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.
There is no need to replace a perfectly good cooking utensil just because it got dirty once, especially if made of cast iron.
If you are queasy about the idea of "only" soap and water, you can also heat the Dutch oven. You can put it in an open flame and take it out before the seasoning is stripped, so at something like 250 to 300 Celsius. After that, if any miniature particles of poo have clung to it, they will have burned to nice clean charcoal. And the contamination level with bacteria will be lower than on your average kitchen utensils, and probably even lower than on the food you have cooked after a day or so in the fridge. So just heat it up and use it.