A mysterious fatal illness in dogs walked on country estates in Norfolk and Nottinghamshire is being investigated by an animal charity.
The illness, which causes vomiting, diarrhoea and lethargy, has affected several dogs since August, the Animal Health Trust said.
All had been on either the royal estate at Sandringham, Thetford Forest, Sherwood Forest or Clumber Park.
The trust wants owners who have walked their dogs there to get in touch.
The illness also struck last autumn, killing eight dogs who had been walked in Sherwood Forest.
A vet in King's Lynn, near Sandringham, also reported two deaths from the condition in 2009.
Two dogs are known to have died in Nottinghamshire since August.
The Animal Health Trust, based in Newmarket, Suffolk, said the cause of the illness has baffled vets and scientists.
It is asking owners who have walked their dogs on any of the four sites to fill in a questionnaire, regardless of whether their dog was taken ill or not.