Canoeist Pathogenic Illness Guide
Rivers and seas in the UK are used as a disposal site for human waste, especially sewage. The ensuing risk of infection to watersports participants has traditionally been accepted with resigned stoicism.
More recently increased environmental awareness, and changes in the law of legal liability, have raised questions about the risk of infection. This study is designed to review published literature in order to assess the risk to canoeists, and to see what advice can be given to reduce that risk.
One category of pathogens which cause infection are those which go into the river with sewage and other human waste. Bacteria and viruses can cause gastroenteritis, and other skin, eye, ear and respiratory tract infections.
Other pathogens are blue green algae, which can contain potentially fatal toxins, and Leptospira spp, which can lead to another potentially fatal illness called Weil's disease.
Tests have been developed to measure the potential infective risk of water using indicator organisms. By enumerating the quantity of certain common human intestinal bacteria in a water sample it is possible to assess the degree of sewage contamination, and hence an indication of potential risk.
Early investigations in America (1953) assessed the risk to bathers on fresh and marine bathing sites. It was found that there was a small illness rate amongst the general population (both gastrointestinal and skin/eye/ear infections) and that bathing increased the illness rate by between 30 and 85%. This study was later used to produce bacterial water quality standards based on faecal streptococci, although the weak epidemiological basis for the standard was widely questioned.
In the UK a PHLS study into cases of poliomyelitis and enteric fever caused by bathing found a negligible risk to health, although as they were only looking at serious notifiable diseases they did not measure the minor infective risks commonly associated with sewage contaminated bathing water. This study was widely used in the UK to justify a lack of action in improving the biological quality of bathing sites.
More recently a number of large scale studies have been carried out all over the world. The study designs were fairly standard, using questionnaires to assess non-bathing illness risks and to enumerate illness symptoms, measuring illness rates in bathers and a non bathing control group, and simultaneously measuring the levels of indicator bacteria in the water. Allowing for illness effects from other causes, such as fast food, was a frequent problem in the statistical analysis.
The many studies produced similar results, with swimmers being typically almost twice as likely to report illness symptoms as non swimmers. The rates of illness increased with increasing bacterial indicator count, and with the number of swimming events, although the indicator organism which proved to be the best index of illness risk was different in each study. Non gastrointestinal illness symptoms such as skin, ear and eye infections were more common than gastrointestinal symptoms.
A small number of watersports related studies have been carried out, especially in white water canoeing. These used the same research designs as the bathing studies, and showed similar illness risks when canoeing on water of a similar biological quality. Canoeing on calm water (ie marathon racing) was shown to have negligible risk, even on more heavily polluted water. Studies at Holme Pierrepont showed that eating/drinking with wet hands increased the risk of infection, and that frequent visitors to the site (ie experienced canoeists) were much less likely to develop illness symptoms.
Primary and secondary sewage treatment processes will disinfect the waste stream to a certain extent, and subsequent dilution and natural disinfection in the river (caused by the UV component of sunlight) will also reduce pathogen levels. If these processes do not provide water of sufficient biological quality it is possible to add a tertiary, disinfection stage to the sewage treatment process. In high rainfall, the sewage works is designed to divert excess flow straight to the river after minimal treatment.
Pathogenic microorganisms also enter the watercourse due to the run-off after a storm, flushing waste from urban and rural land areas into the river. Discharge from storm culverts can have as high a pathogen level as untreated sewage, especially during a storm in the summer after a long dry spell, when solid waste deposits will have built up in culverts and stream beds over a long period.
Studies which measure levels of indicator organisms in the river have shown massive variability, often by several orders of magnitude. There is however an overall pattern which shows low levels in the summer (which in many UK rivers would pass EC Bathing Water Directive water quality standards), higher mean levels in the winter, and isolated maxima associated with storm events, the highest maxima being in the late summer and autumn.
An overall risk assessment shows that there is a risk to white water canoeists from canoeing on river water contaminated with sewage derived microorganism. On water which meets the EC Bathing Water Directive standards, or even slightly exceeds them, the rate of illness or illness symptoms tends to be no more than double the rate of illness or illness symptoms amongst the general population. The illnesses are gastroenteritis, or (more commonly) skin, eye, ear and respiratory tract infections. These are mild and short term, with no reports of illnesses that caused death, serious injury or permanent disablement. By comparison with other commonly accepted risks in sport, the risk to canoeists from pathogenic microorganisms would not appear to be excessive.
It is not possible to assess the short term risk of canoeing at a site on a particular day by using indicator bacteria tests. The variability is too great. It is however possible to estimate the risk using a knowledge of season and rainfall/river flow rate. The risk is low in the summer and medium in the winter (assuming zero or mild rainfall).
After a storm, pathogens flushed into the river will increase the risk. In the early stages of a winter storm the risk will be high, but pathogen dilution and dispersion could lead to lower risk levels as the storm continues. Summer storms have a very high risk due to limited dilution. Further research into the risk to canoeists during storm events could be helpful.
It is possible to reduce the risk of infection by not eating or drinking with wet hands, and by taking part in canoeing activities on calm water with lower water contact.
It is possible that experienced canoeists have a much lower risk of illness, due either to their increased skill level leading to less water contact, or to experienced canoeists as a group tending to be naturally resistant to infection.
There are no statutory water quality standards currently applied to inland rivers, although coastal and estuary beaches are designated under EC Directives. Designation of fresh water under the Directive, or under the Statutory Water Quality Objectives (Water Act 1989) would help to ensure that correct attention was paid to fresh water quality for recreational use.
Simon Dawson - May 1996