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There are a couple of possible causes for these sores between the toes of your dog.
Dogs do develop allergies but they take time to develop, so since she is just visiting in the midwest, it is not terribly likely that she has developed an allergy to something here that she does not have contact with in Seattle. If she has been here for several months or have come here on previous trips, this it would be more likely.
Dogs can develop bacterial infections between their toes and based on your description, that sounds like it is the most likely explanation. If these are mild, you can try just cleaning the affected area twice daily using an antibacterial soap. Be sure to rinse the area well afterwards.
It is also not uncommon for dogs to get small pieces of plant material lodged within their feet - things like prickles from cockleburs or grass awns. If your dog has been running in the fields, this is another possible explanation for these lesions you are seeing. If this is occuring, these may need to be removed manually by your vet
There are a few less common diseases that can affect this area as well. Some of the immune diseases such as pemphigus or lupus can affect these tissues, but these usually cause more severe lesions than what you are describing. Certainly if these continue or worsen or fail to respond to treatment, biopsies can be taken to help determine if that is what is going on.
Since you say that these look like pimples, the bacterial infection is the most likely explanation for what you are seeing. Dogs often will require antibiotics to treat these, so if they do not go away by just cleaning the area, it would be a good idea to have her checked by a vet so they can prescribe the needed medication. You also need to prevent her from licking at the feet as this can make the problem worse.
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