Quotes from the Animal Health Trust faq pages:

'If my dog has experienced signs of SCI but not in Sherwood Forest, Clumber Park, Sandringham or Thetford Forest, should I still report it to AHT?
If your dog experienced clinical signs of SCI (vomiting, diarrhoea and lethargy) after walking in woodland areas other than the four areas referred to above, we may send you a questionnaire for your specific area in due course, depending on numbers of enquiries received. So do please let us know about your case by emailing info@aht.org.uk

What are the clinical signs of SCI and how soon do these signs appear?
The disease comes on very quickly, usually within 24 hours of having walked in a woodland area. The most common clinical signs reported by dog owners are sickness (vomiting), diarrhoea (which can vary from watery to bloody), tummy (abdominal) pain, lethargy (or reluctance to move), loss of appetite, shaking or trembling, and in some cases high temperature (fever). For more information on clinical signs, please click here

What should I do if my dog experiences any of the above mentioned clinical signs?
Contact your vet immediately so that your dog receives treatment as soon as possible.

What are the likely causes?
There are a number of theories regarding the cause for Seasonal Canine Illness; none of them have been confirmed. The research carried out at the AHT and the information gathered will help to build a list of risk factors in affected dogs and compare them to those in non-affected dogs. Hopefully, this will enable us to pinpoint a cause for the disease. For a list of possible causes of SCI,  please click here

Can it be passed to other dogs?
There is no evidence at the moment about this disease being infectious (i.e. transmitted from dog to dog); In some cases dog owners with more than one dog have only had one case, even when all the dogs walked together. However, an infectious cause cannot be ruled out at the moment.

Can it be passed to humans?
There is absolutely no evidence at the moment about this disease being transmitted from dogs to humans. None of the dog owners who had an affected dog showed any clinical signs.

Where have the majority of the deaths occurred?
At the four sites we are investigating and of the cases we know about, the majority of deaths in 2010 occurred in Norfolk.

What general advice would you give on walking through wooded areas?
Our advice is to be vigilant and if you have any concerns, contact your vet immediately. We are reasonably confident that this is a seasonal illness but it is best to stay vigilant at all times of the year. We are not in a position where we can tell you where you should or shouldn’t be walking your dogs, but we are trying to alert dog owners to this potential risk so you can make an educated choice. If you are worried you may wish to walk your dog on a lead so you know exactly where it is at all times. Alternatively contact your local vet to see if any cases have been reported.

Have investigations ruled anything out?
It was suspected a tick or a mite could have caused the deaths, but none were found on the dogs that died.  Poisoning such as strychnine or cyanide was ruled out by Natural England.

What action is being taken in terms of testing?
Natural water sources are being tested by the Environment Agency for the presence of blue-green algae.

The AHT is evaluating data collected from four of the affected areas. We hope this data will enable us to build a list of risk factors in affected dogs and compare them to those in non-affected dogs. This will hopefully enable us to pinpoint a cause for the disease.

The AHT is also storing blood and other samples taken from affected dogs in order to carry out toxicological investigations on them.'