1. Let's help your wife understand MDR1/ivermectin sensitivity
PLEASE share with your wife the WSU page on ivermectin-sensitivity in dogs with the MDR1 gene variant -- this is the university that's done the most research and the best authority in the US on the subject (they also do the genetic testing for it):
The bottom line is that the amount of ivermectin in HW meds is deliberately calibrated to be so low as to be safe for MDR1 dogs
. (This contrasts with the treatment for demodex, where vets give vastly higher doses (like 100x higher) of ivermectin -- that is NOT okay for these dogs.)
Some of us think that the HW prevention low dose may be a problem for the parts of the country with a nasty resistant strain of HW emerging, but it's kept the MDR1 dogs safe. So there's no reason for her to not use some kind of HW protection for this dog (there are also topical HW options that also do flea control -- like Revolution and Advantage Multi [called Advocate in Europe]).
2. It's not just about fleas -- your dog needs heartworm protection and tick protection too
Regardless what you do about fleas, Virginia is an area with heartworms, so you need to find a heartworm prevention product that your wife can be comfortable with (even if it's a topical one): https://www.heartwormsociety.org/pet...incidence-maps
You're also in a tick disease region. Tick diseases are very dangerous for dogs --deadly if untreated, and requiring weeks or even months of antibiotics or other meds to treat.
Frankly, I'd worry a lot more about having to take an MDR1 dog through HW treatment or tick disease treatment than I would about prevention. These are major, drug-heavy treatment protocols that are hard on even dogs that don't have MDR1.
Thus, as you're talking with your wife about a flea protection product that you can both live with, I think you might want to think more comprehensively about protecting your dog from parasites. This would be a really good conversation to have with a vet you trust -- I talk through pros and cons of various products with my vet team regularly, and their expertise in helping navigate the options is very helpful.
3. Vet-Prescribed Flea/Tick Meds are likely to be more effective than older OTC products in some areas
The OTC flea meds that you can buy at the store are older-generation products that the fleas have had a lot of years to become resistant to -- the old products seem to still work in some parts of the US, but not at all in other parts. You likely have discovered that you're in an area where the old products aren't working. There are newer, RX-only products available from your vet that will work far better for you.
An oral med like Nexgard or Simparica would be my first choice, but if they're not an option your wife can be comfortable with, then I would ask the vet for Vectra3D
(a topical, prescription-only product that protects against fleas and ticks). I have friends using Vectra3D in Louisiana, and I know several shelters using it to -- it works.
If your vet doesn't stock Vectra3D, they might be willing to let you fill the RX online through an accredited pet pharmacy like Chewy.com or 800PetMeds.com. Here's the manufacturer's website in case you want to read about it:
4. Follow-up with some testing
After getting a flea infestation under control, I would run a fecal test. Fleas transmit tapeworms, so it's not uncommon to need deworming after going through this.
Also, if your dog hasn't been on HW prevention since moving to VA, you'll likely have to run a HW test now to get a prescription, but then I would ask the vet run a HW test again in about 7 months. It takes 6-7 months for the baby heartworms to grow big enough to be detectable in the blood test, so dogs that haven't been on prevention have to be retested at that interval.
-- this WSU site linked sells a cheek swab test kit for MDR1 that you can order to do at home for $60 if you want to know for sure]