For bracken is the ideal plant to harbour ticks - tiny, blood-sucking parasites - which can carry bacteria that cause Lyme disease, a debilitating condition not unlike arthritis.
In extreme cases, the disease can be fatal. More worrying still, its incidence is increasing and it is almost certain to increase further as climate change brings generally warmer, and sometimes wetter, weather, just what the ticks thrive on in their bracken hideaways.
At the same time, the area of land covered by bracken, a toughie of a fern that has already spread over 2.5 million acres of Britain, is expanding - especially in our national parks and other hill areas popular with walkers.
Named after Old Lyme in Connecticut, where the disease was first diagnosed in the Seventies, Lyme disease affects some domestic and wild animals as well as man, but is carried by others, seemingly without causing them ill health. The first sign is a granular-looking rash in the vicinity if the bite - though what makes diagnosis problematic is that around four in every 10 people infected get no such reddening. The bacteria go on to cause a general flu-like feeling with loss of appetite and insomnia. If not treated at this stage, the disease becomes chronic. The nervous system and joints suffer, particularly the larger ones - hence the severe arthritis that can be so debilitating. Yet treatment with antibiotics early on can stop Lyme disease in its tracks.
Roy Brown, professor of countryside management at Manchester Metropolitan University, who is an expert on bracken and its problems, has followed the growing incidence of the disease. In some parts of the North York Moors he recorded nine ticks per square metre of vegetation in 1979 (when he first began his records; this number has risen consistently over the intervening years, to reach 33 last year. He has recorded a similar pattern in the Quantocks and elsewhere.
"Numbers are looking very high this summer, particularly now, which is their peak time, because the weather has been ideal. Only a few hundred cases of Lyme disease are recorded in Britain each year but many more go unrecorded. It often isn't diagnosed. In the United States, where it is now second to HIV as the most widely reported persistent infection, there were 12,000 new cases last year. In Croatia, where it has really taken off, I know that there were perhaps 20 cases annually 15 years ago. Last year 1,800 were reported there.
"Because bracken is spreading and our climate warming, it is certain to continue to increase here, too."
Bracken is an excellent tick habitat. To survive all stages in their life cycle, these little suckers need high humidity and protection from extremes of temperature - drying out, especially - something that growing bracken can provide par excellence in spring and summer. In winter, the rusty brown, dead growth is equally protective. But bracken is also perfect as a launching-pad for hungry ticks. They can climb up the fronds and simply wait until a human being, dog or another animal brushes past. Bare skin is ideal. And while legs are particularly vulnerable, don't forget that bracken it can grow to 6ft in height; it can easily be chest high.
One of the world's most successful plants, bracken grows on every continent except Antarctica. "In Britain," says Professor Brown, "it's particularly bad in eastern Scotland, Cumbria, the North York Moors, much of Wales and the south west of England. It's spreading on to many roadside verges and in the uplands by as much as 3 per cent in area each year."
Once bracken is established it is difficult to eradicate, because it grows from a dense mass of underground rhizomes.
Over most of Britain, there are now no grants available to eradicate it. Farmers are reluctant to cut it because of the enormous labour involved. Machine cutting may be out of the question, since it often grows on uneven ground. Spraying with a bracken-specific herbicide is expensive, because it usually has to be done by helicopter, and may also run the unacceptable risk of contaminating water supplies.
Longer frost-free periods, more summer sun to provide warm growing conditions, and a damper climate, are just what bracken needs. So, too, do the ticks that spread Lyme disease. In spite of the obvious discomfort on a long, hot walk, the best advice is to wrap up well.
Do's And Don'ts
Do: wear long-sleeved shirts and trousers; wear brightly coloured clothes so that the ticks are visible; tuck trousers into boots or socks; check for ticks on your clothing and body after a walk; remove any with sharp-pointed tweezers using a twisting action (if the mouthparts of the tick break off under the skin, consult your GP immediately); spray dogs with insect-repellent powder
Don't: wear shorts, skirts or rolled up sleeves; burst blood-engorged ticks or squash them with your fingers; ignore any localised rashes around the bite (seek medical attention)