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Tell me did you actually see the sting happen or find the stinger imbedded in your tortoise, this is quite a rare occurrence, I have only suspected this to have happened once in my 25 year career.
I did not see the sting happen nor find the stinger. He was normal when let out in the morning and I found him in his present state by late morning. There are a lot wasps around which sometimes get on the food although I avoid fruit at this time of the year, I find they still are crawling on lettuce and the like. He has been stung before and had an injection and recovered the next day.
OK thanks for that, that makes a sting quite likely. First of all your vet had done just right in giving a steroid shot, that's probably what I would have done myself as steroids massively reduce inflammation. If you have dealt with a dog or cat in this way then the inflammation from the sting would have quickly gone down. But tortoises have a very slow metabolic rate as compared to dogs and cats so it could take three or four days for the steroids to exert their effect. So it could be early days yet ...