Andy, the Wales Outdoors owner has walked the hills for years, taking sips and long draughts alike from mountain streams, and he seems fine ;) However, there is a risk to health and this excellent web page at ultralightoutdoorgear.co.uk explains those risks and offers solutions to wild water drinking.
Drinking water directly from streams and rivers can be hazardous because such sources could contain Pathogens; a Pathogen is a disease causing organism commonly referred to as a germ.
There are four types of water-borne pathogens that are of concern:
Protozoa – single cell organisms often transferred through human and animal faeces. They are highly resistant to chemical treatments but are easily filtered because they are relatively large. They can also be treated with UV Light and heat. Giardia, Cryptosporidium.
Bacteria – single cell organisms that can reproduce in water. Medium sized, they can usually be removed by filters though not all will have this capability. Can also be treated with UV and heat. E. Coli, Salmonella, Cholera.
Viruses – these are spread through water but only reproduce in a host. They can be difficult to filter because they are Small, they are sensitive to chemical, UV and heat treatments. Hepatitis A, Norwalk, Rotavirus.
Multi-cellular parasites – Large and can be easily filtered, can also be treated by boiling. Resistant to UV and chemicals. Hookworm and Roundworm.
LOCATION, LOCATION, LOCATION
The primary threats in water collected from rural, relatively pristine sources are Bacteria and Protozoa, the likelihood of Viruses being present is very low.'
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