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Water contaminants

Algal toxins

Algae in water, usually obvious by the presence of an agal bloom indicated by a green, yellow-brown or red scum on the surface, is potentially very dangerous.  Harmful agal blooms (HABs) are those produced by harmful phytoplankton and are not easy for non-experts to differentiate from less dangerous blooms. The phytoplankton that produces a bloom can be in concentrations of billions of cells per litre of water and can give off dangerous toxins. Depending on the type of algae present, drinking contaminated water can lead to an array of illnesses including in some cases paralysis and death.

Furthermore, blooms in fresh water are usually the result of an abundance of phosphorus from agricultural runoff and thus indicate potentially high levels of fertilizer contamination which can cause ‘blue baby syndrome’ and be fatal to babies.

Though it is generally easy to identify contaminated water because of proximity to agricultural land or golf courses and evidence of the bloom itself, assessing the dangers is very difficult and water with algal blooms is not easily treated using conventional methods and should be avoided at all costs.


Bacteria are generally single cell organisms. Typically, they are 0.3–1.0 microns (1000th of a millimetre) wide by 0.5–8 microns long.  Bacteria have a thin cell wall that surrounds a uniform protoplast composed mostly of gelatinous cytoplasm and RNA and DNA.

Bacteria that have the potential to cause infection are referred to a ‘pathogenic.’

Shape and Movement

Spherical bacteria are called cocci and those with a rod shape are known as bacilli. There is tremendous variation in size and shape within each group.

Some bacteria have thin whip-like flagella (tails) for locomotion, while others move by means of hair-like filaments protruding from their walls called pili, which may also help the bacteria attach themselves to potential host cells.


Bacteria reproduce through binary fission. During this process a cell enlarges and then divides into two identical organisms. Binary fission allows for tremendous growth of a bacterial colony.

For example, under ideal conditions, E. coli divide every 20 minutes. Thus 1,000 organisms become 2,000 and 20 minutes later 4,000 and so on in a doubling process. Within 24 hours one E. coli will theoretically produce 4.74 x 1021 organisms!

Fortunately, such growth rarely occurs in nature because of insufficient nutrients to support unbridled reproduction, temperature swings, unfavourable pH and predatory microorganisms. Not surprisingly, the human intestine is an ideal environment for bacterial reproduction and human infection is invariably via the faecal–oral route.


When faced with adverse environmental conditions, such as a lack of nutrients, some bacteria turn into spores as a survival mechanism. During this process, each bacterium creates a tough round outer shell to protect the vital RNA and DNA components. Once transformed into a dehydrated spore, the bacterium is in a dormant state and cannot reproduce.

Spores can survive for hundreds of years and only reactivate when introduced into a favourable environment. While many spore-forming bacteria are pathogenic, such as C. tetani, which causes tetanus after entering an open wound, spores do not seem capable of causing infection if ingested, and no waterborne-pathogenic bacteria produce spores.

Taxonomy, Nomenclature and Classification

Of the thousands of types of bacteria catalogued by microbiologists, few are actually human pathogens. Microbiologists use several, somewhat confusing, systems to classify bacteria.

The Linnaean hierarchical taxonomic system, which classifies organisms into families, genera and species with Latin names, is insufficient. Bacteria are not studied as individual organisms but rather as ‘pure cultures’ composed of individuals derived from a hypothetical single source organism. These individuals are no longer necessarily identical because of genetic mutation or changes in response to varied environments.

Microbiologists apply the term ‘strain’ to a group of pure cultures derived from either a common source organism, several organisms within one infected person or different organisms from various people in a common source outbreak. Similar strains are then grouped into a species. However, species boundaries, normally defined by the limits of cross-fertility, do not apply to bacteria which reproduce asexually and freely exchange genetic material.

Despite problems with the Linnaean system, in 1980, the International Committee of Systematic Bacteriology (ICSB) published the definitive list of 2,500 species of bacteria to replace a list that had mushroomed to over 30,000 species.

Currently, only the names from the ICSB list, which are Linnaean in origin, are considered valid, and any additions or changes require publication in the International Journal of Systematic Bacteriology.

In the Linnaean system, the first Latin name is capitalized and refers to the genus, while the second Latin name is in lower case letters and identifies the species. For example, in the case of Salmonella typhi, Salmonella identifies the genus and typhi the species. It is customary to abbreviate the singular genus name with a capital letter followed by the species name in lower case, i.e. S. typhi.

Bergey’s Manual

Bacteria are also classified based on characteristics.

In 1923, David H. Bergey developed the most widely used classification system for the American Society for Microbiology (ASM). The most recent edition is Bergey’s Manual of Systematic Bacteriology published in five volumes between 2001 and 2011.

Bergey’s classification system groups bacteria into ‘sections’ according to easily identifiable properties. The sections cut across taxonomic boundaries, have no formal taxonomic status and are purely for ease of use.

There are 33 sections, each with its own list of group properties. These include cell wall composition, shape, aerobic or anaerobic status and whether the bacteria are spore producing. With few exceptions, waterborne bacteria come from section two—aerobic and microaerophilic, motile, helical and vibrioid, Gram-negative bacteria—and most often section five—facultatively anaerobic, Gram-negative, rods.

