Water-Related Illness

6.1 Introduction

  • Juli M. Trtanj
    National Oceanic and Atmospheric Administration
  • Lesley Jantarasami
    U.S. Environmental Protection Agency
  • Joan Brunkard
    Centers for Disease Control and Prevention
  • Tracy Collier
    University Corporation for Atmospheric Research
  • John Jacobs
    National Oceanic and Atmospheric Administration
  • Erin Lipp
    The University of Georgia
  • Sandra L. McLellan
    University of Wisconsin-Milwaukee
  • Stephanie Moore
    University Corporation for Atmospheric Research
  • Hans W. Paerl
    The University of North Carolina at Chapel Hill
  • John Ravenscroft
    U.S. Environmental Protection Agency
  • Mario Sengco
    U.S. Environmental Protection Agency
  • Jeanette Thurston
    U.S. Department of Agriculture

Across most of the United States, climate change is expected to affect fresh and marine water resources in ways that will increase people’s exposure to water-related contaminants that cause illness. Water-related illnesses include waterborne diseases caused by pathogens, such as bacteria, viruses, and protozoa. Water-related illnesses are also caused by toxins produced by certain harmful algae and cyanobacteria (also known as blue-green algae) and by chemicals introduced into the environment by human activities. Exposure occurs through ingestion, inhalation, or direct contact with contaminated drinking or recreational water and through consumption of fish and shellfish.

Factors related to climate change—including temperature, precipitation and related runoff, hurricanes, and storm surge—affect the growth, survival, spread, and virulence or toxicity of agents (causes) of water-related illness. Heavy downpours are already on the rise and increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.1 Projections of temperature, precipitation, extreme events such as flooding and drought, and other climate factors vary by region of the United States, and thus the extent of climate health impacts will also vary by region.

Waterborne pathogens are estimated to cause 8.5% to 12% of acute gastrointestinal illness cases in the United States, affecting between 12 million and 19 million people annually.2,3,4 Eight pathogens, which are all affected to some degree by climate, account for approximately 97% of all suspected waterborne illnesses in the United States: the enteric viruses norovirus, rotavirus, and adenovirus; the bacteria Campylobacter jejuni, E. coli O157:H7, and Salmonella enterica; and the protozoa Cryptosporidium and Giardia.5

Specific health outcomes are determined by different exposure pathways and multiple other social and behavioral factors, some of which are also affected by climate (Figure 6.1). Most research to date has focused on understanding how climate drivers affect physical and ecological processes that act as key exposure pathways for pathogens and toxins, as shown by the arrow moving from the top to the middle box in Figure 6.1. There is currently less information and fewer methods with which to measure actual human exposure and incidence of illness based on those physical and ecologicalmetrics (arrow moving from middle to bottom box in Figure 6.1). Thus, it is often not possible to quantitatively project future health outcomes from water-related illnesses under climate change (bottom box in Figure 6.1).


Figure 6.1: Climate Change and Health–Vibrio

Figure 6.1: Climate Change and Health–<i>Vibrio</i>
This conceptual diagram for an example of infection by Vibrio species (V. vulnificus, V. parahaemolyticus, or V. alginolyticus) illustrates the key pathways by which humans are exposed to health threats from climate drivers. These climate drivers create more favorable growing conditions for these naturally occurring pathogens in coastal environments through their effects on coastal salinity, turbidity (water clarity), or plankton abundance and composition. Longer seasons for growth and expanding geographic range of occurrence increase the risk of exposure to Vibrio, which can result in various potential health outcomes (center boxes). These exposure pathways exist within the context of other factors that positively or negatively influence health outcomes (gray side boxes). Key factors that influence vulnerability for individuals are shown in the right box and include social determinants of health and behavioral choices. Key factors that influence vulnerability at larger scales, such as natural and built environments, governance and management, and institutions, are shown in the left box. All of these influencing factors can affect an individual’s or a community’s vulnerability through changes in exposure, sensitivity, and adaptive capacity and may also be affected by climate change. See Ch. 1: Introduction for more information.

This chapter covers health risks associated with changes in natural marine, coastal, and freshwater systems and water infrastructure for drinking water, wastewater, and stormwater (Legionella in aerosolized water is covered in Ch. 3: Air Quality Impacts). This chapter also includes fish and shellfish illnesses associated with the waters in which they grow and which are affected by the same climate factors that affect drinking and recreational waters (impacts related to handling and post-harvest processing of seafood are covered in Ch. 7: Food Safety). The framing of this chapter addresses sources of contaminations, exposure pathways, and health outcomes when available. Based on the available data and research, many of the examples are regionally focused and make evident that the impact of climate change on water-related illness is inherently regional. Table 6.1 lists various health outcomes that can result from exposure to agents of water-related illness as well as key climate-related changes affecting their occurrence.

