Viruses are a major cause of human waterborne and water-related diseases. Waterborne diseases are caused by water that is contaminated by human and animal urine and feces that contain pathogenic microorganisms. A subject can get infected through contact with or consumption of the contaminated water. Viruses affect all living organisms from single cellular plants, bacteria and animal to the highest forms of plants and animals including human beings. Within a specific kingdom ( Plantae, Animalia, Fungi etc.) the localization of viruses colonizing the host can vary: Some human viruses, for example, HIV, colonizes only the immune system, while influenza viruses on the other hand can colonize either the upper respiratory tract or the lower respiratory tract depending on the type (human Influenza virus or avian influenza viruses respectively). Different viruses can have different routes of transmission; for example, HIV is directly transferred by contaminated body fluids from an infected host into the tissue or bloodstream of a new host while influenza is airborne and transmitted through inhalation of contaminated air containing viral particles by a new host. Research has also suggested that solid surface plays a role in the transmission of water viruses. In experiments that used E.coli phages, Qβ, fr, T4, and MS2 confirmed that viruses survive on a solid surface longer compared to when they are in water. Because of this adaptation to survive longer on solid surfaces, viruses now have a prolonged opportunities to infect humans. Enteric viruses primarily infect the intestinal tract through ingestion of food and water contaminated with viruses of fecal origin. Some viruses can be transmitted through all three routes of transmission. Water virology started about half a century ago when scientists attempted to detect the polio virus in water samples. Since then, other pathogenic viruses that are responsible for gastroenteritis, hepatitis, and many other virus strains have replaced enteroviruses as the main aim for detection in the water environment.
History
Major outbreaks Water virology was born after a large hepatitis outbreak transmitted through water was confirmed in New Delhi between December 1955 and January 1956. Viruses can cause massive human mortality. The smallpox virus killed an estimated 10 to 15 million people per year until 1967. Smallpox was finally eliminated in 1977 by extinction of the virus through vaccination, and the impact of viruses such as influenza, poliomyelitis and measles are mainly controlled by vaccination. Despite advances in vaccination and prevention of viral diseases, it is estimated that in the 1980s a child died approximately every six seconds from diarrhea confirmed by WHO. Many cases of hepatitis A and/or E, both of which are enteric viruses, are typically transmitted by food and water. Extreme examples include the outbreak of 300,000 cases of hepatitis A and 25,000 cases of gastroenteritis in 1988 in Shanghai caused by shellfish harvested from a sewage polluted estuary. In 1991, an outbreak of 79,000 cases of hepatitis E in Kanpur was ascribed to drinking polluted water. A more recent outbreak of Hepatitis E in South Sudan killed 88 people. Medecins Sans Frontieres (MSF) said it had treated almost 4,000 patients since the outbreak was identified in South Sudan in July 2012. In this outbreak, Hepatitis E, which causes liver infections, and was thought to be spread by drinking water contaminated with feces. Even more recently In 2014, another Hepatitis E outbreak occurred in south Sudan refugee camp situated in Ethiopia. The outbreak, which began in April 2014 and ended in January 2015, claimed a total of twenty-one lives. Sewage contaminated water contains many viruses, over one hundred species are reported and can lead to diseases that affect human beings. For example, hepatitis, gastroenteritis, meningitis, fever, rash, and conjunctivitis can all be spread through contaminated water. More viruses are being discovered in water because of new detection and characterization methods, although only some of these viruses are human pathogens.
Virus survival in water Viruses need a suitable environment to survive in. There are many characteristics that control the survival of viruses in water such as temperature, light, pH, salinity, organic matter, suspended solids or sediments, and air–water interfaces.
Temperature Temperature has the highest effect on virus's survival in water since lower temperatures are the key to longer virus survival. For instance, an article published in 2018 noted that it takes one year for certain viruses including poliovirus and echovirus to decrease by a 5log unit at a temperature of 4°C, while it takes only a week to obtain same result at a temperature of 37°C (human body temperature). The rate of protein, nucleic acid denaturation and chemical reactions that destroy the viral capsid are increased at higher temperatures, thus viruses will survive best at low temperatures. Hepatitis A, adenoviruses and parvoviruses have the highest survival rate in low temperatures amongst enteric viruses.
Light Ultraviolet light (UV) is the light in sunlight and can inactivate viruses by causing cross-linking of the nucleotides in the viral genome. Many viruses in water are exterminated in the presence of sunlight. The combination of higher temperatures and more UV in the summer time corresponds to shorter viral survival in summer compared to winter. Double stranded DNA viruses like adenoviruses are more resistant to UV light inactivation than enteroviruses because they can use their host cell to repair the damage caused by the UV light. Visible light can also affect virus survival by a process called photodynamic inactivation but the length and intensity of the light exposure can change the inactivation rate.
pH The pH of most natural water is between 5–9. Enteric viruses are stable in these conditions. On the other hand, many enteric viruses are more stable at pH 3-5 than at pH 9 and 12. Enteroviruses can survive at pH 11–11.5 and 1–2, but for only short periods. Adenoviruses and rotaviruses are delicate to a pH of 10 or greater and leads to inactivation.
Salts and metals In a general perspective, viruses don't survive in areas with high concentration of salt. Thus, viruses can live longer in a freshwater habitat than water bodies with high salt concentration. It is also known that certain heavy metals are toxic to viruses.
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