Section five has three families of bacteria: the enterics, the vibrios, which are curved rods and the pasteurellas. Only the enterics and vibrios are important as waterborne pathogens. Gram-negative or positive refers to how bacteria cell walls react to Gram’s method of differential staining performed to enhance viewing under a microscope.

When microbiologists talk about bacteria they usually call them by their formal Linnaean terms for genera and species but describe them using Bergey’s scheme. For example, bacteria in the genera Salmonella are often referred to as Gram-negative, flagellated, non-sporulating, aerobic, bacilli. This means that they give up violet dye in Gram’s test and must be stained red to be seen, move by means of a whip-like flagella (tail), do not form spores, use oxygen and are rod shaped.

Waterborne Pathogenic Bacteria

The following tables summarize common and uncommon but important waterborne pathogenic bacteria found in North America and Europe, their associated illness and treatment.

Common Pathogenic Bacteria Worldwide
Bacterium Illness Treatment
Campylobacter jejuni Gastroenteritis, dysentery, possible extra-intestinal complications Fluid and electrolyte replacement and anti-microbial drugs
Salmonella species Gastroenteritis, small intestine dysentery, enteric fever, possible extra-intestinal complications Fluid and electrolyte replacement and anti-microbial drugs
Shigella species Shigellosis (bacillary dysentery) and possible extra-intestinal complications Fluid and electrolyte replacement and anti-microbial drugs
Escherichia coli Gastroenteritis, shigellosis-like bacillary dysentery Fluid and electrolyte replacement and anti-microbial drugs
Y. enterocolitica Gastroenteritis, shigellosis-like bacillary dysentery Fluid and electrolyte replacement and anti-microbial drugs
Other Worldwide Pathogenic Bacteria, Uncommon in North America
and Europe
Bacterium Illness Treatment
Vibrio cholerae Cholera characterized by severe dehydration often resulting in death Massive oral and / or intravenous fluid and electrolyte replacement
Aeromonas Asymptomatic excretion, gastroenteritis, and possibly dysentery Fluid and electrolyte replacement
Plesiomonas Gastroenteritis, severe dehydration, and fever Fluid and electrolyte replacement
Leptospira interrogans Asymptomatic excretion, flu-like illness, and Weil’s disease Immediate drug therapy, and fluid and electrolyte replacement
Legionella pneumophila Pontiac fever and Legionnaire’s disease Anti-microbial drugs
Listeria monocytogenes Listeriosis, spontaneous abortion or still birth in pregnant women Anti-microbial drugs


Most waterborne pathogenic bacteria are also foodborne pathogens. Invariably, the transmission of waterborne pathogenic bacteria is by the faecal–oral route, meaning that bacteria are ingested in water contaminated with human or animal faeces. The amount of bacteria shed per gram of faeces from an infected person or animal varies according to the severity of infection and the type of bacteria.

Infective Dose

The infective dose is the minimum number of pathogenic bacteria that can cause an infection in humans. It varies widely between organisms from as few as ten to several million. The infective dose is lower when consumed in water than food because water passes through the acid bath of the stomach faster, so many bacteria make it into the small intestine alive.

In the case of almost all pathogenic bacteria, the severity of infection is directly related to how many organisms are eaten. Low doses may cause asymptomatic infection, while heavy doses cause the worst illness.


Humans and animals are the natural habitat or reservoirs for pathogenic bacteria. Abundant nutrients, an optimum temperature, and good pH make our intestines the ideal bacterial breeding grounds.

While many pathogenic bacteria can multiply on food, few can multiply in water because of low nutrient content and variable temperature and pH. Consequently, water is a natural reservoir for only a few bacteria including Vibrio cholerae, which causes cholera, and Salmonella typhi, which causes typhoid fever.

This is good news because when bacterially-contaminated faeces get into water sources, the number of organisms begins to decline almost immediately. Dilution, UV radiation, water temperature, pH and predatory microorganisms all help to disperse or kill bacteria in untreated water.

Another advantage is that because humans are the only natural reservoirs for some bacterial pathogens, unless infected humans are constantly depositing faeces in the water the bacteria will eventually die off.


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

8) pathology

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.

Common Pathogenic Viruses Worldwide
Virus Illness 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
Other Worldwide Pathogenic Viruses, Uncommon in North America and Europe
Poliovirus Abortive poliomyelitis, aseptic meningitis, and paralytic poliomyelitis None, prevention is with polio vaccine
Echovirus and coxsackievirus Aseptic meningitis and encephalitis amongst others None
Coronavirus Possibly gastroenteritis 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.

Infective Dose

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.

Viral Illnesses

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.

Protozoa Parasites

All parasitic protozoa are complex single celled organisms.

Many, including Cryptosporidium, Giardia lamblia, and Entamoeba histolytica, transform from free living forms into dormant, but highly infectious cysts when expelled in the host’s faeces.

Cysts are usually round or oval shaped and have tough shells allowing them to survive long periods in water sources while they await ingestion by a suitable host in which they can reactivate into free living organisms and reproduce.

Compared to viruses, and even most bacteria, parasitic protozoa are veritable microscopic giants.

Cryptosporidium parvum oocycts (the cycst form of Crypto) are 2–6 microns in diameter, while Giardia lamblia trophoites (the mobile form) are 10–20 microns in length with cysts 8–14 microns in diameter.