Table 6.1: Climate Sensitive Agents of Water Related Illness

Click on a table row for more information.

Pathogen or Toxin Producer Exposure Pathway Selected Health Outcomes & Symptoms Major Climate Correlation or Driver (strongest drivers listed first)
Algae: Toxigenic marine species of Alexandrium, Pseudo-nitzschia, Dinophysis, Gambierdiscus; Karenia brevis Shellfish Fish
Recreational waters (aerosolized toxins)
Gastrointestinal and neurologic illness caused by shellfish poisoning (paralytic, amnesic, diarrhetic, neurotoxic) or fish poisoning (ciguatera). Asthma exacerbations, eye irritations caused by contact with aerosolized toxins (K. brevis). Temperature (increased water temperature), ocean surface currents, ocean acidification, hurricanes (Gambierdiscus spp. and K. brevis)
Cyanobacteria (multiple freshwater species producing toxins including microcystin) Drinking water
Recreational waters
Liver and kidney damage, gastroenteritis (diarrhea and vomiting), neurological disorders, and respiratory arrest. Temperature, precipitation patterns
Enteric bacteria & protozoan parasites: Salmonella enterica; Campylobacter species; Toxigenic Escherichia coli; Cryptosporidium; Giardia Drinking water
Recreational waters Shellfish
Enteric pathogens generally cause gastroenteritis. Some cases may be severe and may be associated with long-term and recurring effects. Temperature (air and water; both increase and decrease), heavy precipitation, and flooding
Enteric viruses: enteroviruses; rotaviruses; noroviruses; hepatitis A and E Drinking water
Recreational waters Shellfish
Most cases result in gastrointestinal illness. Severe outcomes may include paralysis and infection of the heart or other organs.


Heavy precipitation, flooding, and temperature (air and water; both increase and decrease)
Leptospira and Leptonema bacteria Recreational waters Mild to severe flu-like illness (with or without fever) to severe cases of meningitis, kidney, and liver failure. Flooding, temperature (increased water temperature), heavy precipitation
Vibrio bacteria species Recreational waters
Varies by species but include gastroenteritis (V. parahaemolyticus, V. cholerae), septicemia (bloodstream infection) through ingestion or wounds (V. vulnificus), skin, eye, and ear infections (V. alginolyticus). Temperature (increased water temperature), sea level rise, precipitation patterns (as it affects coastal salinity)

Whether or not illness results from exposure to contaminated water, fish, or shellfish is dependent on a complex set of factors, including human behavior and social determinants of health that may affect a person’s exposure, sensitivity, and adaptive capacity (Figure 6.1; see also Ch. 1: Introduction and Ch. 9: Populations of Concern). Water resource, public health, and environmental agencies in the United States provide many public health safeguards to reduce risk of exposure and illness even if water becomes contaminated. These include water quality monitoring, drinking water treatment standards and practices, beach closures, and issuing advisories for boiling drinking water and harvesting shellfish.

Many water-related illnesses are either undiagnosed or unreported, and therefore the total incidence of waterborne disease is underestimated (see Ch. 1: Introduction for discussion of public health surveillance data limitations related to “reportable” and “nationally notifiable” diseases).6,7 On average, illnesses from pathogens associated with water are thought to be underestimated by as much as 43-fold, and may be underestimated by up to 143 times for certain Vibrio species.7

6.2 Sources of Water-Related Contaminants

The primary sources of water contamination are human and animal waste and agricultural activities, including the use of fertilizers. Runoff and flooding resulting from expected increases in extreme precipitation, hurricane rainfall, and storm surge (see Ch. 4: Extreme Events) may increase risks of contamination. Contamination occurs when agents of water-related illness and nutrients, such as nitrogen and phosphorus, are carried from urban, residential, and agricultural areas into surface waters, groundwater, and coastal waters (Figure 6.2). The nutrient loading can promote growth of naturally occurring pathogens and algae. Human exposure occurs via contamination of drinking water sources, recreational waters, and fish and shellfish.


Figure 6.2: Links between Climate Change, Water Quantity and Quality, and Human Exposure to Water-Related Illness

Figure 6.2: Links between Climate Change, Water Quantity and Quality, and Human Exposure to Water-Related Illness
Precipitation and temperature changes affect fresh and marine water quantity and quality primarily through urban, rural, and agricultural runoff. This runoff in turn affects human exposure to water-related illnesses primarily through contamination of drinking water, recreational water, and fish and shellfish.

