Norwalk Virus (Bar=50 nanometers)
From EPA Microbiology Homepage
Introduction
Detection/Indicators
Transport
Survival
Remediation/Treatment
References
Viruses are disease-causing agents that insert themselves into living tissues and reproduce, causing great damage to host organisms. Outside of a host, the virus is unable to reproduce and is dormant. Viruses are much smaller than bacteria and other life forms, making detection of viruses in water supply nearly impossible. Furthermore, a small quantity of a particular virus in a large volume of water can be epidemic because of its capacity to reproduce within a host and spread from host to host. Controversy and public concern has risen about possible viral contamination in drinking water due to the fact of difficulty in detection and isolation. Adding to the confusion, some research reports have produced contradictory results. This uncertainty has made it difficult for regulatory agencies to develop policies to protect public health and for engineers to develop a treatment for disinfection of specific viruses in drinking water. The primary sources of viruses are from sewage, industrial waste, and urban and rural runoff. These viruses become water-borne and contaminate ground water and surface water supplies, thus the drinking water supply is affected as well. Some viral agents found in drinking water supplies are Hepatitis-A, Norwalk-like agents, and Rotaviruses. In the United States, there are few reported cases of viral outbreaks. Since all drinking water is treated before consumption, most viruses are destroyed in this process. Even though scientists are uncertain about some aspects of viral contamination, there should be little public concern, because of the few outbreaks occurring from water-borne viruses.
There are two types of methods to check for the possibility of virus transmission, direct and indirect methods.
Direct methods consist of those which directly check for the concentrations of certain viruses in water. One such method is adsorption to filters. Through chemistry, it is known that it is possible to associate certain viruses with a certain type of filter material. The viruses are caught on the filter and their concentrations are measured. Problems arise with this method when other foreign object get caught in the filter material and restrict the flow of the water through the filter.
The indirect method of looking for viruses in ground water is to look for other organisms which point to the presence of viruses. The most traditional microorganism group looked for in possibly infected waters is the coliform group. This is possibly not a good way to test for the presence of viruses for a number of reasons. Viruses are more resistant to disinfection than coliforms, they are more stable in different conditions, and they are removed less easily by treatment processes.
The transport of viruses through soils to ground water and then to the public has been a topic of great concern. Many epidemics of infectious diseases has been contributed to the consumption of contaminated ground water. The factors controlling viral transport through soils are Soil Type, pH, Conductivity of the Percolating Water, Soluble Organic Materials, and Saturated versus Unsaturated Flow of Water.
The soil has a lot to do with the amount of viral transport that occurs. Fine textured soils tend to absorb viruses more than coarsely textured soils. In the fine textured soils viral transport is diminished because of the absorptivity. The high sorptive properties of a clay soil will impede viruses from transporting, where a coarse soil will not uphold the movement of the viruses to another medium, such as ground water.
Viruses are negatively charged, and will be attracted to a positive charge. In neutral to alkaline soil situations, the viruses will not bind and will be left to move. In acidic situations the viruses will bind to the soil and transportation is over. This is a good rule of thumb but it is not always true. With varying viral types, there will be exceptions to the rule.
Conductivity is a measure of the ionic strength of a solution. The virus absorption to the soils is affected by the concentrations of cations present. Cations allow absorption to take place between the virus and the soil by reducing their repulsive forces. The presence of sewage waste water provides an environment that allows viral retention to soils to rise, where as the retention in distilled water would be low. Distilled water may actually lead to desorption of viruses from the soil. This would mobilize the virus and transportation would begin.
Soluble Organic Materials will also try and absorb to the soil. In this way they will compete with the virus to find an absorption site. Humic and Fulvic acids also compete with the virus to find a spot to absorb. These acids have been shown to reduce the absorption of viruses to the soil and in some cases eliminate the absorption.
When the soil is saturated, all pores are filled with water. This allows viruses to pass through the soil more easily because contact with the soil has been lessened. When the flow is unsaturated, the viruses are in closer contact with the soil. The close contact promotes absorption of the virus to the soil. Unsaturated flow will be a direct method for absorbing viruses to the soil.
