Retired Old ■■■■:
And if you want to be really helpful you could leave them to soak in soapy water until you get home!
Just leave them sloshing about in soapy water for 500 miles or so. That should do the trick. 
On a (very slightly) serious note- did anyone make a “washing machine” thing for trips to uncivilised places? I imagine some sort of lidded/sealed tank with a couple of cricket balls in it. Something like that.