Nana, if the dog has been started on ivermectin, that doesn't mean you have to continue that.
As you know
I am a recent convert, and quite a fervent one, to giving preventive. Nothing like a case of heartworm to cause me to find religion!
My dogs are getting Sentinel (milbemycin oxime), and I will switch them to Interceptor as soon as I fill the scrips I have for that. My vet clinic only carries Sentinel and Heartgard.
I am not sure I'm comfortable with the 45-day interval, so it's interesting you bring up Mary Strauss's view. I may wind up going 30-day intervals for all my dogs, but I doubt I will give it in the dead of a Chicago winter.
I did a fair amount of reading about the MDR1 sensitivity -- did you know that 75% of Collies now carry that gene mutation?! -- and also talked to a friend who is a vet tech of 20 years, and I learned that, as this article says, "Given the mechanism for toxcity, there is no reason to consider milbemycin oxime safer for dogs with the mdr1-1Δ mutation than ivermectin." This is because "So-called "ivermectin" sensitivity is actually sensitivity to a broad class of compounds due to a basic defect in the blood-brain barrier. Normal dogs are protected from acute and often fatal neurotoxicoses when these compounds are administered as pharmaceuticals (including ivermectin) by P-glycoprotein, an ATP-dependent drug transporter that moves a broad spectrum of substrates across several tissue borders throughout the body. The normal gene encoding for P-glycoprotein is MDR1.
http://www.awca.net/drug.htm
Basically, both preventives should be considered to have the same potential effects, one does not have a distinct advantage in that area, my reading told me.
However, that is not bad news necessarily, as the dose in heartworm preventive is now so much lower than it was 20 years ago -- when I absorbed my 'heartworm preventive is toxic!' message -- that a dog would have to consume an entire vet clinic of preventive tablets in order to poison itself. Given at the dose of monthly preventives, neither ivermectin nor milbemycin oxime has been shown to cause problems.
The same is not true of moxidectin, which is used in that injectable 6-month preventive, but you wouldn't give that anyway.
Once my dogs all gobbled down their preventive in May, when I started giving it this year (following Rowley's hw diagnosis in April) and nobody had any reaction at all, I relaxed and forgot about it. My vet tech friend assured me that sensitivity to the preventive would present itself immediately, in 99.9% of cases of reaction; she also told me the drug is out of their systems in 72 hours or less, so giving liver support for a few days is of course a good idea.
I love your new girl, Nana! How is she settling in with you, and you with her?
) and her spectacular MP-ODO story 
, but have a quick practical question before I see my vet on Wednesday. 
; there was something elsewhere (vin.com and some PDF from an old talk from the early 2000s, I think) about how a 12-month schedule also brings down the possibility of infection down to 0% (versus ±4%). Anyway! I'll read up on the heartwormsociety page—thank you! 
So I hope you and TBD settle in with each other and come to discover all kinds of nice things about one another. I did love MP's willingness to consume every food on offer, but that's kind of unusual, so you might not have come across that again in TBD.