The gigantic Balantidium coli, the largest protozoan, measures in at 45 microns by 60 microns, which is just on the edge of being visible to the naked eye.

Given their relatively large size, parasitic protozoa are easily seen with normal light microscopes.


Microbiologists divide parasitic protozoa into four groups—Sarcodina (amoebas), Mastigophora (flagellates), Ciliata or Ciliophora, and Sporoza—based on shape, means of locomotion, and other physical characteristics.

Within these groups are numerous parasitic protozoa, but only a few infect humans through the faecal–oral route in food and drinking water.

Sarcodina (amoebas), such as Entamoeba histolytica, the cause of amoebic dysentery, are the classics of high school biology. Amoebas look like translucent microscopic bags of jelly. They move by projecting a pseudopod and then flowing into it.
Mastigophora (flagellates), such as Giardia lamblia, have more defined shapes and move by means of one or more whip-like flagella.
Ciliophora move by wiggling rows of small cilia projecting from their sides. The only human parasite in this phyla is the large Balantidium coli, which infects the intestines of humans and pigs.

Sporoza have no special means of moving but have complex lifecycles that often involve alternating sexual and asexual reproduction in different hosts. Cryptosporidium parum is the most important waterborne member of this group.

Common Pathogenic Protozoa Worldwide
Protozoan Illness Treatment
Giardia lamblia Giardiasis (gastroenteritis) Fluid and electrolyte replacement and anti-microbial drugs
Cryptosporidium parvum Gastroenteritis Fluid and electrolyte replacement
Other Pathogenic Protozoa Worldwide, Uncommon in North America and Europe
Protozoan Illness Treatment
Entamoeba histolytica Amoebic dysentery and severe dehydration Fluid and electrolyte replacement and anti-microbial drugs
Isospora belli Dysentery Fluid and electrolyte replacement and anti-microbial drugs
Balantidium coli Dysentery Fluid and electrolyte replacement and anti-microbial drugs
Entamoeba polecki Gastroenteritis, possibly dysentery Fluid and electrolyte replacement and anti-microbial drugs


Depending on the species, parasitic protozoa reproduce either sexually, asexually, or using both methods. As with bacteria, rapid asexual division among protozoa produces staggering numbers of organisms in a short period of time.


Waterborne parasitic protozoa are transmitted by the faecal–oral route meaning that the organisms or cysts are ingested in water or food contaminated with infected human faeces. People, or animals, with heavy Giardia infections can pass 106 cysts per gram of faeces.

Infective Dose

While most bacteria and viruses require relatively high numbers to cause infection, because of their acid resistant shells, most protozoa cysts can infect humans in numbers of ten or less.

Given the high volumes of cysts in the faeces of infected people and animals, the potential for easy waterborne transmission and the low infectious dose, the potential for infection is massive.

As with other pathogens, the severity of infections caused by protozoa are usually related to the number of organisms ingested. Low doses may cause only asymptomatic infection, while heavy doses cause the worst illness.

Distribution and Reservoirs

All important waterborne parasitic protozoa are naturally present in the intestinal tracts of humans and other animals. Global infection rates with Giardia and Cryptosporidium are enormous.

For example, Cryptosporidium has historically infected the water supply of St. Petersburg, Russia. Estimates in the 1990s were that 90% of the city’s population was infected with the protozoan.

In North America, Giardia and Cryptosporidium are the most prevalent protozoa. Giardia have been isolated from domestic cats and dogs, bears, deer, bighorn sheep, rodents and, of course, beavers, which lend giardiasis the popular name ‘Beaver Fever.’

Cryptosporidium have also been isolated from herd animals including domestic sheep, cattle, goats and wild deer and elk. Given the high volumes of cysts in infected faeces, the huge natural reservoirs and the ability of cycsts to persist in the environment for weeks or months, it is obvious why Giardia and Cryptosporidium are the most common of all waterborne pathogens in North America and Europe.

Studies have found Giardia cysts in 97% of all surface water in North America, including high quality mountain streams and lakes even in remote areas of the Yukon. A study of 10,000 waterways found Cryptosporidium in about 75% of all rivers and lakes in the western US. Currently, but probably not for long, Cryptosporidium oocysts may only be absent in the most remote lakes of northern Canada and Alaska while all European waterways are likely infected.

Concentrations of cysts vary depending on the water source and the exact location of faeces in the water. On average, concentrations in water in wilderness areas of North America are usually below one cyst per litre of water. However, water in a beaver pond can average well over 100 Giardia cysts per litre.

Raw sewage or run-off from land populated by cattle infected with Cryptosporidium can exceed 5,000 oocysts per litre.

In 1996, spring run-off in Cranbrook, British Columbia, Canada flushed infected cattle faeces into the town’s reservoir, which caused a massive outbreak of cryptosporidiosis that involved several thousand people.

In 1993, in the largest ever North American outbreak, 400,000 people became infected with Cryptosporidium in Milwaukee, Wisconsin from contaminated drinking water. Adding to the potential for severe outbreaks is the strong resistance of Giardia cysts and total immunity of Cryptosporidium oocysts to the chlorine used to treat urban drinking water.


