Kittens peed in my bed

So kittens peed in my bed when i was at work, not sure how long its been there since i didnt really notice it untill i took of the cover sheats to wash them and saw all the yellow spots. It doesn't smell but having all those yellow spots looks very nasty if nothing else, best way to get rid of them? Doesn't seem to dissapear in normal washer. :X

Or should one just buy new covers?