Viruses are the smallest known creatures, and those that cause sickness in humans are referred to as ‘pathogenic.’
While bacteria measure in the 0.3–8 micron range, viruses are a full magnitude smaller measuring in the 25–300 nanometre (nm) (0.025–0.3 micron) range. Thus, a 0.4 micron wide by 1.5 micron long V. cholerae bacterium is about 55 times as long and 15 times as wide as a 27 nm (0.027 micron) diameter Norwalk virus.
Viruses are so small that individual viral particles cannot be seen with a normal light microscope and must be viewed with an electron microscope. This makes detecting viruses in water very difficult and often no viral agent can be isolated from people who are ill.
Some microbiologists argue that viruses are not true organisms because they do not have a metabolism and cannot reproduce by themselves. Indeed, viruses are little more than either an RNA or DNA nucleic acid core encased in a protein capsid. Instead of being labeled as ‘alive’ or ‘dead,’ viruses that can replicate are referred to as ‘active’ and those that have lost the ability to replicate are deemed ‘inactivated.’ Whether they are organisms is not as important to the traveller as the fact that viruses cause some of the world’s deadliest diseases.
Viruses are classified according to their capsid symmetry, which is determined by the shape of the individual units or capsomeres that make up the capsid (outer surface).
Cubic symmetry refers to viral capsids that have a near spherical shape composed of twenty identical triangles—these viruses look similar to soccer balls. Helical symmetry refers to a hollow cylinder-shaped formation of spirally-arranged capsomeres. Viruses with more elaborate structures are referred to as having complex capsomeres.
Instead of reproducing by division like bacteria, viruses reproduce by invading a host cell and hijacking the cell’s reproductive processes to make it into a virus production factory. This process has five steps:
1) a single virus attaches itself to the host cell in a process called adsorption
2) the viral genome (genetic information) enters the cell in a process called penetration
3) the viral components replicate in a process called viral synthesis, which produces nucleic acid and protein
4) the viral components reassemble into new viruses in a process called maturation
5) the new viruses leave the cell in a process of release that often kills the cell
Compared to bacteria, the taxonomic system for viruses is simple. In 1966, the International Committee of Taxonomy of Viruses (ICTV) created a system for classifying viruses that is similar to the traditional Linnaean scheme.
The ICTV system assigns virus names in three hierarchical levels. The most general category is the family and a few subfamilies, followed by the genus, and finally the species.
All family names end in viridae, all genus names finish in virus, and all species names are in plain English. Currently the ICTV has named about 2000 species of viruses in 61 families.
Classification and Identification
Viruses are classified and described using the following characteristics listed in order of importance:
1) whether they have RNA or DNA nucleic acid type
2) size and type of symmetry (sometimes this includes the number of capsomeres and the presence or absence of a membrane)
3) response to chemical agents, especially ether
4) presence of specific enzymes
5) immunologic characteristics
6) natural means of transmission
7) host and tissue
9) symptomatology (the disease they cause)
Waterborne Pathogenic Viruses
Currently about 300 viruses are known to infect humans causing at least 50 different syndromes. Probably only a small number are waterborne viruses and the symptoms are almost always related to some form of gastroenteritis. Research suggests that that unknown or undetected viruses in untreated drinking water may actually cause the bulk of gastroenteritis in North America and Europe. We just do not know because viruses are so small and hard to detect.
The following chart summarises the most common and important known waterborne pathogenic viruses, the illnesses they cause and the general treatment.
|Rotavirus||Gastroenteritis with severe dehydration||Fluid and electrolyte replacement|
|Norwalk virus||Gastroenteritis, often with vomiting and abdominal cramps||Fluid and electrolyte replacement|
|Other calicivirus-like viruses||Gastroenteritis, often with vomiting and abdominal cramps||Fluid and electrolyte replacement|
|Hepatitis A||Infectious hepatitis||Fluid and electrolyte replacement and rest|
|Adenovirus serotypes 40 & 41||Gastroenteritis||Fluid and electrolyte replacement and rest|
|Poliovirus||Abortive poliomyelitis, aseptic meningitis, and paralytic poliomyelitis||None, prevention is with polio vaccine|
|Echovirus and coxsackievirus||Aseptic meningitis and encephalitis amongst others||None|
Invariably, waterborne pathogenic viruses are transmitted by the faecal–oral route, meaning they are ingested in water contaminated with infected human faeces.
As with bacteria, viruses are shed in the faeces of infected individuals in volumes that can exceed 108 organisms per gram. These massive quantities allow for easy contamination of water sources.
The infective dose is the minimum number of pathogenic viruses that can cause an infection in humans. It varies by virus type and is usually lower in water than food because water carries viruses quickly through the stomach’s acid bath.
The severity of infection is also related to the number of viruses ingested. Low doses may cause only asymptomatic infection while heavy doses cause the worst illness.
Current research strongly suggests that humans are the only natural hosts for the waterborne enteroviruses that infect our gastrointestinal systems. This does not guarantee that water will be free of viruses if no human faeccal contamination is present.
Animals such as dogs can ingest and shed human enteroviruses without being infected just as humans often unknowingly ingest and shed viruses that infect plants and animals.
With millions of dogs depositing hundreds of thousands of tonnes of faeces in parks and recreational areas the potential for viral contamination of water is enormous. Even remote lakes and rivers can be contaminated with pathogenic viruses dropped in the faeces of birds, dogs, rats and deer.
Water in wilderness or recreational areas downstream of towns and cities can harbour huge numbers of pathogenic viruses. In Canada approximately 90% of the urban population have access to sewage collection. Yet an incredible 40% of this sewage is not treated before being discharged directly into rivers or oceans. In the U.S., 27% of sewage receives little or no treatment before being discharged into waterways. In Germany, enteroviruses have been isolated from water samples 25 kilometres downstream from a single sewage outlet.
The news is not all bad. Fortunately, viruses cannot replicate without human host cells, and environmental factors such as UV radiation can inactivate viruses and make a waterway virus-free over time if no new viruses are introduced.
Even in contaminated water, viruses tend to clump together or with inorganic or organic particles that often settle to the bottom of lakes and ponds. While these viruses are still active, they are trapped in bottom sludge so drinking water taken from the top is much less likely to be contaminated.
The bulk of pathogenic waterborne viruses affect the gastrointestinal system. These viruses cause viral gastroenteritis with symptoms and treatments that are basically identical to gastroenteritis caused by bacteria.
Some viruses can cause infections that spread out of the intestine and involve other organs. Important known waterborne viruses that pose a risk to backcountry and recreational travellers are detailed in separate posts.