Plentiful and essential, water is considered the universal solvent for biological life. Indeed, life is thought to have originated in the aqueous environment of the planet, and living organisms rely on blood, extra- and intracellular solutions and digestive juices for biological processes. Yet, water can also be a place where dangerous and deadly microbes reside, waiting for a host to arrive.
Waterborne illnesses are likely to occur more frequently with the ongoing changes to our weather and climate patterns. The health effects of climate change could undercut public health advances and developments made in the last 50 years. Rain events, storm surges and the rise and fall of temperatures and humidity levels all impact the survival of microbes on land and in our seas. Some have evolved strategies to survive at temperatures higher than ambient body or host temperatures. Furthermore, changes in precipitation patterns can mobilize or concentrate pathogens and compromise water and sanitation infrastructure. Flooding and runoff can lead to the dissemination of raw sewage, harmful chemicals and pathogens, and drought can concentrate pathogens in areas of limited water supply. These factors favor the growth, reproduction, spread and survival of the major microbial taxa (parasites, helminths, bacteria, viruses and toxins created by harmful algae). Ultimately, global populations will likely meet these agents, leading to a rise in waterborne illnesses through ingestion, inhalation and skin contact, as well as via the consumption of contaminated fish and shellfish.
Examples of Waterborne Pathogens
In the December 2021 issue of Emerging Infectious Diseases, the Centers for Disease Control and Prevention (CDC) reported that a 56-year-old Maryland woman hospitalized on Sept. 20, 2019 had likely acquired melioidosis, a disease characterized by a range of symptoms, including localized, pulmonary, bloodstream and disseminated infection, via novel transmission ofBurkholderia pseudomallei from a freshwater home aquarium. The woman is the first person known to have acquired the severe tropical infection by this transmission route.
One of the reasons that aquatic infections are not immediately considered in differential diagnoses is that many aquatic zoonoses, including B. pseudomallei, are not endemic in the U.S. Melioidosis, the disease caused by B. pseudomallei and formerly known as Whitmore’s disease, was first described in 1912. Cases were historically identified primarily in northern Australia and areas of Southeast Asia, such as Thailand, and almost all cases of infection reported in the U.S. have been related to international travel to endemic regions.
In the U.S., 11.5 million households have pet fish, and most exotic aquarium animals are caught in the wild and transported to a major hub for transport to the U.S., where a quarantine may not be in place. This could be why outbreaks have recently been reported in the U.S.
Mycobacterium marinum is another bacterium that is commonly found in both salt and fresh water. It is the causative agent of disease in many species of fish, and the disease occasionally reaches humans. The infection is commonly known as fish tank granuloma, and it produces nodular or ulcerating skin lesions on the extremities of healthy hosts. Fishhook injury is a common route of infection. Diagnosis is usually delayed because individuals do not consider this a likely microbial route of infection, and invasion into deeper structures, such as synovia, bursae and bone, occur in approximately 30% of reported cases. M. marinum is not a nationally notifiable condition in the U.S.
Fishing and/or cleaning and preparing fish or shellfish may provide additional exposure risk to water-loving bacteria. For example, Streptococcus iniae has caused cellulitis, arthritis, endocarditis and meningitis following superficial or puncture injuries, notably from cleaning tilapia. Other infections from contact with fish include, primarily topical skin infections from Erysipelothrix rhusiopathiae, and gastroenteritis from Plesiomonas shigelloides, Campylobacter spp. and Salmonella spp.
How Do Environmental Conditions Influence Pathogens?
How does water become contaminated with pathogens in the first place? Most people are aware that water is usually treated to ensure safe drinking quality, so how do outbreaks of waterborne illness occur?
Much of the existing research, including a recent extensive literature review from a team of collaborators at Emory University and University of Colorado, examines historical relationships between observed weather and disease incidence, with a more limited number of studies projecting future disease rates. Positive associations between ambient temperature and diarrheal diseases (excluding viral diarrhea), as well as an increase in diarrheal disease following heavy rainfall and flooding events, have been recognized. The effects of drought on diarrhea have been less supported, due to a lack of evidence.
Heavy rainfall typically increases in intensity, duration and frequency due to climate changes. With increased intensity and/or amount of rainfall come increased runoff and flooding, especially in river and coastal environments. As one might expect, massive rainfalls can wash substances at the surface level, like chemicals, animal waste containing pathogens, gasoline and other dangerous items, into sources of drinking water. Changes in the Great Lakes’ rainfall rates, higher lake temperatures and low lake levels have all been linked to increases in fecal bacteria levels in those waters. During summers following heavy rainfalls, freshwater streams in the southeastern U.S. show increased levels of the bacterial pathogens Salmonella and Campylobacter.
Additionally, Cryptosporidium parvum, a parasitic protozoan that causes diarrhea, cramping, abdominal pain and fever, is responsible for the deadliest waterborne outbreak in U.S. history in Milwaukee. Cryptosporidium infects the intestines of people and animals and is resistant to chlorination. Due to this resistance, one often must filter the water and utilize ultraviolet radiation or ozone to make the drinking water safe. Possible sources of this outbreak included cattle, slaughterhouses and human sewage along 2 rivers that flow into the Milwaukee harbor. Rivers that were swelled by spring rains and snow runoff may have transported oocysts into Lake Michigan, and from there to the intake of the southern water plant. The outbreak caused 403,000 illnesses and over 50 deaths.
