The smell comes from the excrement of, and the dead and dying critters that grew in it.
An oxidizer such as bleach 'burns' them up.
Start off with a small amount per load. If it still smells, use more the next time.
'Colorfast bleaches' work the same way, but with a peroxide and some 'laundry boosters' instead of chlorine. They are less aggressive and may work. Medicine cabinet style hydrogen peroxide will work too.
Dilute any of them before using on clothing. Concentration is entirely experimental. Contact time is as effective as higher concentrations.
Consider using a series of PLASTIC buckets with your cleaner of choice in it (Do not mix chlorine with any others), moving clothes progressively from dirtier to cleaner, then finally to the washer. Use the first bucket until nothing happens, then the second becomes the first.
Don't mix whites with colors, and too high a chlorine concentration can yellow instead of whiten.
Good luck. If nothing else, think of all the new shop and yard clothes you have!