Isles of Scilly vet Dr Heike Dorn has warned that it is still not safe for people to walk their dogs on the Garrison.
Heike previously advised islanders and visitors to avoid taking their pets to the popular spot in July after more than 20 were struck down with vomiting and diarrhoea.
Although the situation subsequently improved, she said it has now started up again with two more cases in the last three days.
She told This is Scilly: "If your dog does start with vomiting and diarrhoea, it doesn't make sense to wait. If it is young and healthy the dog may be able to get over it without antibiotics but if it's older or not in the best health it will need them so come and see me."
Heike said she is not sure exactly what the problem on the Garrison is, but believes it to be a kind of bacteria.
She added: "You can't see it so it's hard to say where it is but I'm sure it was brought in by a visitor's dog. Both the dogs I've seen in the last three days became ill after walking on the Garrison so if you want to be safe, it's best not to take the risk."