Seasonal canine illness (SCI) was first reported in the autumn of 2009, when dogs walked in woodland at the Royal Estate at Sandringham and Thetford Forest began showing symptoms.

Affected animals suffer sickness, diarrhoea and lethargy within 72 hours of walking in woodland, with some dying from the condition.

While the cause is not known for certain, it is believed harvest mites may carry the infection, which occurs between August and November.

Posters have now been put up at Sandringham, where there were more than 15 cases last year. They alert visitors to the disease and its symptoms and giving advice on what to do when visiting woodland with dogs.

So far, there have been Helen Walch, the estate's public access manager, said: 'We are anxious to do what we can to help prevent further cases this year, whether here or anywhere else.'

The Animal Health Trust (AHT) says dogs should be walked on leads in woodland at this time of year so that owners can keep a close eye on them, and they also suggest that spray treatments for mites may be helpful.

Spokesman Farrah Owens said: 'There were harvest mites found on a number of cases, that's why we advise dog owners to make sure their topical flea and mite protection is up to date.

'The best thing dog owners can do is be aware of the symptoms and access their vets as soon as they see signs of sickness, diarrhoea and lethargy. Dogs that visit their vet quickly tend to recover.'

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