Have had this a few times and usually I sponge it with a wet cloth or sponge with a little washing up liquid or laundry detergent. Let it dry completely. Then I spray it with M&S Carpet and Fabric Freshener (all natural and smells like lavender).
My biggest fear is that ds gets into bed with us and wees on the brand-new fluffy and expensive mattress topper!
I think if you turn it over it will be okay, but you should try to get the stains out if possible.