As with bacteria and viruses, intestinal protozoa cause ailments ranging from asymptomatic infection to gastroenteritis, dysentery, and, in some cases, tissue invasion and organ damage. Treatment is usually in the form of hydration and electrolyte replacement. Drug therapy might be available and necessary depending on the pathogen.

High Risk Groups

No treatment currently exists for Cryptosporidium infection, making it an important health risk for the elderly, children and other people with weak immune systems, especially AIDS patients.

Normally, the human immune system can handle occasional oocysts ingested in unfiltered municipal water, however the immune systems of many AIDS sufferers cannot handle even a few oocysts. The end result can be complete intestinal colonization and eventually death.

Some estimates suggest that Cryptosporidium infections contracted from municipal drinking water cause up to seven percent of all deaths among AIDS patients.

Parasitic Worms

Parastic worms or ‘helminths’ are worms that have a lifecycle that involves living in a human host

Nothing turns the stomach more of the traveller to distant lands than the thought of enormous worms living for years in your intestine without you knowing it. Yet this is the case for billions of people around the world including many of us in hyper-sanitary North America and Europe.

Worldwide, an estimated one to one and a half billion people are infected with an average of seven 15–60 centimetre long Ascaris lumbricoides roundworms while another 600–800 million people have hookworms. Perhaps two billion people have the tiny pinworm living in their bowel and millions of others have various beef and fish tapeworms in their intestines, some of which can reach 10–20 metres in length.

Unlike their microbial parasitic counterparts, helminthes (parasitic worms) are only microscopic in the egg or larvae stage, with most oval-shaped eggs measuring in the 25 micron by 30 micron to 50 micron by 150 micron range.

North America and northern Europe are relatively free of most types of parasitic worm species with a few notable exceptions. The knowledge that parasitic worm eggs are not naturally found in North American and northern European waterway may add to your peace of mind. Nevertheless, raw sewage and waterways polluted with sewage often contain enormous numbers of eggs from infected individuals, particularly in areas frequented by tourists.

Fortunately, only a few types of eggs can infect humans without intermediary hosts that are not indigenous to North America and Europe. This means that water in wilderness areas, and to a lesser extent in more frequented recreational areas, can be considered generally free of parasitic worm eggs and larvae.

Even if eggs or larvae are present, they are easily filtered or destroyed by heat. The greatest threat in North America and Europe is from worms that produce eggs that can infect humans immediately after being passed in the faeces, without needing an intermediary host or specific soil conditions.

There are also several worm parasites that infect animals, such as dogs, which can live in humans but die without maturing or reproducing. Since parasitic worms are rarely acquired from drinking water in North America and Europe and realtively easily dealt with while travelling outside of these areas this section contains only be a brief summary of waterborne parasitic worms.


Biologists divide helminthes into the two main categories of platyhelminths (flatworms), which includes the subdivisions of cestodes (tapeworms) and trematodes (flukes), and nematodes (roundworms).


Humans get worm infections in two ways. The first is via oral ingestion of eggs in undercooked meat or fish, or in food and water, or from hands and objects contaminated with eggs.

The second method involves certain types of microscopic larvae burrowing through the skin in body parts exposed to contaminated soil or water—conditions not found in North America and Europe.

As with microbial parasites, the faeces of people infected with intestinal worm parasites can contain an enormous number of microscopic eggs. For example, each Ascaris lumbricoides worm produces over 200,000 eggs per day, which are shed in the host’s faeces. Eggs can remain viable for long periods of time. Eschinoccus granulosus (dog tapeworm) eggs are infectious for up to one year and Ascaris lumbricoides eggs can remain infectious for up to nine years.

Infectious Dose and Reservoirs

In theory, infection can start after ingesting one egg. Human parasitic worms are generally restricted to humans with occasional crossovers from other mammals. Many types of helminthes live in people, in North America, but only a small number can reproduce effectively because of unfavourable environmental conditions. For example, all trematodes—intestinal, liver, lung and blood flukes—require specific species of freshwater snails to complete their lifecycles before they can infect humans. These snails do not live in North America and Europe.

Cestodes (Tapeworms)

Taenia (Beef and Pork Tapeworms)

Two versions of Taenia infect humans. The larvae of T. solium are found in pigs, and T. saginata live in cattle. Humans get infected by eating raw or undercooked flesh from animals that have infective cysticerci (larvae) imbedded in their flesh.

While Taenia can exceed 20 metres in length in the human intestine they have only a minimal effect on infected people. There is also the possibility that the round, 35–40 micron diameter T. solium eggs might hatch into cysticerci in humans. Because cysticerci cannot develop in the flesh of humans they die in bodily tissues, including the brain, and are calcified by the body within a year. Known as cysticercosis, this syndrome can be acquired by ingesting eggs passed in human faeces.

Echinococcus granulosus

Humans get infected with Echinococcus granulosus only by accident. Normally the worm’s lifecycle involves a larvae stage in sheep, goats, cattle or horses and an adult phase in dogs. Generally only humans who work closely with sheep and dogs, such as sheep ranchers, get infected. Echinococcus granulosus eggs are roundish in shape and 35–40 microns in diameter.

Hymenolepis nana

Hymenolepis nana is the most common tapeworm in humans, particularly in Asia. Mature worms measure only 1.5–4 centimetres long and Hymenolepis nana are the only tapeworms that do not require intermediary hosts before infecting humans.

