Dog owners were today being warned to stay vigilant for signs of a mystery illness which can prove fatal to pets.

Last autumn dozens of dogs died after being walked in the countryside in Norfolk, Suffolk and the East Midlands.

Animals which had been walked in woodlands, including Sandringham Park, on the Royal Estate, suddenly became ill.

Royal aides called in vets from the Animal Health Trust (AHT) to investigate the cause of the bug, which has been called Seasonal Canine Illness.

Symptoms included vomiting, diarrhoea and lethargy - with some animals dying within hours.

Today the AHT said any dog owners who see these signs in their pet should seek veterinary advice immediately.

AHT vet Richard Newton said: 'We weren't aware of any cases in 2011, then suddenly at the start of September we had a flurry of reports.

'Vets and dog owners seem to be more aware of this illness this year and this has helped enormously with our investigation.

'We want to arm dog owners with as much information as we can.

'Unfortunately we are looking for a small needle in a very large hay stack so this could take some time.'

Mr Newton said the AHT's investigation into the disease was now focussed on specific areas in Norfolk, and Clumber Park and Sherwood Forest in Nottinghamshire.

Dog owners who walked their pets at Sandringham and Thetford were among hundreds completing qeuestionnaires about any symptoms seen.

Mr Newton added: 'We do not believe that dogs are at any more risk walking at the four sites we're investigating than at any other woodland site. We simply need to limit our investigation area in order to get the most useful data.

'Nearly 400 people completed questionnaires last autumn after walking dogs in these areas but we were also alerted to other areas, across the whole of the UK, where dog owners believe cases had occurred.'

The AHT is asking dog owners who have walked their dogs at any of the four study sites since August 1, 2011, to complete and return a questionnaire.

Follow the links, top right, for more information.