It's really interesting that you mention that Minnie has a plastic allergy - it further supports my suspicion that what she is dealing with is Flea Allergy Dermatitis (FAD). When an animal has one allergy, it often means she will be prone to other allergies, as it indicates she has a hyper-alert immune system.
Essentially a FAD means that Minnie is sensitive or allergic to flea bites. It's kind of the same as how some people are allergic to bee stings. For most of us, a bee sting hurts a bit but isn't really a big deal. For others, however, a single bee sting can make their whole arm swell up or even cause an anaphylactic reaction that can be life -threatening.
In the same way, animals who have FAD get insanely itchy over a single flea bite. In this situation, you may have only a few fleas in the house, and may not even see any on the pets. However, if a single flea hops on, takes a bite and then hops off, Minnie could be insanely itchy for days!
The reason that I am so suspicious of FAD is because of the *pattern* of hair loss and the tiny scabs. It is just so typical of FAD. It is by far the most common thing causing this pattern.
I would recommend getting a flea comb from the pet store or even your local Walgreen's ( where they are sold as nit combs for parents to check their kids for lice ). Comb each animal down their back and under the chin. Look in the comb for black specks, or even live fleas.
If you get black specks out when you comb, put them on a white tissue or paper towel and add a drop of water. If they stain the tissue rust coloured it confirms that the specks are "flea dirt" which is a nice way of saying flea poo. Fleas eat blood, so their poop is digested blood and that is why it stains the tissue.
It is quite difficult to control a flea problem since for every ONE flea on your pet, there can be ONE THOUSAND fleas in the home! I strongly recommend using either Advantage or Revolution on the pets (available in Canada through your vet, but I believe in the USA you can get it at pet stores or even on-line) and then using a product to treat your home.
For more information about flea allergy dermatitis, you can check out these links:
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