Water contamination by human waste is tied to failure of local urban or rural water infrastructure, including municipal wastewater, septic, and stormwater conveyance systems. Failure can occur either when rainfall and subsequent runoff overwhelm the capacity of these systems—causing for example, sewer overflows, basement backups, or localized flooding—or when extreme events like flooding and storm surges damage water conveyance or treatment infrastructure and result in reduction or loss of performance and functionality. Many older cities in the Northeast and around the Great Lakes region of the United States have combined sewer systems (with stormwater and sewage sharing the same pipes), which are prone to discharging raw sewage directly into surface waters after moderate to heavy rainfall.8 The amount of rain that causes combined sewer overflows is highly variable between cities because of differences in infrastructure capacity and design, and ranges from 5 mm (about 0.2 inches) to 2.5 cm (about 1 inch).9,10 Overall, combined sewer overflows are expected to increase,11 but site-specific analysis is needed to predict the extent of these increases (see Milwaukee Case Study). Extreme precipitation events will exacerbate existing problems with inadequate, aging, or deteriorating wastewater infrastructure throughout the country.12,13 These problems include broken or leaking sewer pipes and failing septic systems that leach sewage into the ground. Runoff or contaminated groundwater discharge also carries pathogens and nutrients into surface water, including freshwater and marine coastal areas and beaches.14,15,16,17,18,19,20,21

Water contamination from agricultural activities is related to the release of microbial pathogens or nutrients in livestock manure and inorganic fertilizers that can stimulate rapid and excessive growth or blooms of harmful algae. Agricultural land covers about 900 million acres across the United States,22 comprising over 2 million farms, with livestock sectors concentrated in certain regions of the United States (Figure 6.3). Depending on the type and number of animals, a large livestock operation can produce between 2,800 and 1,600,000 tons of manure each year.23,24 With the projected increases in heavy precipitation for all U.S. regions,1 agricultural sources of contamination can affect water quality across the Nation. Runoff from lands where manure has been used as fertilizer or where flooding has caused wastewater lagoons to overflow can carry contamination agents directly from the land into water bodies.23,24,25


Figure 6.3: Locations of Livestock and Projections of Heavy Precipitation

Interact with the Figure Below

This figure compares the geographic distribution of chicken, cattle, and hog and pig densities to the projected change in annual maximum 5-day precipitation totals (2046–2065 compared to 1981–2000, multi-model average using RCP8.5) across the continental United States. Increasing frequency and intensity of precipitation and subsequent increases in runoff are key climate factors that increase the potential for pathogens associated with livestock waste to contaminate water bodies. (Figure sources: adapted from Sun et al. 2015 and USDA 2014).31,32

Management practices and technologies, such as better timing of manure application and improved animal feeds, help reduce or eliminate the risks of manure-borne contaminant transport to public water supplies and shellfish harvesting waters and reduce nutrients that stimulate harmful algal blooms.23,25,26,27 Drinking water treatment and monitoring practices also help to decrease or eliminate exposure to waterborne illness agents originating from agricultural environments.

Water contamination from wildlife (for example, rodents, birds, deer, and wild pigs) occurs via feces and urine of infected animals, which are reservoirs of enteric and other pathogens.27,28,29 Warmer winters and earlier springs are expected to increase animal activity and alter the ecology and habitat of animals that may carry pathogens.1 This may lengthen the exposure period for humans and expand the geographic ranges in which pathogens are transmitted.1,30

6.3 Exposure Pathways and Health Risks

Humans are exposed to agents of water-related illness through several pathways, including drinking water (treated and untreated), recreational waters (freshwater, coastal, and marine), and fish and shellfish.

Drinking Water 

Heavy rain

Extreme precipitation events have been statistically linked to increased levels of pathogens in treated drinking water supplies.

Although the United States has one of the safest municipal drinking water supplies in the world, water-related outbreaks (more than one illness case linked to the same source) still occur.33 Public drinking water systems provide treated water to approximately 90% of Americans at their places of residence, work, or schools.34 However, about 15% of the population relies fully or in part on untreated private wells or other private sources for their drinking water.35 These private sources are not regulated under the Safe Drinking Water Act.36 The majority of drinking water outbreaks in the United States are associated with untreated or inadequately treated groundwater and distribution system deficiencies.33,37

