Is it possible for rats to get mites from aspen wood shavings ? How can I tell ?

I got kiln-dried aspen shavings from pet smart and I didn't know you had to freeze them first and i used them. Someone else who has snakes told me he always uses them and his snakes don't have mites. But I have no idea what to look for. They scratch themselves with their back legs like dogs do but then they lick the foot , Is this normal ? Is there a specific amount they should be doing this ? How can I tell if they are just cleaning themselves or scratching themselves ? What signs should I look for ?