4 Dogs Excessive Itching!?

My two shiba inus, my friends dog, and my moms dog, who have all been together for over a week have all just started itching. All of them have been treated for fleas and have had flea baths, no signs of fleas, is there anything else that it may be? The vet guessed Allergies, but is it common for all 4 of them to have it?