In 2000, heavy rainfall in Walkerton, Ontario, Canada, carried agricultural runoff containing E. coli into the town’s primary water source: a shallow well. This extreme weather-related event caused 2,300 illnesses and 7 deaths. Since groundwater wells receive limited water treatment because the rock and sediment layers act as natural filters, they are more at risk for contamination with heavy precipitation events, increasing the possibility of waterborne illnesses for anyone who drinks water from those common wells.
Data indicate that the intersection of flooding and higher temperatures will not only transport pathogens into recreational waters, but may also help facilitate a microbial population explosion. Numerous examples of temperature-induced microbial growth have been reported for Naegleria fowleri, commonly referred to as the “brain-eating amoeba;” Salmonella; Campylobacter and Legionella, which causes Legionnaires’ Disease, a respiratory illness consisting of cough, shortness of breath, high fever, muscle aches and headaches.
Massive sewage overflow from events such as heavy rainfalls can also contaminate seafood. This type of event occurs repeatedly in California, where an outdated sewage treatment plant is unable to accommodate heavy rainfall and subsequently causes sewage to flow from the Tijuana River into the adjacent Pacific Ocean, spreading pathogens across regions of beaches along the California-Mexico border. Fish and shellfish can accumulate viruses found in sewage and put consumers at risk for gastrointestinal viruses, such as norovirus and hepatitis A. Each of these can result in stomach pain, nausea, diarrhea and vomiting. Hepatitis A can also cause acute liver failure, which is associated with high mortality.
Increases in the frequency and range of harmful algal blooms is another concern associated with rising sea surface temperatures and altered rainfall distribution. Certain algal blooms produce potent toxins that can contaminate seafood and lead to dangerous health issues. Paralytic shellfish poisoning is the most common and severe form of shellfish poisoning and is caused by ingesting shellfish contaminated with saxitoxins that are produced by phytoplankton. Symptoms include numbness, tingling sensations in the body, headache, nausea, vomiting and diarrhea. A large dose of the toxin may lead to the inability to control bodily movements, difficulty swallowing, change in mental status, flaccid paralysis (feeling weak or paralyzed) and respiratory failure. Ciguatera fish poisoning, or ciguatera, is an illness caused by eating fish that contain toxins produced by a marine microalga called Gambierdiscus toxicus. People who have ciguatera may experience nausea, vomiting and neurologic symptoms such as tingling fingers or toes.
Prevention and Treatment of Waterborne Infections
Keeping some basic safety measures in mind while interacting with water outdoors or indoors can help limit the exposure and spread of waterborne infections. For example, when swimming or playing in recreational facilities, it is important to keep water out of the mouth and ears. Likewise, individuals who have been sick with diarrhea in the past 2 weeks should stay out of the water. Disinfection with chlorine or bromine and properly regulated pH is the first defense against the germs that cause recreational water illnesses in pools, hot tubs/spas and water playgrounds. If one suspects something is not right with the body of water they are about to enter, such as it smells bad or is cloudy, they should stop and report it to someone who can inspect the facility.
People should not participate in outdoor water sports in lakes, rivers and streams without proper eye and nasal protection to protect against the rare, but deadly, parasite N. fowleri. Also, the importance of careful hand hygiene when caring for aquariums cannot be stressed enough. Aquarium filters, filter floss, biofilm, charcoal and gravel might have exceptionally high concentrations of bacteria. Gloves are critical when cleaning aquariums, and immunocompromised individuals should avoid this task altogether. Finally, one should never drink water from an unknown source or if there is uncertainty about whether the water has been properly treated.
Individuals who acquire waterborne illnesses usually have primarily gastrointestinal symptoms, but upper respiratory symptoms and skin manifestations may also appear. Gastrointestinal symptoms are usually self-limited, and supportive treatment may be all that is necessary. However, some infections can cause significant morbidity and mortality.
In some infections, specific antimicrobial agents may be required to limit the pathology. For example, Vibrio vulnificus can cause a dangerous infection known as cellulitis, which can lead to amputation or death. With Legionnaires' disease, if it is unrecognized and progresses, the mortality rate can be as high as 10%. Lastly, there could occasionally be medical and public health reasons to utilize prophylaxis to prevent the spread of disease in outbreak situations (e.g., long-term care facilities, daycare centers and among immunocompromised populations).
In 2012, the WHO estimated that 12.6 million deaths (23% of global mortality) were attributable to modifiable environmental factors, many of which could be influenced by climate change or are related to the driving forces of climate change. While this article does not provide a comprehensive look at climate change, one can conclude that environmental conditions do influence pathogens via direct impacts of climate change that result in higher temperatures (e.g., heatwaves) and increases in the frequency of complex, extreme weather events (e.g., windstorms, floods, droughts).