Your veterinarian should contact manufacturers to report a possible product failure. They will have protocols for additional testing and treatment at their expense if it is a true product failure.
Borderline positive for heartworm is like being borderline pregnant. I suspect your dog is infected. Preventative failure is becoming more common even if you dose every 30 days. Not sure the manufacturer will pay for treatment anymore as heartworm larvae is becoming resistant to ivermectin. Many websites promoted extending the time interval between dosing to a month and a half and this contributes to drug resistance. Always dose every thirty days. There is also controversy about the correct reporting of ivermectin effectiveness in the first place.