Factors Influencing Virus Transport in Soil
| Flow Rate | Rate of movement increases with increased water rate. |
| Hydraulic Condition | Rate of movement is greater in saturated than unsaturated flow. |
| Soil Texture | Fine grained soils retain move viruses than coarse grained soils. |
| Soil Solution | Greater ionic strength means greater adsorption of viruses. |
| pH | Higher pH leads to greater adhesion to soil. |
| Virus Type | Adsorption varies according to the strain and type of virus. |
| Humic Substances | Organic matter may retard virus adhesion to soils. |
| Cations | Adsorption increases in the presence of cations. |
Viral survival, replication, and die off rates in soils influenced by different factors. These factors include temperature, soil moisture, sunlight, and soil characteristics. Table Factors Influencing Virus Survival in Soils
This is the most detrimental factor in affecting viral survival in soils. As temperature changes so do the chemical and biological processes in the soils. With the changes in the processes, viral communities are affected. Although many virus communities will be destroyed with intense temperature changes, there are some thermoresistant members that will be able to endure the change for a extended period of time. The reason for the different effects on the virus colonies is the heterogeneity of viral populations.
The amount of moisture that is in the soil is proportional to the amount of virus in the soil. For example, enteric viruses survive for 15-25 days in an air-dried soil as compared to 60-90 days in a sample with 10 percent moisture by content. One of the pathways for viruses to escape from soil is through evaporation. This would account for the loss of viral pathogens from the dry soil.
The ultraviolet light from the sun is destructive to viruses. This light has been shown to inactivate viruses at the surface of the soil but as the viruses move deeper, the ultraviolet light plays a minor role. The deeper the virus goes the less of an effect the light plays.
The most prevalent component of a soil that would help viruses to survive is clay minerals. Viruses survive better in an absorptive state rather than a suspended state. The clay helps by absorbing water, and applying a high cohesive bond with it. Even when the soil is dry there may be some water left that is held by the clay minerals. This water can be utilized by the viruses to survive the dry environment. This absorption of the clay helps the viral components absorb to the soil and allows the viral particles to stabilize in the soil. As a result of this, viral genomes are protected from destruction by the clay minerals.
Die-off Rate of Certain Viruses
| Virus | Die-off rate(per day) |
| Poliovirus 1 | 0.046 to 0.77 |
| Coxsackievirus | 0.19 |
| Rotavirus SA-II | 0.36 |
| Coliphage T7 | 0.15 |
| Coliphage f2 | 0.39 to 1.42 |
There are two main ways that viruses replicate themselves, and note they replicate themselves instead of reproduce because they are not living organisms. The two types of replication are called the lysogenic and lytic cycles. Most of the actions of the viruses in these cycles are the same up until the end of the cycles. The first step of both cycles is that a virus moves around in the open atmosphere in a protective protein casing called a capsid. Then the virus attaches itself to a host cell and absorbs to it. The DNA of the virus is injected into the cytoplasm of the host cell through its outer and cytoplasmic membranes. The virus' DNA is then transcribed onto the host cells' RNA.
In the lysogenic replication cycle the host cell replicates and goes through many generations with the virus' DNA within its own DNA, because of a repressor that maintains the viral DNA as inactive. However, when the host cell is threatened by ultraviolet light or something else that can kill it, the viral repressor is no longer produced and the viral DNA becomes active, and the lytic cycle begins. The host cell begins to replicate viral DNA and viral structures, so that the new viruses will be able to survive in the outside atmosphere. The viral DNA is packed into the protein cases and the viruses are released into the open atmosphere through cell lysis of the host cell. It is possible for a virus to go through the lytic cycle without first going though the lysogenic cycle.
Figure 1: Lysogenic and Lysic Cycles
Drawing by: Jim Merriman
There are many different ways to treat affected water supplies. One such way is though biological processes. Using either activated sludge or a trickle filter to adsorb and contain the viruses. Other, less effective, methods use either sedimentation or coagulation in conjunction with filtration. Another method which is not very reliable is the use of carbon adsorption. This method relies heavily on filter aids to assist it. Methods which go straight to disinfection include the use of chlorine, ozone, and in some cases bromine or iodination.
American Society of Civil Engineers. The Incidence, Monitoring, and Treatment of Viruses in Water Supply Systems - A State of the Art Review. 1983.
Berg, Gerald. Transmission of Viruses by the Water Route. John Wiley & Sons. New York. 1965.
C. Hagedorn and R. B. Reneau, Jr. Waste Disposal Technologies In Soil; Class Notes. 1996-1997
Pelczar, Michael J. et al. Microbiology Concepts and Applications. McGraw-Hill, Inc. New York. 1993
Send comments or suggestions to: jmerrick@vt.edu
Student Authors: Tina Harris, Jason Johns, Jim Merriman, and
Jason Merricks
Faculty Advisor: Naraine Persaud, npers@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