Most eggs released by the worms in the ileum of the intestine are capable of hatching and reinfecting the already infected host. Oval-shaped 30 micron by 45 micron eggs are passed in the faeces and can survive up to two weeks. Given the concentration of this parasite in Asia and the short survival period of eggs, North American and European water outside of urban or recreational areas frequented by Asia tourists is probably egg free. Water within Asia has a considerably higher probabilty of containing the eggs.

Diphyllobothrium latum

Diphyllobothrium latum is a fish tapeworm common in Scandinavia and northern North America. Humans contract it by eating infected raw or undercooked fresh water fish.

Adult worms are several metres long and release oval-shaped 45 micron by 65 micron eggs in the host’s faeces. Eggs hatch in fresh water and infect fresh water copepods, which are then eaten by fish. The larvae develop in the fish, and if infected fish are eaten raw or in an undercooked state, the larvae can develop into mature egg-producing adults, in the human intestine.

Trematodes (Flukes)

Intestinal Flukes

These flukes are common in East Asia in humans and pigs, and produce eggs in the 80 micron by 140 micron range.

The eggs of the most common intestinal fluke, Fasciolopsis buski, are passed in the faeces and hatch into larvae in fresh water where they infect particular snails and develop into cercariae. The cercariae then escape and become encysted as metacercariae on aquatic vegetation such as water caltrop or water chestnut, which are eaten by humans.

The metacercariae encyst in the human intestine and eventually develop into seven centimetre long flukes that attach themselves to the intestinal wall. Similar intestinal flukes include Heterophyes and Metagonimus.

Liver Flukes

Fasciola hepatica are parasitic liver flukes of the bile ducts that are found in sheep, cattle, and often humans. They produce eggs in the 80 micron by 140 micron range, which eventually hatch into larvae that are ingested on contaminated watercress. Once they reach the intestine, they encyst before hatching into tiny flukes that penetrate the intestinal wall, reach the liver, and finally take up residence in the bile ducts. Eggs are passed via the faeces and the cycle continues. Other liver flukes with similar lifecycles include Clonorchis sinensis, Opisthorchis felineus and Opisthorchis viverrini.

Lung Flukes

Paragonimus westermani are lung flukes usually isolated in East Asia, West and Central Africa, and Central and South America.

Humans acquire the fluke by eating larvae encysted in fresh-water crabs and crayfish that are raw or undercooked. After ingestion, the larvae encyst in the small intestine and penetrate the gut wall before migrating to the lungs. Adults grow in the lungs within fibrous cysts to about one and a half centimetres long. Oval-shaped 35 micron by 85 micron eggs are coughed up in sputum, and are spit out or are swallowed to eventually pass in the faeces.

In fresh water, the eggs hatch to release miracidia that enter a particular snail where they develop into cercariae. The cercariae then escape into the water and penetrate the flesh of fresh water crabs or crayfish, where they await ingestion by humans.

Schistosomes (Blood Flukes)

An estimated  200–300 million people, primarily in Africa, South America and East Asia, are infected with this fluke which causes up to 1.5 million deaths annually. Three species of schistosomes infect humans. The fluke is pervasive in areas such as the Nile River delta and other common traveller destinations in Africa and Asia.

The adults exist as thin threadlike worms in the abdominal veins of humans and a few domestic animals. Eggs are laid in the capillaries that drain the intestine and most eggs pass into the intestine or bladder and are excreted in the faeces or urine with a small amount of blood. With severe infection, blood loss can be extreme and is easily visible in the urine and faeces.

Some eggs are carried in the blood to the liver or lungs, depending on the species, and become encased in a fibrous granuloma. Eggs passed from the body hatch in fresh water and infect particular snails before being released as 80 micron by 200 micron cercariae.

The cercariae eventually infect humans by burrowing through the skin of people standing in or handling infected water and the cycle continues. In North America and Europe, duck schistosomes commonly penetrate and die in the skin of people, causing the well-known ailment ‘swimmer’s itch.’

Nematodes (Roundworms)

Ascaris lumbricoides

Ascaris lumbricoides live in 20% of the world’s population with the average infected human carrying seven worms. Infection is usually limited to tropical areas but can occur anywhere an infected person deposits untreated faeces so infection is common. The worm’s oval-shaped, 30 micron by 60 micron eggs are tough and can survive months or years in the environment while they await ingestion by a human host.

Once eaten, Ascaris lumbricoides eggs hatch in the intestine. Larvae breach the intestinal wall and are carried to the lungs in the blood stream. In the lungs, they enter the alveolar spaces and begin to mature before being coughed up and swallowed. Once in the intestine for the second time, the worms mature into adults that resemble 15–60 centimetre long earthworms. Adult worms live for one to two years.

Trichuris trichiura

Trichuris trichiura simply hatch in the intestine and penetrate the colon wall where they mature. The worms then re-enter the large intestine, with their tails remaining in the lumen, where they lay 20 micron by 50 micron eggs that pass out in the faeces. The worms live for several years.

Ancylostoma duodenale, Necator americanus (Hookworm) and Strongyloides stercoralis

These roundworms live in humans in warm tropical areas and produce eggs in the 30 micron by 60 micron size.

Eggs in the case of Ancylostoma duodenale and Necator americanus and larvae in the case of Strongyloides stercoralis, are passed in the faeces and develop into infective larvae in warm moist soil. The larvae then burrow through any human flesh that comes into contact with infected soil including bare feet.

The larvae enter the bloodstream and are carried to the lungs. In the lungs, they break into the alveolar spaces and are coughed up and swallowed.

Back in the intestine, the larvae grow into egg-producing adults. Strongyloides stercoralis larvae often migrate from the intestine, through body tissues, causing inflamation and often reinfecting the host. In these cases, infection can continue for a decade or more and, in immunocompromised people, the end result can be complete tissue infestation and death by septicaemia.

Enterobius vermicularis (Pinworm)

Enterobius vermicularis are known popularly as pinworms or threadworms. These tiny one to two centimetre worms are common worldwide, especially in children.

Adult worms live in the lumen and squirm out of the anus when the infected person is resting, usually at night, and lay thousands of 25 micron by 50 micron eggs on the skin surrounding the anus.

The light eggs are easily ingested from bedding lint from under the fingernails of the infected person who scratches the irritated area or from door handles, furniture, recreational equipment or other objects handled by the infected person. Once eggs are swallowed they quickly develop into adults that continue the cycle.

Trichinella spiralis

Trichinella spiralis infect some rodents, hyenas, polar bears and occasionally humans who have eaten raw or poorly cooked meat including pork.

These roundworms live for six to eight weeks in the intestine where the females pass large numbers of 6 micron by 100 micron larvae that burrow through the intestinal wall and enter the bloodstream. Once in the blood, they are carried to voluntary muscles where they encyst and die as they are calcified by the body. Larvae that escape in faeces can be reingested and mature in animal and human hosts.

Filarial Nematodes

Among these parasitic worms are members of the genera Wuchereria, Brugia, Onchocera, Loa and Dracunculus (Guinea worm).

All of these parasites are restricted by environmental considerations to areas in Central and South America, Southeast Asia, Central Africa, parts of the Middle East, India, the West Indies and South Pacific islands.

People become infected when 7 micron by 200 micron microfilariae get into the bloodstream via the bite of mosquitoes or blackflies. Only Dracunuculus, which are found in West Africa, parts of the Middle East and India, are transmitted by drinking water that contains small fresh water crustacea that act as an intermediary host.

Nematodes with Non-Human Hosts

Several round worms that infect non-human mammals can infect humans and die without reproducing.

Most importantly, the larvae of Ancylostoma braziliensis (dog hookworms) can penetrate the human skin and migrate for several months beneath the skin, causing allergic reactions, before dying.

The eggs of Toxocara canis, which are similar to those produced by its close relative Ascaris, are common in dogs and cats and sometimes hatch in humans. The larvae can migrate around the body before dying in an immature state.

Soil in parks, playgrounds and beaches that is contaminated with dog faeces is the most common source of this canine parasite.

Chemicals and toxins

Continued economic growth and industrial activity has contaminated much of the world’s fresh water supply with manufactured chemicals.

The range of contaminants includes toxic heavy metals, such as lead, mercury and cadmium, agricultural chemicals such as pesticides and fertilizers (nitrates) and volatile organic chemicals such as gasoline / petrol and solvents.

Sources of contaminants include agricultural and golf course run-off, industrial run-off from pulp mills, chemical plants, manufacturing plants, run-off from roads, acid rain from airborne pollution and leaching from landfill sites, mines and septic fields.

Generally, the more people who live in an area the greater the level of chemical pollution. Many recreational areas, and some wilderness areas, are downstream or downwind from industrial zones or agricultural areas, which ensures continuous contamination.

For example, 39% of all groundwater wells in South Dakota have levels of nitrate that exceed US national drinking water standards. The nitrate is from fertilizer contamination of aquifers and is in high enough concentrations to harm children. Across eastern Canada, the US and eastern Europe, acid rain has rendered thousands of lakes essentially lifeless. Ice core samples from the Canadian arctic and Antarctica confirm that chemicals are transported all over the world by weather systems.

Some chemicals that are now banned in North America and Europe, such as DDT, are sometimes still manufactured and used in developing countries. Even after a chemical is banned it can often persist for years, decades or potentially indefinitely in the environment of any country. Furthermore, mining or mill slag heaps, abandoned industrial areas, lake and river sludge adjacent to industrial areas and other sources will remain sources of chemical leaching into groundwater, rivers and lakes far into the future causing local and trsnboundary problems.

A Few Words on Risks

Given the proliferation of chemicals in the environment and the scale of the problem, the purpose of this section is to outline the chemicals and elements that may be present in water in recreational and wilderness areas and to assess the potential dangers of drinking such water.

It must be noted that very few chemical and elemental contaminants are found in water in recreational and wilderness areas in sufficient concentrations to cause immediate illness or death. Many of the effects are longer term and relate to organ damage and cancer.

However, the most difficult task in assessing the non-microbiological risk of drinking is determining what harmful chemicals or elements might be in the water without sending a sample to a lab.

How exactly do you assess the quality of water when you are out adventuring? The short answer is that you can’t. However you can do some research before heading out to help you predict with reasonable certainty what contaminants you might encounter and how to deal with them. The section of the website on treatment systems offers some advice on these points.