Pathogen and Algal Toxin Contamination

Between 1948 and 1994, 68% of waterborne disease outbreaks in the United States were preceded by extreme precipitation events,38 and heavy rainfall and flooding continue to be cited as contributing factors in more recent outbreaks in multiple regions of the United States.39 Extreme precipitation events have been statistically linked to increased levels of pathogens in treated drinking water supplies40 and to an increased incidence of gastrointestinal illness in children.21,41 This established relationship suggests that extreme precipitation is a key climate factor for waterborne disease.42,43,44,45 The Milwaukee Cryptosporidium outbreak in 1993—the largest documented waterborne disease outbreak in U.S. history, causing an estimated 403,000 illnesses and more than 50 deaths46—was preceded by the heaviest rainfall event in 50 years in the adjacent watersheds.10 Various treatment plant operational problems were also key contributing factors.47 (See future projections in the Milwaukee Case Study). Observations in England and Wales also show waterborne disease outbreaks were preceded by weeks of low cumulative rainfall and then heavy precipitation events, suggesting that drought or periods of low rainfall may also be important climate-related factors.48

Small community or private groundwater wells or other drinking water systems where water is untreated or minimally treated are especially susceptible to contamination following extreme precipitation events.49 For example, in May 2000, following heavy rains, livestock waste containing E. coli O157:H7 and Campylobacter was carried in runoff to a well that served as the primary drinking water source for the town of Walkerton, Ontario, Canada, resulting in 2,300 illnesses and 7 deaths.43,44,50 High rainfall amounts were an important catalyst for the outbreak, although non-climate factors, such as well infrastructure, operational and maintenance problems, and lack of communication between public utilities staff and local health officials were also key factors.44,51

Likewise, extreme precipitation events and subsequent increases in runoff are key climate factors that increase nutrient loading in drinking water sources, which in turn increases the likelihood of harmful cyanobacterial blooms that produce algal toxins.52 The U.S. Environmental Protection Agency has established health advisories for two algal toxins (microcystins and cylindrospermopsin) in drinking water.53 Lakes and reservoirs that serve as sources of drinking water for between 30 million and 48 million Americans may be periodically contaminated by algal toxins.54 Certain drinking water treatment processes can remove cyanobacterial toxins; however, efficacy of the treatment processes may vary from 60% to 99.9%. Ineffective treatment could compromise water quality and may lead to severe treatment disruption or treatment plant shutdown.53,54,55,56 Such an event occurred in Toledo, Ohio, in August 2014, when nearly 500,000 residents of the state’s fourth-largest city lost access to their drinking water after tests revealed the presence of toxins from a cyanobacterial bloom in Lake Erie near the water plant’s intake.57   

Water Supply

Climate-related hydrologic changes such as those related to flooding, drought, runoff, snowpack and snowmelt, and saltwater intrusion (the movement of ocean water into fresh groundwater) have implications for freshwater management and supply (see also Ch. 4: Extreme Events).58 Adequate freshwater supply is essential to many aspects of public health, including provision of drinking water and proper sanitation and personal hygiene. For example, following floods or storms, short-term loss of access to potable water has been linked to increased incidence of illnesses including gastroenteritis and respiratory tract and skin infections.59 Changes in precipitation and runoff, combined with changes in consumption and withdrawal, have reduced surface and groundwater supplies in many areas, primarily in the western United States.58 These trends are expected to continue under future climate change, increasing the likelihood of water shortages for many uses.58

Future climate-related water shortages may result in more municipalities and individuals relying on alternative sources for drinking water, including reclaimed water and roof-harvested rainwater.60,61,62,63 Water reclamation refers to the treatment of stormwater, industrial wastewater, and municipal wastewater for beneficial reuse.64 States like California, Arizona, New Mexico, Texas, and Florida are already implementing wastewater reclamation and reuse practices as a means of conserving and adding to freshwater supplies.65 However, no federal regulations or criteria for public health protection have been developed or proposed specifically for potable water reuse in the United States.66 Increasing household rainwater collection has also been seen in some areas of the country (primarily Arizona, Colorado, and Texas), although in some cases, exposure to untreated rainwater has been found to pose health risks from bacterial or protozoan pathogens, such as Salmonella enterica and Giardia lamblia.67,68,69

Projected Changes

Runoff from more frequent and intense extreme precipitation events will contribute to contamination of drinking water sources with pathogens and algal toxins and place additional stresses on the capacity of drinking water treatment facilities and distribution systems.10,52,59,70,71,72,73 Contamination of drinking water sources may be exacerbated or insufficiently addressed by treatment processes at the treatment plant or by breaches in the distribution system, such as during water main breaks or low-pressure events.13 Untreated groundwater drawn from municipal and private wells is of particular concern.  

