eventually lead to a cost-effective early warning system so public health authorities can direct resources to protect people from contaminated drinking water.
While treatment of drinking water and wastewater substantially lowers rates of waterborne infectious disease, systems can still be contaminated, a problem the authors said will likely worsen as infrastructure ages. And pathogen-specific water-quality monitoring is not often used because it is resource-intensive and expensive.
"Understanding the factors that give rise to these infections could eventually lead to a cost-effective early warning system so public health authorities can direct resources to protect people from contaminated drinking water," first author Victoria Lynch, PhD, said in a Columbia press release.
The authors noted that they couldn't include data on specific water quality, which are critical in the evaluation of likely route of exposure for pathogens (eg, E coli) that can contaminate food. They also couldn't include data from much of the Southeast.
They urged future research to incorporate this information and monitor outbreaks tied to extreme weather to clarify the links between waterborne infections and weather and geographic factors.
"As most cases of waterborne disease are not hospitalized, future work should also expand to include all reportable cases; this is particularly important for understanding the burden of community-acquired pneumonia due to biofilm-forming pathogens," the researchers concluded.
