By Charles Nicolson
As most of our world continues to live under the pandemic viral cloud of Covid-19, the three previous articles in ‘Getting Technical’ have looked at what viruses are, some of their properties and the effects viruses can cause when they use human body cells as hosts.
Information was also given on how these ultra-tiny entities can survive for periods suspended in the atmosphere or in contact with various surfaces for several hours or even several days. This final section is concerned with viruses in water volumes and water flows.
In common with all other organisms, micro -, macro – or larger, viruses do not dissolve in water but remain in suspension although they will gradually sink downwards by gravity in static water volumes. There are also many variables that affect the survival of viruses in water such as temperature, light, pH, salinity, organic matter content, other suspended solids or sediments and air–water interfaces.
Temperature has a direct effect on how long viruses can survive in water. In general, survival time is longer at lower temperatures, since lower temperatures are the key to continuing virus survival. Research data published in 2018 showed that it takes up to a year for amounts of viruses such as poliovirus in water volumes to reduce by one 5log unit at a temperature of 4°C, whereas at normal human body temperature of 37°C the same reduction occurs within a few days.
Ultraviolet light such as UV in sunlight can deactivate viruses. Many viruses in water are exterminated in the presence of sunlight. The combination of higher temperatures and increased UV in summer time corresponds to shorter viral survival in summer compared to winter. Visible light can also affect virus survival by a process called photodynamic inactivation. The rate of inactivation depends on the duration of exposure to and intensity of the light.
The pH of most natural water is between 5–9. Enteric viruses are stable in these conditions although some of these viruses are more stable at pH 3-5 than at pH 9 and 12. Enteroviruses can also survive at high pH levels of 11–11.5 as well as low values of 1–2, but for only short periods. Adenoviruses and rotaviruses are sensitive to a pH of 10 or greater which results in progressive de-activation.
In general, viruses do not survive in water containing high concentrations of dissolved salts. Therefore, viruses tend to live longer in freshwater habitats than water bodies with high salt concentrations. Also, certain heavy metals dissolved or dispersed in water are toxic to viruses.
Another effect which is often seen is aggregation which is the one of the most common methods used for survival by viruses. In a liquid environment, viruses tend to clump together forming aggregations. These aggregations usually result in a reduced rate of virus inactivation indicating that viral particles that do not aggregate are more easily destroyed. Aggregation may form spontaneously or may result by nucleation on particles in water.
Example of viral aggregation
Water that is intended for drinking should go through some treatment to reduce pathogenic viral and bacterial concentrations to levels which have proven to be generally safe. Reducing the amounts of viruses in drinking water is accomplished by various treatments that are typically part of drinking water treatment systems in developed countries.
In recent years there has been considerable progress in the quality of drinking water which is ensured through a framework of water safety plans incorporating the safe disposal of human waste so that drinking water supplies are not contaminated. However, most studies agree that improving the sanitation, hygiene and management of water resources could prevent at least a further ten percent of total global disease.
About half of the hospital beds occupied in the world are related to the lack of safe drinking water. Unsafe water leads to the over 80% of global cases of diarrhoea and 90% of the deaths of diarrhoeal diseases in children under five years old. Most of these deaths occur in developing countries due to poverty and the high cost of safe water. An article published in 2003 by the CDC – the Centre for Disease Control, a United States federal agency under the Department of Health and Human Services – concluded that the death of children at under five years of age caused by rotaviruses ranges between 352 000 to 592 000 per annum on a global scale.
Over 1 billion people do not have access to improved water and more than 2 billion people do not have access to sanitation facilities. This situation leads to 2 million preventable deaths each year.
Cynthia Mathew, a researcher at the University of Canberra, recently wrote a short, lucid, introductory article on viruses from which some extracts and comments are presented below.
Viruses are mostly regarded as having aggressive and infectious natures which is true in the sense that most viruses have a pathogenic relationship with their human hosts in that they cause diseases ranging from a mild cold to serious conditions like severe acute respiratory syndrome (SARS).
But not all viruses are bad. Some viruses kill bacteria, while others can fight against more dangerous viruses. In fact, there are several protective viruses in human bodies, often referred to as ‘probiotics’.
Bacteriophages, or ‘phages’, are viruses that infect and destroy specific bacteria. They are found in the mucus membrane lining sections in human digestive, respiratory and reproductive tracts.
Mucus is a thick, jelly-like material that provides a physical barrier against invading bacteria and protects the underlying cells from being infected. Recent research suggests the phages present in the mucus are part of natural immune systems protecting the human body from invading bacteria.
Phages have, in fact, been used to treat dysentery, sepsis caused by Staphylococcus aureus, salmonella infections and skin infections for nearly a century. Early sources of phages for therapy included local water bodies, dirt, air, sewage and even body fluids from infected patients. The viruses were isolated from these sources, purified, and then used for treatment. Phages have attracted renewed interest as the rise of drug-resistant infections continues. Recently, there was a report on a teenager in the United Kingdom who was close to death when phages were successfully used to treat a serious infection that had been resistant to antibiotics.
Nowadays, phages are genetically engineered. Individual strains of phages are tested against target bacteria, and the most effective strains are purified into potent concentrations. These are stored as either bacteriophage stocks (cocktails), which contain one or more strains of phages and can be used to target broad ranges of bacteria. Alternatively, particular phages can be selected from phage stocks to target specific bacteria. Treatment can be safely administered orally, applied directly onto wounds or bacterial lesions, or even spread onto infected surfaces. Clinical trials for intravenous administration of phages are ongoing.
Viral infections at a young age are important to ensure the proper development of our immune systems. In addition, the immune system is continuously stimulated by systemic viruses at low levels sufficient to develop resistance to other infections. Some viruses protect humans against infection by other pathogenic viruses. For example, latent (non-symptomatic) herpes viruses can help human natural killer cells which are a specific type of white blood cell, identify cancer cells and cells infected by other pathogenic viruses. They arm the natural killer cells with foreign substances known as antigens which cause immune responses in the human body that will enable them to identify tumour cells.
This is both a survival tactic by the viruses to last longer within their host, and to get rid of competitive viruses to prevent them from damaging the host. In the future, modified versions of viruses like these could potentially be used to target cancer cells.
Image credit: Freepick.com
There are viruses such as Pegivirus C or GBV-C that do not cause clinical symptoms. Studies have shown that HIV patients infected with GBV-C live longer in comparison to patients without it. Pegivirus C slows disease progression by blocking the host receptors required for viral entry into the cell and promotes the release of virus-detecting interferons and cytokines – proteins produced by white blood cells that activate inflammation and removal of infected cells or pathogens.
Modern technology has enabled us to understand more about the complexities of the microbial communities that are part of the human body. In addition to ‘good’ bacteria, we now know there are beneficial viruses present in the gut, skin and even blood.
Cynthia concludes her article by noting that “Our understanding of this viral component is largely in its infancy. But it has huge potential in helping us understand viral infections, and importantly, how to fight the bad ones. It could also shed light on the evolution of the human genome, genetic diseases, and the development of gene therapies.”