[Ellie May]

The first rule of vet med is do no harm. Prescribing a drug before without the proper testing would have them going against this.

I understand what you're saying, but the test is important, things do go legal (and yes, vets are constantly concerned about it), and most are going to feel a certain responsibility to do things correctly. Heartgard may not kill a HW positive dog, but the disease can still progress. The owner can sign a waiver (and some vets don't think testing is necessary at all), but vets do tend to want to know what is actually going on with their patients.

And doxy is becoming very popular before treatment to kill the wolbachia. They wouldn't know to use this without testing.