Dog-owners walking their pets at a beauty spot have been warned to keep a tight rein on their pets after the deaths of animals who had been exercised there.

Posters calling for pet-owners to deter their pets from eating anything and ensure vaccinations are kept up-to-date have been erected at Ogden Water nature reserve, near Denholme, in Calderdale.

Estate manager David Wilson said a small number of animals had collapsed and died after being exercised around the reservoir and surrounding country park.

Mr Wilson said: "These incidents did not actually happen in Ogden but did involve some of the walkers exercising their pets in the area.

"A small number of animals have come into contact with something. It appears to have happened to dogs who have been walked up and down the valley. They have not actually picked anything up while walking in Ogden itself."

He said some animals had suffered fits and a small number had died. Mr Wilson said the ranger service had been working with the police and Yorkshire Water to find the source of the problem.

He said: "We have carried out tests in the area to find a cause for the problem but have found nothing.

"Unfortunately, vets did not carry out post-mortems on the dead animals so we have no clues there either.

"But the last reported incident was before Christmas. We have had thousands of dogs here since then.

"The reported incidents are rather strange but certainly nothing to do with us."

PC Roman Soltan, police wildlife officer for Calderdale, said inquiries were continuing.

A Yorkshire Water spokesman said Ogden Water itself, a 34.5 acre reservoir which provides much of Halifax with its drinking water, had "in no way been affected by the issue".

"We have been working in partnership with the Ogden ranger and the police wildlife officer and have warned our tenants about the incidents," said the spokesman.

"We can reassure customers that water supplies have in no way been affected by this

issue. We will continue to work with the council and the police if required to do so."

Dog owner Joan Jones, of Ovenden, who walks her border collie cross Tiny at Ogden Water, said she had not been put off by the incidents.