The Special Case of Nitrate

The one chemical that is often present in water at high enough levels to cause immediate harm is nitrate, which gets into the environment through chemical fertilizers, animal wastes and septic systems.

Nitrate poses a greater risk for recreational travellers than any other chemical because of widespread agriculture and livestock ranching, and the potentially deadly affect on babies.

In the digestive system of an infant, a certain type of bacteria converts nitrate into nitrite. Nitrite reacts with hemoglobin, which carries oxygen in the blood, to form methemoglobin, which cannot carry oxygen. The end result is oxygen depletion in the baby.

This illness is called methemoglobinemia, or ‘blue-baby’ disease or syndrome, and is characterized by symptoms of suffocation including bluish skin, particularly around the eyes and mouth.

The infant must be immediately hospitalized for treatment to convert the methemoglobin back into hemoglobin.

After infants reach six months of age, their stomachs usually no longer harbour large numbers of the bacteria that convert nitrate into nitrite because of an increased level of hydrochloric acid. Infants under this age should not be exposed to any water sources that have even a remote possibility of nitrate contamination, especially water from agricultural and ranching areas, golf courses or near septic fields.

Elements, Chemicals and Heavy Metal Contaminants

The following chart, derived from Health Canada and US EPA guidelines, summarizes the major chemical and elemental contaminants that have been detected in North American water supplies. Both Health Canada and the US EPA use specific terms to quantify the levels of chemical contaminants in drinking water. European systems are broadly similar if perhaps tighter following recent EU regulations on chemicals.

Health Canada’s levels are based upon an average daily intake of one and a half litres of water by a 70 kilogram adult and a large safety factor of up to ten is considered when determining the levels. Important definitions for the chart are: Health Canada’s Maximum Acceptable Concentration (MAC), which is the maximum level of concentration permitted for ‘substances that are known or suspected to cause adverse effects on health.’

The US EPA’s equivalent term, Maximum Contaminant Level (MCL), which ‘is the highest amount of a specific contaminant allowed in the water delivered to any customer of a public water system.’

When possible the Health Canada MAC or US EPA MCL is provided. Most importantly, a brief description of the possible sources of contamination and possible health effects is given. This should help you to assess the quality of water sources based on geographic location and make judgement calls about the risks of drinking water that may contain possible contaminants.

Elements and Inorganic Chemicals
MSC / MCL in mg per litre
Potential Health Effects
Sources of Contamination
Antimony – / 0.006 Cancer Fire retardants, ceramics, electronics, fireworks, pipe-solder
Arsenic 0.025 (IMAC) / 0.05 Skin and nervous system toxicity Natural deposits, smelters, glass, electronic wastes, orchards
Asbestos – / 7 (million fibres/litre) Cancer Natural deposits, asbestos cement in water systems
Barium 1.0 / 2 Affects circulatory system Natural deposits, pigments, epoxy sealants, burned coal ash
Beryllium – / 0.004 Damage to bones and lungs Electrical, aerospace, defence manufacturing
Cadmium 0.005 / 0.005 Affects kidneys Galvanized pipe corrosion, natural deposits, batteries, paint
Copper – / 1.3 Gastrointestinal irritation Natural and industrial deposits, wood preservatives, plumbing pipes
Chromium 0.05 / 0.1 Liver, kidney and circulatory systems disorders Natural deposits, mining, electroplating, fertilizer
Lead 0.01 / 0.005 Damage to kidneys and nervous system Natural deposits, pipe-solder, leaded gasoline car exhaust
Mercury 0.001 / 0.002 Kidney and nervous system disorders Agricultural run-off, natural deposits, batteries, electrical switches
Nitrate 45 / 10 Methemoglobulinemia (blue baby disease) Animal faeces, fertilizers, septic fields and natural deposits
Nitrite – / 1 Methemoglobulinemia (blue baby disease) Animal faeces, fertilizers, septic fields and natural deposits
Selenium 0.01 / 0.05 Liver damage Natural deposits, mining, smelting, coal and oil burning
Thallium – / 0.002 Kidney, liver, brain, intestinal damage Electronics, drugs, alloys, glass
Organic Chemicals
Contaminant MSC / MCL in mg per litre Potential Health Effects Sources of Contamination
Acrylamide – / – Cancer, affects nervous system Polymers used in sewage treatment
Adipate – / 0.4 Weight loss Synthetic rubber, food packaging, cosmetics
Alachlor – / 0.002 Cancer Herbicide run-off from corn, soyabean and other crops
Atrazine – / 0.003 Breast tumors Herbicide run-off from corn and other crop and non-crop fields
Carbofuran 0.09 / 0.04 Affects nervous and reproductive systems Soil fumigants on corn and cotton
Chlordane – / 0.002 Cancer Leaching from termite insecticide
Chlorobenzene – / 0.1 Affects liver and nervous sysetm Waste solvent from metal degreasing
Dalapon – / 0.2 Affects kidneys and liver Herbicides on orchards, beans, lawns and road and railway ditches
Dibromochloropropane – / 0.0002 Cancer Soil fumigant on soybeans, cotton and orchards
o-Dichlorobenzene –       0.6 Liver, kidney, and blood cell damage Paint, engine cleaners, dyes and chemical wastes
trans-1,2-Dichloroethylene – / 0.1 Affects liver, kidneys and nervous and circulatory systems Waste from industrial extraction solvents
cis-1,2-Dichloroethylene – / 0.07 Damage to liver, kidneys, and nervous and circulatory systems Waste from industrial extraction solvents
Dichloromethane 0.05 / 0.005 Cancer Paint stripper, metal degreasers and propellants
trans-1,2-Dichloropropane – / 0.005 Cancer and liver and kidney damage Soil fumigant and industrial solvents
Dinoseb 0.01 / 0.007 Damage to thyroid and organs Herbicide run-off from crops
Dioxin – /  0.00000003 Cancer Chemical production byproduct, pulp-mills, herbicide impurity
Diquat 0.07 / 0.02 Affects liver, kidneys and eyes Herbicide run-off from land and aquatic weeds
2, 4-D – / 0.07 Damage to liver and kidneys Herbicide run-off from wheat, corn, rangelands and lawns
Organic Chemicals continued
Contaminant MSC / MCL in mg per litre Potential Health Effects Sources of Contamination
Endothall – / 0.1 Liver, kidney and gastrointestinal problems Herbicide on crops and land and aquatic weeds
Endrin – / 0.002 Damage to heart, liver, and kidneys Pesticide (currently restricted in the US)
Epichlorohydrin – / - Cancer Water treatment chemicals, epoxy resins
Ethylbenzene – / 0.7 Damage to liver, kidneys and nervous system Gasoline, insecticides, waste from chemical manufacturing
Ethylen edibromide – / 0.00005 Cancer Leaded gasoline additives, soil fumigant
Glyphosate 0.28 /
IMAC) 0.7
Damage to liver and kidneys Herbicide run-off from grass, weeds and brush
Heptachlor – / 0.0004 Cancer Termite insecticide leaching
Heptachlor epoxide – / 0.0002 Cancer Biodegradation of heptachlor
Hexchlorobenzene – / 0.001 Cancer Pesticide manufacturing byproduct
– / 0.05 Damage to kidney and stomach Pesticide production byproduct
Lindane – / 0.0002 Affects liver, kidneys, and nervous, immune, and circulatory systems Incesticide used on cattle, lumber, and gardens (restricted since 1983)
Methoxychlor 0.9 / 0.04 Affects growth, liver, kidneys, and nervous system Insecticide run-off from fruit, vegetable, alfalfa crops, and livestock
Oxamyl (Vydate) – / 0.2 Kidney damage Insecticide run-off from apples, potatoes and tomatoes
PAHs (benzo(a)pyrene) – / 0.0002 Cancer Tar coatings, ashes from burning organic matter, fossil fuels
PCBs – / 0.0005 Cancer Coolants from electrical transformers and plasticizers
Pentachlorophenol 0.06 / 0.001 Cancer and affects liver and kidneys Wood preservatives, herbicides and cooling tower waste
Phthalate (di(2-ethylhexyl)) – / 0.006 Cancer PVC and other plastics
Picloram 0.19 / (IMAC) 0.5 Damage to liver and kidneys Herbicide run-off from broadleaf and woody plants
Organic Chemicals continued
Contaminant MSC / MCL in mg per litre Potential Health Effects Sources of Contamination
Simazine – / 0.004 Cancer Herbicide run-off from sod, some crops and aquatic algae
Styrene – / 0.1 Damage to liver and nervous system Plastics, rubber, resins, drug manufacturing, leachate from city landfills
Tetrachloroethylene 0.03 / 0.005 Cancer Dry-cleaning solvents
Toluene – / 1 Damage to liver, kidneys, and nervous and circulatory systems Gasoline additive, fabric seam-sealant, solvents
Toxaphene – / 0.003 Cancer Insecticide run-off from cattle, cotton, soybeans (restricted in the US since in 1982)
2,4,5-TP – / 0.05 Damage to liver and kidneys Herbicide on crops, right-of-ways, and golf courses (restricted in the US since 1983)
1,2,4-Trichlorobenzene – / 0.07 Damage to liver and kidneys Herbicide manufacturing and dyes
1,1,2-Trichloroethane – / 0.005 Kidney, liver, and nervous system damage Solvent in rubbers, and chemical manufacturing waste
Xylenes – / 10 Damage to liver, kidneys, and nervous system Byproduct of oil refining, paints, inks and detergents
Volatile Organic Chemicals
Contaminant MSC / MCL in mg per litre Potential Health Effects Sources of Contamination
Benzene – / 0.005 Cancer Gasoline, pesticides, paint, plastic manufacturing
Carbon Tetrachloride 0.005 / 0.005 Cancer Solvents
p-Dichlorobenzene – / 0.075 Cancer Room and water deodorants, mothballs
1,2-Dichloroethane 0.005 / 0.005 Cancer Leaded gasoline, fumigants, paint
1,1-Dichloroethylene 0.014 / 0.007 Cancer Plastics, dyes, paints
Trichloroethylene 0.05 / 0.005 Cancer Adhesives, metal degreasers
1,1,1-Tichloroethane – / 0.2 Affects liver and nervous system Adhesives, aerosols, paints, inks, metal degreasers
Vinyl Chloride 0.002 / 0.002 Cancer PVC piping, breakdown of solvents