At this time of year, with the heat of summer passed, walking on our moors and hills is not only pleasurable but healthy too. Mile after mile of rough grassland, heather and gorse; the sound of cascading streams; big gulps of invigorating air; and the earthy smell of bracken as it crushes underfoot.
But, beware! What most autumn walkers and horseriders are blissfully ignorant of is that far from being the epitome of healthy exercise, walking close to bracken at this time of year is a health hazard to be avoided. One of the world's most successful plants, growing on every continent except Antarctica, this robust and attractive fern is also one of the most poisonous. It harbours ticks which transmit diseases including the arthritis-causing Lyme disease and Louping III. A killer of livestock, its spores - vast numbers of which are produced from the plants in early autumn - are carcinogenic to a range of laboratory animals and there is little doubt that they can have the same effect on humans.
This autumn, according to the Bracken Advisory Commission (BAC), a group of experts who draw attention to the health threat posed by bracken, could be a bumper year for spores because of the combination of a warm summer and short wet spells. It is recommending a range of precautions which should be adopted by anyone walking among, or working near, bracken. The Commission is also concerned that large quantities of bracken spores will end up in public water supplies as they wash into reservoirs in heavy autumn rains, yet another potential health hazard if the supply isn't adequately treated.
"In experiments I have done," says Dr Alan Heyworth, a botanist and one of the four BAC commissioners, "A 10-minute walk through a bracken bed resulted in my inhaling 50,000 spores. They're too big to get into the lungs so they are all swallowed. Like the bracken plant, the spores contain ptaquilosides (a complex group of organic chemicals) which cause cancers, often in the digestive tract. Feeding bracken spores to mice and other animals quickly produces digestive tract and other cancers," he adds.
Japan, where young bracken fronds are eaten as a delicacy, has the world's highest incidence of stomach and oesophagus cancers. Sheep, if they eat the plant by accident (like cattle, they avoid it) suffer from sarcomas and mouth cancers. According to a study commissioned in 1989-1990 by the Ministry of Agriculture, the cost of bracken to the British livestock sector - in terms of animals killed, vets bills, and animals lost in dense areas of bracken - was about pounds 8m a year.
Avoiding bracken when it is sporing in early autumn isn't difficult. But walking through bracken at any time of the year runs risks. Bracken harbours ticks, some of which carry diseases. Lyme disease, the worst, can affect humans, dogs and horses. It is seldom fatal, but can persist in the body for years causing prolonged symptoms including chronic arthritis. The first sign is a rash around the tick bite, followed by a more extensive rash, tiredness, headaches and fever. Some victims develop heart problems. In the US, Lyme disease is second to HIV as the most widely reported disease caused by persistent bacterial or viral infection.
Louping III, a viral infection, can also be an unforeseen consequence of a bite from a bracken-lurking tick. It produces high temperatures and fits because it attacks the central nervous system. Though it can kill sheep, Louping III isn't lethal to humans. Its effects can persist for several weeks all the same.
If bracken was as uncommon as some other ferns, the health risk it poses wouldn't merit a second thought. But the plant is widespread in the uplands, around woods, on river banks and across heathlands in lowland areas too. After woodland and heath, dense stands of bracken dominate more land in the New Forest than any other vegetation. The area covered by it in Britain is equivalent to the size of Yorkshire, 2.5m acres. What's more, it is still spreading, in places by up to three per cent a year, according to Roy Brown, Professor of Sustainable Land Use at Bishop Burton College and a BAC commissioner.
"It's particularly bad in eastern Scotland, Cumbria, in the North York Moors, parts of Wales and southwest England," says Professor Brown. "It is spreading on many lowland roadside verges, sometimes into pastures. In the uplands, if heather moorland is burnt too often, or grazed too heavily, bracken takes over. It spreads very quickly, especially on deeper soils, by its underground rhizomes. Once bracken is established getting rid of it is difficult," he adds.
