There is nothing wrong with you keeping a dog for the purpose mentioned (to alert its owner that a seizure is imminent), whilst taking precautions to avoid getting the impurity of its saliva on your body, clothes and so on, as much as possible.

1. If pure water is mixed with some pure substance, and its colour, taste and smell does not change, then it remains pure, because it is still called water. 2. If pure water is mixed with a pure substance, so that it is no longer called water, it is not valid to do Wudu or Ghusl with this [altered] water. 3. If ordinary water is changed with the addition of any pure substance, but it is still called water – such as water that is mixed with soap and its colour changes, or hummus falls into water and changes its taste