I have found a paper written in the journal of nutrition Ref. 1, which investigates the break down of thiamine in wet feed stock, not a mouse, but it would be a similar trend. They found that freezing slows the breakdown of the thiamine by oxidation or reduction by sulphur dioxide or casein. The freezing also inactivates the thiaminase which cleaves the thiamine. Mice do not contain casein, but do produce sulphur dioxide. Therefore freezing is the best way of preserving the thiamine for long term storage.

A fresh mouse will contain higher thiamine as it is replaced naturally whilst it is alive as it eats seeds and grain and but will quickly breakdown after death if it is warm almost 50% in 1 hour. A snake can handle lower concentrations of thiamine due to its efficient use of water, thiamine is very soluble and will only accumilate in the muscles for a short period in mammals as they deficate and urinate often. Whilst in the muscle it is converted into less soluble thiamin pyrophosphate Ref . 2, which is a co-enzyme that helps other enzymes in the digestion of carbohydrate (something a mouse has fairly little of anyway). In snakes it will remain in the muscle tissue longer and more will be converted.