We have a very large cast iron dutch oven which can not fit into our house oven. Have not used it for 4 years and has not moved from our potting shed, in wanting to utilize it it has been pooped on by birds and chances are a mouse or two. It is the outside only which includes the lid. What is the best way to clean it so we have no issues fixing food in it. Or should we just replace it with a new one? 
Wash with soap and water...use normally.  Is the seasoning still in tact?  If the surface of the pan/seasoning is failing, you'll want to go through the steps for re-seasoning your pan. 