Veterinary questions? Ask a Vet for Answers ASAP
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You are through to Dr Coles. I am a UK based vet. I am sorry to hear you are having problems with your terrapin. The symptoms you are describing are quite concerning. Red discolouration to the shell is often a symptom of septicaemia. This usually is caused by overwhelming bacterial infection, in the blood stream too. Usually in terrapins this can be due to either vitamin A deficiency, calcium deficiency, lack of UVB lighting, low temperatures, inadequate water temperature (too low) or inadequate basking area temperature. I would also recommend checking your water quality (nitrates, ammonia etc) as poor water quality can predispose to infections. It is also important to supplement the diet with vitamin A and calcium using supplements such as avipro and calcidust. Sometimes it is better to feed in a separate water tank to avoid contamination of the water with food material & faecal matter.
However I would advise that he needs to be seen by a vet ASAP as it is likely he will need antibiotic treatment, usually given by injections. The orange spot is possibly a fungal growth which can be secondary to poor water qualities as well.