Four people have been trampled to death by cows in just over eight weeks this summer, prompting British farmers and the Ramblers Association to warn yesterday of the potential dangers.

The spate of incidents is regarded as highly unusual; in the past eight years there have only been 18 deaths in total caused by cattle of all kinds – including incidents involving bulls, which have always been known to present risks.

Cows have been thought to be generally docile, and this remains true, the National Farmers' Union emphasised yesterday. However, the NFU pointed to the fact that at least two of the four deaths involved walkers with dogs, which may be a factor in turning cows from placid cud-chewing bystanders into potential killers.

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"Cows can get aggressive in the presence of dogs, especially if they have their calves with them," Robert Sheasby, the NFU's rural surveyor, said yesterday. "They see the dog as a threat, and take exception to it. Cows are generally placid and docile, but when a mother animal feels the protection of her offspring is at risk, temperaments can change."

The first of this summer's fatal incidents seems to have conformed to that pattern: it occurred on 21 June in the Yorkshire Dales when a veterinary surgeon from Warrington in Cheshire, Liz Crowsley, was walking along the Pennine Way near Hawes with her two dogs, a spaniel and a collie. She was found dead, having apparently been trapped against a wall and then trampled. Police speculated that Ms Crowsley's dogs may have sparked the attack.

In what may have been a similar incident, 63-year-old Anita Hinchey was trampled to death on 18 July when walking her dog in a field on the outskirts of Cardiff. Three days earlier, 65-year-old Barry Pilgrim was killed when walking with his wife in the countryside near Sheldon in Derbyshire. It is believed Mr Pilgrim did not have a dog with him, but he was attacked by cows who were accompanied by their calves.

This summer's fourth fatal incident occurred on 11 August, when a 75-year-old farmer, Harold Lee from Burtle in Somerset, was trampled and killed by his own herd. It is thought the animals may have been frightened by a passing fire engine.

This total of four deaths, in such a short time, has caused concern. "It's certainly something we're not used to seeing. This year has been very unusual," Mr Sheasby said. "It is very unfortunate that three walkers can be killed in the space of a few weeks, and we would hope that this is an unfortunate coincidence."

There are about 7.5 million cows in the fields of Britain, who share the landscape with millions of rambling Britons in the summer. No one knows just how many walkers are out on a given day, but the numbers are clearly very high – each of Britain's 18 national parks will chalk up between 20 and 40 million day visits every year.

As a fully-grown cow can weigh anything from three-quarters of a tonne to a tonne, it does not take much for serious injury to be inflicted on anyone unfortunate enough to be on the end of a sudden bout of bovine bad temper.

Someone who can testify to this painful truth is David Blunkett, the former Home Secretary, who was attacked by a cow while walking in the Peak District on 6 June, his 62nd birthday, and suffered bruising and a broken rib. Once again, a dog was involved – in this case, the blind MP's trusted guide dog, Sadie.

The NFU joined with the Ramblers yesterday in offering a key piece of advice: if you feel menaced by cows when you have a dog with you, let it go, as it may well be the dog that is causing the problem.

"Your dog will outrun the cattle and you can then make your own way to safety," Mr Sheasby said. "At most times it is important to keep your dog on a lead in the countryside, but if you feel under threat because of it, let it go and put it back on its lead later."

Cows are not generally dangerous, he added. "They are naturally curious, rather than naturally aggressive, as well as short-sighted, and they will often come right up to you just to see who is in the field with them." But it was important, he said, not to let your dog get between a cow and its calf.

Wild animals: Creatures on the attack

*Cows are not the only livestock causing problems this summer. Yesterday a farmer described his ordeal after becoming one of the first people in Britain to be bitten by an alpaca, the South American relative of the llama.

Rob Rawlins, 52, had to be airlifted to hospital after an animal at the Wellground Alpaca Stud Farm, near Westbury, Wiltshire, sank its teeth into his lower left arm as he tried to stop a "dominance fight" with another male.

"I tried to intervene, which was when I was caught on the arm by one of them," said Mr Rawlins. "The bite had not been intended for me but unfortunately my arm got in the way. I was able to remove my arm using my other hand and that really was the end of the incident, although I did end up with quite a nasty wound." Allen Brown, treasurer of British Camelids Association, representing llamas and alpaca farmers, said he had never heard of anyone being bitten in that way by the animals.

*Two people have died in recent days after being stung by wasps in separate attacks. Farmer Mark Evison, 47, of Ellerker, East Yorkshire, disturbed a wasps' nest while clearing a dyke and was attacked by the swarm. Mr Evison was allergic to wasps and when emergency services reached him, he was already in the advanced stages of anaphylactic shock – a life-threatening allergic reaction – and died at the scene. Joan Russell, 81, of Holt, Norfolk, collapsed within minutes of being stung on her hand while washing up and never regained consciousness.