Climate change is not expected to substantially increase the risk of contracting illness from drinking water for those people who are served by treated drinking water systems, if appropriate treatment and distribution is maintained. However, projections of more frequent or severe extreme precipitation events, flooding, and storm surge suggest that drinking water infrastructure may be at greater risk of disruption or failure due to damage or exceedance of system capacity.6,58,70,74,75 Aging drinking water infrastructure is one longstanding limitation in controlling waterborne disease, and may be especially susceptible to failure.6,13,74 For example, there are more than 50,000 systems providing treated drinking water to communities in the United States, and most water distribution pipes in these systems are already failing or will reach their expected lifespan and require replacement within 30 years.6 Breakdowns in drinking water treatment and distribution systems, compounded by aging infrastructure, could lead to more serious and frequent health consequences than those we experience now.

Recreational Waters 

Family jumping in lake

In areas where increasing temperatures lengthen recreational swimming seasons, exposure risks are expected to increase.

Humans are exposed to agents of water-related illness through recreation (such as swimming, fishing, and boating) in freshwater and marine or coastal waters. Exposure may occur directly (ingestion and contact with water) or incidentally (inhalation of aerosolized water droplets).

Pathogen and Algal Toxin Contamination

Enteric viruses, especially noroviruses, from human waste are a primary cause of gastrointestinal illness from exposure to contaminated recreational fresh and marine water (Table 6.1).76 Although there are comparatively few reported illnesses and outbreaks of gastrointestinal illness from recreating in marine waters compared to freshwater, marine contamination still presents a significant health risk.39,77,78,79,80 Illnesses from marine sources are less likely to be reported than those from freshwater beaches in part because the geographical residences of beachgoers are more widely distributed (for example, tourists may travel to marine beaches for vacation) and illnesses are less often attributed to marine exposure as a common source.39,76

Key climate factors associated with risks of exposure to enteric pathogens in both freshwater and marine recreational waters include extreme precipitation events, flooding, and temperature. For example, Salmonella and Campylobacter concentrations in freshwater streams in the southeastern United States increase significantly in the summer months and following heavy rainfall.81,82,83 In the Great Lakes—a freshwater system—changes in rainfall, higher lake temperatures, and low lake levels have been linked to increases in fecal bacteria levels.10 The zoonotic bacteria Leptospira are introduced into water from the urine of animals,84,85 and increased illness rates in humans are linked to warm temperatures and flooding events.86,87,88,89,90

In marine waters, recreational exposure to naturally occurring bacterial pathogens (such as Vibrio species) may result in eye, ear, and wound infections, diarrheal illness, or death (Table 6.1).91,92,93 Reported rates of illness for all Vibrio infections have tripled since 1996, with V. alginolyticus infections having increased by 40-fold.91 Vibrio growth rates are highly responsive to rising sea surface temperatures, particularly in coastal waters, which generally have high levels of the dissolved organic carbon required for Vibrio growth. The distribution of species changes with salinity patterns related to sea level rise and to changes in delivery of freshwater to coastal waters, which is affected by flooding and drought. For instance, V. parahaeomolyticus and V. alginolyticus favor higher salinities while V. vulnificus favors more moderate salinities.94,95,96,97,98,99

Harmful algal blooms caused by cyanobacteria were responsible for nearly half of all reported outbreaks in untreated recreational freshwater in 2009 and 2010, resulting in approximately 61 illnesses (health effects included dermatologic, gastrointestinal, respiratory, and neurologic symptoms), primarily reported in children/young adults age 1–19.100 Cyanobacterial blooms are strongly influenced by rising temperatures, altered precipitation patterns, and changes in freshwater discharge or flushing rates of water bodies (Table 6.1).101,102,103,104,105,106,107 Higher temperatures (77°F and greater) favor surface-bloom-forming cyanobacteria over less harmful types of algae.108 In marine water, the toxins associated with harmful “red tide” blooms of Karenia brevis can aerosolize in water droplets through wind and wave action and cause acute respiratory illness and eye irritation in recreational beachgoers.109,110 People with preexisting respiratory diseases, specifically asthma, are at increased risk of illness.111,112 Prevailing winds and storms are important climate factors influencing the accumulation of K. brevis cells in the water.77,113 For example, in 1996, Tropical Storm Josephine transported a Florida panhandle bloom as far west as Louisiana,114 the first documented occurrence of K. brevis in that state.

Projected Changes

Overall, climate change will contribute to contamination of recreational waters and increased exposure to agents of water-related illness.10,81,115,116,117,118,119 Increases in flooding, coastal inundation, and nuisance flooding (linked to sea level rise and storm surge from changing patterns of coastal storms and hurricanes) will negatively affect coastal infrastructure and increase chances for pathogen contamination, especially in populated areas (see also Ch. 4: Extreme Events).70,120 In areas where increasing temperatures lengthen the seasons for recreational swimming and other water activities, exposure risks are expected to increase.121,122

As average temperatures rise, the seasonal and geographic range of suitable habitat for cyanobacterial species is projected to expand.123,124,125,126,127 For example, tropical and subtropical species like Cylindrospermopsisraciborskii, Anabaena spp., and Aphanizomenon spp. have already shown poleward expansion into mid-latitudes of Europe, North America, and South America.106,128,129 Increasing variability in precipitation patterns and more frequent and intense extreme precipitation events (which will increase nutrient loading) will also affect cyanobacterial communities. If such events are followed by extended drought periods, the stagnant, low-flow conditions accompanying droughts will favor cyanobacterial dominance and bloom formation.102,130

In recreational waters, projected increases in sea surface temperatures are expected to lengthen the seasonal window of growth and expand geographic range of Vibrio species,95,131 although the certainty of regional projections is affected by underlying model structure.132 While the specific response of Vibrio and degree of growth may vary by species and locale, in general, longer seasons and expansion of Vibrio into areas where it had not previously been will increase the likelihood of exposure to Vibrio in recreational waters. Regional climate changes that affect coastal salinity (such as flooding, drought, and sea level rise) can also affect the population dynamics of these agents,96,98,133 with implications for human exposure risk. Increases in hurricane intensity and rainfall are projected as the climate continues to warm (see Ch. 4: Extreme Events). Such increases may redistribute toxic blooms of K. brevis (“red tide” blooms) into new geographic locations, which would change human exposure risk in newly affected areas.

Fish and Shellfish 

Water-related contaminants as well as naturally occurring harmful bacteria and algae can be accumulated by fish or shellfish, providing a route of human exposure through consumption (see also Ch. 7: Food Safety).134,135,136 Shellfish, including oysters, are often consumed raw or very lightly cooked, which increases the potential for ingestion of an infectious pathogen.137

Pathogens Associated with Fish and Shellfish

Enteric viruses (for example, noroviruses and hepatitis A virus) found in sewage are the primary causes of gastrointestinal illness due to shellfish consumption.138,139 Rainfall increases the load of contaminants associated with sewage delivered to shellfish harvesting waters and may also temporarily reduce salinity, which can increase persistence of many enteric bacteria and viruses.140,141,142,143 Many enteric viruses also exhibit seasonal patterns in infection rates and detection rates in the environment, which may be related to temperature.144,145,146

Among naturally occurring water-related pathogens, Vibrio vulnificus and V. parahaemolyticus are the species most often implicated in foodborne illness in the United States, accounting for more than 50% of reported shellfish-related illnesses annually.139,147,148,149,150 Cases have increased significantly since 1996.91,147 Rising sea surface temperatures have contributed to an expanded geographic and seasonal range in outbreaks associated with shellfish.95,151,152,153,154

Precipitation is expected to be the primary climate driver affecting enteric pathogen loading to shellfish harvesting areas, although temperature also affects bioaccumulation rates of enteric viruses in shellfish. There are currently no national projections for the associated risk of illness from shellfish consumption. Many local and state agencies have developed plans for closing shellfish beds in the event of threshold-exceeding rain events that lead to loading of these contaminants and deterioration of water quality.155

Increases in sea surface temperatures, changes in precipitation and freshwater delivery to coastal waters, and sea level rise will continue to affect Vibrio growth and are expected to increase human exposure risk.95,133,151,156 Regional models project increased abundance and extended seasonal windows of growth of Vibrio pathogens (see Research Highlight on seasonal Vibrio abundance).131 The magnitude of health impacts depends on the use of intervention strategies and on public and physician awareness.157

Harmful Algal Toxins

Harmful algal blooms (HABs) that contaminate seafood with toxins are becoming increasingly frequent and persistent in coastal marine waters, and some have expanded into new geographic locations.158,159,160,161,162 Attribution of this trend has been complicated for some species, with evidence to suggest that human-induced changes (such as ballast water exchange, aquaculture, nutrient loading to coastal waters, and climate change) have contributed to this expansion.161,163

Among HABs associated with seafood, ciguatera fish poisoning (CFP) is most strongly influenced by climate.164,165,166 CFP is caused by toxins produced by the benthic algae Gambierdiscus (Table 6.1) and is the most frequently reported fish poisoning in humans.167 There is a well-established link between warm sea surface temperatures and increased occurrences of CFP,164,165,166 and in some cases, increases have also been linked to El Niño–Southern Oscillation events.168 The frequency of tropical cyclones in the United States has also been associated with CFP, but with an 18-month lag period associated with the time required for a new Gambierdiscus habitat to develop.164,165

Paralytic shellfish poisoning (PSP) is the most globally widespread shellfish poisoning associated with algal toxins,169 and records of PSP toxins in shellfish tissues (an indicator of toxin-producing species of Alexandrium) provide the longest time series in the United States for evaluating climate impacts. Warm phases of the naturally occurring climate pattern known as the Pacific Decadal Oscillation co-occur with increased PSP toxins in Puget Sound shellfish on decadal timescales.170 Further, it is very likely that the 20th century warming trend also contributed to the observed increase in shellfish toxicity since the 1950s.171,172 Warm spring temperatures also contributed to a bloom of Alexandrium in a coastal New York estuary in 2008.173 Decadal patterns in PSP toxins in Gulf of Maine shellfish show no clear relationships with long-term trends in climate,174,175,176 but ocean–climate interactions and changing oceanographic conditions are important factors for understanding Alexandrium bloom dynamics in this region.177

There is less agreement on the extent of climate impacts on other marine HAB-related diseases in the United States. Increased abundances of Pseudo-nitzschia species, which can cause amnesic shellfish poisoning, have been attributed to nutrient enrichment in the Gulf of Mexico.178 On the U.S. West Coast, increased abundances of at least some species of Pseudo-nitzschia occur during warm phases associated with El Niño events.179 For Dinophysis species that can cause diarrhetic shellfish poisoning, data records are too short to evaluate potential relationships with climate in the United States,158,180 but studies in Sweden have found relationships with natural climate oscillations.181

The projected impacts of climate change on toxic marine harmful algae include geographic range changes in both warm- and cold-water species, changes in abundance and toxicity, and changes in the timing of the seasonal window of growth.182,183,184,185 These impacts will likely result from climate change related impacts on one or more of 1) water temperatures, 2) salinities, 3) enhanced surface stratification, 4) nutrient availability and supply to coastal waters (upwelling and freshwater runoff), and 5) altered winds and ocean currents.182,184,185,186,187

Limited understanding of the interactions among climate and non-climate stressors and, in some cases, limitations in the design of experiments for investigating decadal- or century-scale trends in phytoplankton communities, makes forecasting the direction and magnitude of change in toxic marine HABs challenging.183,185 Still, changes to the community composition of marine microalgae, including harmful species, will occur.182,188 Conditions for the growth of dinoflagellates—the algal group containing numerous toxic species—could potentially be increasingly favorable with climate change because these species possess certain physiological characteristics that allow them to take advantage of climatically-driven changes in the structure of the ocean (for example, stronger vertical stratification and reduced turbulence).184,187,189,190,191

Climate change, especially continued warming, will dramatically increase the burden of some marine HAB-related diseases in some parts of the United States, with strong implications for disease surveillance and public health preparedness. For example, the projected 4.5°F to 6.3°F increase in sea surface temperature in the Caribbean over the coming century is expected to increase the incidence of ciguatera fish poisoning by 200% to 400%.165 In Puget Sound, warming is projected to increase the seasonal window of growth for Alexandrium by approximately 30 days by 2040, allowing blooms to begin earlier in the year and persist for longer.171,184,192

6.4 Populations of Concern

Climate change impacts on the drinking water exposure pathway (see “Drinking Water”) will act as an additional stressor on top of existing exposure disparities in the United States. Lack of consistent access to potable drinking water and inequities in exposure to contaminated water disproportionately affects the following populations: tribes and Alaska Natives, especially those in remote reservations or villages; residents of low-income rural subdivisions known as colonias along the U.S.–Mexico border; migrant farm workers; the homeless; and low-income communities not served by public water utilities—which can be urban, suburban, or rural, and some of which are predominately Hispanic or Latino and Black or African American communities in certain regions of the country.200,201,202,203,204,205,206,207,208 In general, the heightened vulnerability of these populations primarily results from unequal access to adequate water and sewer infrastructure, and various environmental, political, economic, and social factors jointly create these disparities.201  

Children, older adults (primarily age 65 and older), pregnant women, and immunocompromised individuals have higher risk of gastrointestinal illness and severe health outcomes from contact with contaminated water.4,209,210,211,212,213 Pregnant women who develop severe gastrointestinal illness are at high risk for adverse pregnancy outcomes (pregnancy loss and preterm birth).214 Because children swallow roughly twice as much water as adults while swimming, they have higher recreational exposure risk for both pathogens and freshwater HABs.100,119 Recent cryptosporidiosis and giardiasis cases have frequently been reported in children aged one to nine years, with onset of illness peaking during the summer months.215 In addition, 40% of swimming-related eye and ear infections from Vibrio alginolyticus during the period 1997–2006 were reported in children (median age of 15).92

Razor clam dig

Water-related contamination of shellfish may reduce consumption and contribute to loss of tribal cultural practices tied to shellfish harvest.

