Impact of Climate Change on Water-borne Diseases and Toxic Algae

Water-borne diseases: types and climate change effects.

Waterborne diseases like cholera and typhoid are still a leading cause of human morbidity and mortality worldwide. As climate change increasingly impacts global temperatures and weather events, the risk of these infectious diseases will only worsen. The most prevalent water-borne diseases are cholera, typhoid, diarrheagenic Escherichia coli (DEC), and leptospirosis. There is an additional range of water-related vector-borne diseases such as malaria, dengue fever, etc., that are covered in our information hub section on vector-borne diseases.

Cholera

Vibrio cholerae: a commensal of copepods (zooplankton) that thrives in water along the coast and in large bodies of water inland. According to WHO: V. cholerae infects 3-5 million people/yr (with diarrhea symptoms) and 100,000 deaths/yr globally. Peaks in cholera occurrence correspond with low river discharge in the spring, which allows bacteria-laden tidal seawater to wash in, and high river discharge in the autumn, leading to the cross-contamination of water supplies. Hot weather and above-average rainfall in the context of poor infrastructure and crowding. Warmer nutrient-rich waters support thriving populations of phytoplankton, consequently leading to a bloom of zooplankton. It can never be eradicated, because the host is an integral part of the environment. Sea surface temperature, sea surface height, and, most recently, salinity can predict likely outbreaks of cholera in a given region, because all these variables affect the timing, location, and severity of an outbreak. The disease appears in two different outbreak patterns: endemic, in coastal areas directly influenced by tidal cycles, and epidemic, occurring more in inland areas, which can go years without many cases and then experience a huge outbreak.

Elevated sea surface temperature (SST) in brackish water provides ideal environmental growth conditions for Vibrio species. These conditions can be found during the summer months in areas of water with moderate salinity such as the Baltic Sea, or the East China Sea around Shanghai. The SST of enclosed bodies of water and estuaries has increased more rapidly as a result of climate change than that of oceans. The number of Vibrio-associated illness cases has been found to increase in line with a rise in sea surface temperature during summer. Rainfall also has a direct influence on the transmission of cholera. High rainfall can increase the risk of wastewater contaminating either raw or treated water (person-to-environment transmission). In addition, low rainfall can increase the concentration of pathogens in water (environment-to-person transmission).

Typhoid

Typhoid fever is a life-threatening infection caused by the bacterium Salmonella typhi. It is usually spread through contaminated food or water. Once Salmonella typhi bacteria are ingested, they multiply and spread into the bloodstream. Primarily affects children and adolescents in low- and middle-income countries. Global Burden of Disease estimates in 2019: 9 million cases and more than 110,000 deaths due to typhoid worldwide. Urbanization and climate change have the potential to increase the global burden of typhoid. In addition, increasing resistance to antibiotic treatment is making it easier for typhoid to spread in communities that lack access to safe drinking water or adequate sanitation.

Studies have suggested that an increase of 1°C in the mean weekly minimum temperature can lead to a 5.8% rise in the weekly number of typhoid cases. Especially consequential is bacterial proliferation in food during production, storage and distribution. Floods also create vulnerability to faecal‐oral transmission of typhoid and other pathogens due to obstructed urban drains and the release of untreated sewage or faecal matter. In situations where access to clean water is challenging, people are more inclined to consume water from surface sources, which are contaminated by floodwaters, potentially carrying Salmonella. Floods have been recognized as the most frequent and devastating natural disaster worldwide and are projected to increase due to changing precipitation patterns and rising sea levels. Research shows that compared to no floods, the risk of typhoid fever infection is 1.76 times higher during floods.

Weather extremes can enhance the transmission of S. typhi. For example, across various intensity levels, tropical cyclones elevate the risk of infectious diarrhoea linked to typhoid. Additionally, insects may serve as transient vectors during hurricanes, broadening the range of transmission. Extreme weather events, including tsunamis and hurricanes, wield substantial impacts on human production, livelihoods and ecosystems. Displacement of families from their homes due to extreme weather frequently results in overcrowded and unsanitary conditions, thereby amplifying the mortality rate associated with typhoid.

Diarrheagenic Escherichia coli (DEC)

E. coli consists of a diverse group of bacteria. Pathogenic E. coli strains are categorized into pathotypes. Six pathotypes are associated with diarrhea and collectively are referred to as diarrheagenic E. coli. Pathogenic bacteria can cause illness, either diarrhea or illness outside of the intestinal tract. The types of E. coli that can cause diarrhea can be transmitted through contaminated water or food, or through contact with animals or persons. A 8% increase in the incidence of diarrheagenic E. coli has been found for each 1°C increase in mean monthly temperature.

Leptospirosis

Leptospirosis is a disease that affects humans and animals, caused by bacteria of the genus Leptospira. In humans, it can cause a wide range of symptoms. Without treatment, leptospirosis can lead to kidney damage, meningitis, liver failure, respiratory distress, and even death. The bacteria that cause leptospirosis are spread through the urine of infected animals, which can get into water or soil. The bacteria can enter the body through skin or mucous membranes, especially if the skin is broken from a cut or scratch. Drinking contaminated water can also cause infection. Outbreaks of leptospirosis are usually caused by exposure to contaminated water, such as floodwaters.

Higher annual mean temperatures and higher rainfall levels enhance the growth and activity of Leptospira spp., and at the same time lengthen the infectious season and expand the geographical distribution of the bacteria. These climate change effects are expected to increase the disease burden. Heavy rainfall, storms and associated flooding events also increase human exposure to contaminated water, particularly in combination with poor sanitation, insufficient healthcare or crowded conditions, thus exposing people to increased infection risks. Drought episodes on the other hand stimulate recreational activities like swimming and bathing, and the risks of occupational exposure for example when farms use alternative, contaminated water sources in periods of drought. Both could lead to an increase in leptospirosis infections.