The causes of SCI are unclear but the potentially fatal disease is contracted by dogs walking in woods in the autumn and causes vomiting, diarrhoea and lethargy. Many have had harvest mites on them.
School administrator Jo Raby, 35, said her five-month-old labrador Chester fell ill the day after his first walk at the site on October 8. He was in "immense pain" and spent two days on a drip at a vet's, costing her more than £400 (NZD$907).
When the mother of two, from Snettisham, contacted the estate's visitor centre a member of staff told her there was only one sign warning dog owners.
She said:"They said they had hundreds of acres of woodland with five car parks and they couldn't put signs up everywhere.
"The member of staff then told me, 'Do you expect us to put up a poster on every other tree?' I said I just wanted more signs put up on car parks and trails where dog walkers could see them."
Terry Wenn, 65, had two dogs fall seriously ill after a visit on September 27. Scruffy, a six-year-old terrier, and Charlie, an 11-year-old dachshund, spent several days on a drip at a cost of £700 (NZD $1588).
The foster carer, who lives with wife Linda, 60, in Walpole Highway, said: "We didn't see any signs at all on the estate. If they had more signs up it would make people think twice about walking their dogs there at this time of year."
Buckingham Palace said the estate took any reports of Seasonal Canine Illness seriously.
A spokesman added: "There is notification via posters and flyers at the visitor centre and country park, including the main dog walking areas, the car parks and the camping and caravan sites. There is also a notice on the estate website and the campsite gives advice when taking bookings."
Vets confirm suspected cases of Seasonal Canine Illness
Vets in the Sandringham area confirmed that they had treated another dozen dogs with suspected SCI after being walked on the estate in recent weeks.
Cases were first identified at Sandringham and other wooded areas in 2009. The Animal Health Trust was asked by the Queen, its patron, to conduct a study the following year. This found 35 dogs fell ill at Sandringham in autumn 2010 and six of them died. A further 51 cases, including two deaths, were linked to the estate the following year.
Buckingham Palace said there had been a drop-off in reported cases of SCI over the past two years.
- Daily Mail