They sound like mites and they can easily be blown in on the wind or brought in in food / bedding / shavings etc. They are a devil to get rid of and you need to get a house spray and spray round the cages and in the house etc. They can bite humans too - well certain types can and give you nasty rashes. Some breeders here have found the Johnson's spray to be good and constant treatment is needed. You can spray the bedding with it and mix around before you put the hammies back.
Not long till Friday now and I hope the vet sorts them out.
This is the spray
Pest Control / JOHNSONS ANTI MITE SPRAY 100ML - Petstuffgalore
Ivermectin drops can also be used
http://www.animalmedicationdirect.co...6-pr-5735.html