The injection itself rehydrates the animal. For anyone else who is going to read this and has these problems there are many books at the library that can be very helpful in regards to this subject. This method of treatment is available and does work. I know I have experienced it. An animal can actually live to 20 years old now a days.
Cats can live even longer that that.
There are many options like I said. Don't just take one person's advice. Research it yourself and get many opinions.