Traditional tribal consumption of fish and shellfish in the Pacific Northwest and Alaska can be on average 3 to 10 times higher than that of average U.S. consumers, or even up to 20 times higher.216 Climate change will contribute to increased seafood contamination by toxins and potentially by chemical contaminants (see section 6.5), with potential health risks and cultural implications for tribal communities. Those who continue to consume traditional diets may face increased health risks from contamination.217 Alternatively, replacing these traditional nutrition sources may involve consuming less nutritious processed foods and the loss of cultural practices tied to fish and shellfish harvest.218,219

6.5 Emerging Issues

A key emerging issue is the impact of climate on new and re-emerging pathogens. While cases of nearly-always-fatal primary amoebic meningoencephalitis due to the amoeba Naegleria fowleri and other related species remain relatively uncommon, a northward expansion of cases has been observed in the last five years.220,221 Evidence suggests that in addition to detection in source water (ground and surface waters), these amoebae may be harbored in biofilms associated with water distribution systems, where increased temperatures decrease efficacy of chlorine disinfection and support survival and potentially growth.222,223,224

Climate change may also alter the patterns or magnitude of chemical contamination of seafood, leading to altered effects on human health—most of which are chronic conditions. Rising temperatures and reduced ice cover are already linked to increasing burdens of mercury and organohalogens in arctic fish,225 a sign of increasing contamination of the arctic food chain. Changes in hydrology resulting from climate change are expected to alter releases of chemical contaminants into the Nation’s surface waters,226 with as-yet-unknown effects on seafood contamination.

6.6 Research Needs

In addition to those identified in the emerging issues discussion above, the authors highlight the following potential areas for additional scientific and research activity on water-related illness, based on their review of the literature. Enhanced understanding of climate change impacts will be facilitated by improved public health surveillance for water-related infectious diseases and expanded monitoring and surveillance of surface and coastal water quality. In addition, improved understanding of how human behaviors affect the risk of waterborne diseases can facilitate the development of predictive models and effective adaptation measures. Predictive models can also help identify major areas of uncertainty and refine key research questions.

Future assessments can benefit from research activities that

  • assess the interactions among climate drivers, ecosystem changes, water quality and infectious pathogens, including Vibrio spp., N. fowlerii, chemical contaminants, and harmful algal blooms;
  • increase understanding of how marine and terrestrial wildlife, including waterfowl, contribute to the distribution of pathogens and transmission of infectious disease and assess the role of climate;
  • explore how ocean acidification affects toxin production and distribution of marine HABs and pathogens;
  • analyze the hydrologic (discharge, flow-residence time, and mixing) thresholds for predicting HAB occurrences; and
  • increase understanding of how the impacts of climate change on drinking water infrastructure, including the need for development of new and emerging technologies for provision of drinking water, affect the risks of waterborne diseases.


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Very Likely
≥9 in 10
≥2 in 3
As Likely as Not
≈ 1 in 2
≤ 1 in 3
Very Unikely
≤1 in 10

Confidence Level

Very High Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus
High Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus
Medium Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought
Low Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts

Documenting Uncertainty: This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Appendix 4: Documenting Uncertainty for more on assessments of likelihood and confidence.

Key Finding 1: Seasonal and Geographic Changes in Waterborne Illness Risk

Increases in water temperatures associated with climate change will alter the seasonal windows of growth and the geographic range of suitable habitat for freshwater toxin-producing harmful algae [Very Likely, High Confidence], certain naturally occurring Vibrio bacteria [Very Likely, Medium Confidence], and marine toxin-producing harmful algae [Likely, Medium Confidence]. These changes will increase the risk of exposure to waterborne pathogens and algal toxins that can cause a variety of illnesses [Medium Confidence].

Key Finding 2: Runoff from Extreme Precipitation Increases Exposure Risk

Runoff from more frequent and intense extreme precipitation events will increasingly compromise recreational waters, shellfish harvesting waters, and sources of drinking water through increased introduction of pathogens and prevalence of toxic algal blooms [High Confidence]. As a result, the risk of human exposure to agents of water-related illness will increase [Medium Confidence].

Key Finding 3: Water Infrastructure Failure

Increases in some extreme weather events and storm surges will increase the risk that infrastructure for drinking water, wastewater, and stormwater will fail due to either damage or exceedance of system capacity, especially in areas with aging infrastructure [High Confidence]. As a result, the risk of exposure to water-related pathogens, chemicals, and algal toxins will increase in recreational and shellfish harvesting waters and in drinking water where treatment barriers break down [Medium Confidence].