Come on in, the water's fine

Weil's disease is the secret weapon of whatever dark forces are opposed to wild swimming," wrote Roger Deakin, Britain's most famous freshwater swimmer and author of Waterlog.

Previous generations have always had things to scare them away from the water, with folklore about selkies and river hags such as Jenny Greenteeth pulling unsuspecting folk to their deaths, but in the 21st century we have other names for our deep water fear — blue- green algae and Weil's disease.

Here's the truth. I've spent my life swimming outdoors and run a nationwide network of outdoor swimmers. No one I know has ever got it, and no one I know knows anyone who has got it.

Despite this week's headlines, following the death of Olympic rower Andy Holmes — "rat disease kills rower", "how Weil's disease puts competitors at risk", "rower dead in days from waterborne illness" — Weil's is not peculiarly a disease of water users and it's not, medically, particularly associated with rats.

There is less than a one in 10 million chance of contracting Weil's, and most cases are successfully treated with antibiotics. "Weil's disease is actually not a disease, but a development of a waterborne bacterial infection called leptospirosis," says Dr Robert Smith, a clinical scientist from the Health Protection Agency who specialises in this area. "Around 50 people a year in England and Wales become infected with leptospirosis, and only 10 per cent of those go to on to develop Weil's."

Weil's has traditionally been an occupational disease — affecting animal handlers, farmers and people who work in waste management and fishing. Building contractors for Canary Wharf and anyone who develops property near standing water are aware of it. One case involved a street cleaner using a jet wash after a street market. Another involved a rugby player, thought to have contracted it drinking from a beer bottle contaminated in a pub cellar.

According to Smith (who analyses infections for the HPA), of the 2009 cases of leptospirosis in England and Wales, 14 were from travelling abroad (it's very much a tropical and subtropical disease, rare in temperate climates), 11 were occupational (farmers, vets), seven recreational (canoeing, fishing, sailing, windsurfing, swimming), six acquired it residentially (from rats and mice around the house, cleaning out ditches, streams or drains), and two from accidental immersion. It has even been acquired at the Glastonbury festival.

"Leptospirosis is no more common in urban that rural watercourses," says Smith. You can find it in crystal clear and clean salmon and trout streams as well as urban docks or Camden Lock.

Even where leptospirosis is present it is very hard to be infected. You cannot be infected by swallowing water, and "there is little or no risk of transmission through intact skin," says Smith. The most common route of infection with leptospirosis is through a cut or abrasion, although the mucous membranes in the eyes or mouth could provide an entry point, particularly if there are cuts, bleeding gums, or de-roofed blisters.

In terms of avoiding higher risk areas, the bacteria are likely to be concentrated at the edge of rivers, docks and lakes, where animal urine runs into the water, and the water runs slower. Floods may flush more infection into the water, raising risk. Water carries infection but something like cutting a foot in the mud beside a river and lakeside could lead to bacteria in a wound.