Professor Brown believes that removing the grants that used to be paid to farmers to eradicate bracken has done us no favours. Some funding is available in specially designated places, like Environmentally Sensitive Areas, but in the rest of England the grants have been removed. Until two years ago, farmers could obtain half of the cost. That was reduced to 30 per cent, then abandoned. For now grants are available in Wales.
Regular cutting of bracken, preferably three times a year, will eventually weaken it so much that it gives up the ghost. But on steep, rocky or uneven land, cutting is impossible. This is where the herbicide, Asulox, comes in. Non-toxic to animals, it kills any fern it's sprayed on. For large areas, spraying by helicopter is the only practical option. The problem is that most farmers won't stump up the cost of such a scheme unless agricultural grants are available.
So bracken is spreading. The BAC is concerned at the way it has spread in many of the catchment areas of our reservoirs in the uplands of west and north Britain. Ironically this spread, increasing as it is the chances of large numbers of carcinogenic bracken spores washing into the water, is partly the result of an EU Directive aimed at preventing the contamination of water supplies.
The Directive, which stipulates that no more than one part of any pesticide in 10 billion parts of drinking water is permissible, was intended to eliminate any risk from highly toxic organochlorine pesticides such as those used for dipping sheep. But it also ensnares the bracken herbicide, Asulox. So the chemical isn't sprayed in the catchments of reservoirs for fear of drinking water failing the Directive, a prospect no water company relishes. Not surprisingly, in many reservoir catchments, bracken is on the march.
The BAC believes that this could pose a problem, especially if there is a surge of vast numbers of spores in the early autumn, washed down by heavy rain. Dr Heyworth admits that the dilution is usually substantial, and that filters can remove most of the spores. "But if the spores aren't filtered out and filters changed regularly," he cautions, "the ptaquilosides are released into the water as the spores decompose. We also don't know if these carcinogens are washed out of the bracken plants themselves by rainfall. Anyone on an unfiltered [usually, small local] supply could be particularly at risk."
Dr Brian Crathorne, Director of the National Centre for Environmental Toxicology at the Water Research Centre agrees that large numbers of spores can enter reservoirs but that filters and coagulants used in the purifying process will remove them. "I don't believe that the spores degrade quickly," comments Dr Crathorne, "so I don't consider that there will be any sudden release of the carcinogens they contain."
But if cancer-causing substances getting into our water supplies is a possibility, albeit a contentious one, there is now less chance than there was of them being ingested via milk. In developing countries in which bracken is common - parts of South America for instance - dairy cows often feed more widely, roaming over larger land areas where they can inadvertently eat bracken mixed in with other plants, and bracken carcinogens can contaminate milk. Years ago, the same extensive grazing of dairy cattle was common in some parts of Britain. Today, almost all British dairy cattle are confined to pastures, hardly getting a taste of anything but ryegrass. In countries like Costa Rica there is published evidence that human digestive-tract cancers are being caused by drinking milk from cattle which have eaten bracken.
But not everything about bracken is bad news. It provides a habitat for some insects, although not many, and it is important for some upland breeding birds like whinchats. It's also valued by walkers and horseriders as an attractive component of our hills and uplands, one that adds texture and the changing colours of fresh green in early summer, the olive-green tinges of late summer and the rust browns of autumn and winter.
The problem is that this dangerous plant is spreading too fast to be good for us. Come global warming, mild winters will boost its growth even further and warm summers will stimulate more sporing. Country walks could become a serious danger to our health.
BRACKEN: THE SPORE LAWS
Wear a face mask near bracken beds from late August to early October.
Wear long-sleeved shirts and trousers, preferably brightly coloured ones so that any ticks that jump on you are visible.
Tuck your trousers into boots or socks.
Spray dogs with insect-repellent powder before letting them walk through bracken.
Check for ticks on your body after walking through bracken, removing them with a pair of sharp-pointed tweezers in a twisting action. If the mouth breaks off under the skin, consult your doctor immediately.
Burst blood engorged ticks or squash them between your fingers.
Wear shorts, skirts or rolled up sleeves.
Walk in, or downwind of, bracken beds from late August to early October unless you are wearing a face mask.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies