What are waterborne pathogens? (cont.)
Aeromonads
Aeromonads are bacteria that are found naturally in lakes and streams. In large numbers, they can cause wound infections. Usually, the concentration of aeromonads in the water is too low to pose any health risk. But certain conditions-warm water combined with excess loadings of nutrients-can cause aeromonads to grow and multiply explosively. Such massive growth of aeromonads can put persons with compromised immune systems at risk for wound infections.
Viruses
Viruses are tiny bundles of genetic material-either DNA or RNA-carried in a protein shell that protects them from environmental hazards. Ranging in size from 0.02 to 0.09 ?m, they are much smaller than bacteria and making them visible requires sophisticated electron microscopy techniques. Viruses are inanimate until they come in contact with a suitable host cell. They exist for one purpose only--to reproduce. To do that, they inject their genetic material into a host cell and take over its reproductive machinery. Several viral agents for waterborne diseases have been detected in nearshore waters of the Great Lakes. They include hepatitis A virus, which is the agent causing infectious hepatitis. Norwalk, rotavirus, and some other types found in Great Lakes waters can cause viral gastroenteritis.
Protozoa
While most protozoa (“little animals” in Greek) in the water are part of the natural community in a lake, some are intestinal parasites in animals that can be transmitted to humans and cause disease. In the Great Lakes region, Giardia lamblia and Cryptosporidium are the protozoans that are of most concern as agents of waterborne disease. When discharged by an infected host, these intestinal parasites enter a resting stage and form durable cysts that can survive chlorination and many other disinfection methods. Filtration is the most reliable technique to remove Giardia and Cryptosporidium from the water. Giardia cysts are approximately 8-12 ?m in size, Cryptosporidium cysts 4-6 ?m.
In 1993, an operation failure at a water treatment plant in Milwaukee led to a massive outbreak of cryptosporidiosis. The outbreak was the largest documented water-borne disease outbreak in the United States since record keeping began in 1920. An estimated 403,000 persons became ill, of whom 4,400 were hospitalized. Several deaths of immunocompromised patients were reported as a result of the infection. During the nineties, there were also several smaller outbreaks of cryptosporidiosis in Ontario due to contaminated drinking water. Giardia lamblia gained notoriety some years ago when an outbreak occurred in Banff National Park. This giardiasis outbreak was termed by the media as beaver fever because the local beavers were thought to be the source of contamination of the water supply. Giardia lamblia is a growing concern since the number of reported infections in the Great Lakes region has been increasing in recent years.
Blue Green Algae
Concentrations of nutrients (from fertilizers, manure, septic systems, etc.) in lakes above the natural levels can cause algae to grow and reproduce at high rates in what is called an “algal bloom”. Some species of blue-green algae (or cyanobacteria) can release toxic substances as they bloom. One example is microcystis, a toxic blue-green alga that has recently reappeared in Lake Erie. Exposure to the toxins of microcystis may lead to gastrointestinal distress. Risk is greatest when algal blooms are thickest and for those who are most likely to ingest lake water (i.e. children and pets). Even so, there have never been any reports of individuals becoming ill from exposure to microcystis. However, microcystis blooms in ponds have reportedly caused the death of cows, sheep, and even dogs drinking from the water.
(Back to part one)
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