Need formula that finds the last ", " in a text string & substitutes it w/ the word "and"

Hello, I have a text string that might have any number of words in it, all separated by a comma and a space. So one text string might be "dog, cat, boy, girl" and another one might be "dog, cat". I need a formula that will find the last comma and space, and substitute that with the word "and". So instead of the text string stating "dog, cat, boy, girl" it will read "dog, cat, boy and girl".

Any help would be much appreciated!