If Turbo was tested within 6 months of getting him and this is the first time since then that he was tested, I would strongly suspect that he already had heartworms but was not far enough along to test positive. Many, many times a year, we got dogs in that had come from the shelter and had tested negative, but 8 months later they turned out to be positive after all.
I corresponded with someone who claimed to work at the plant where Heartgard was made. They said that the ivermectin and the meat paste carrier are mixed in a vat then pressed into molds of the appropriate size. That tells me that other than not giving enough or allowing air to spoil the meds, splitting should not be a problem. Example, if I have a 35 lb dog and a 15 lb dog, I should be able to split a chew for a 60 lb dog into thirds and give one dog 1/3 and the other dog 2/3 and it should be fine as long as I can split them accurately. I should not save some for the next month though, because the drugs could oxidize. The vet and the company make more if I have to buy a pack of the appropriate size for each and since it is a prescription drug, the laws don’t allow sharing anyways, but trimming off excess should not make the whole dose ineffective. My dogs weigh 42 lbs and usually I have to buy drugs for 60 lbs. Trimming off 1/4 should still leave enough drug for a 45 lb dog, but if it is a drug that comes in sealed pods/blister packs, I assume that it should be kept away from oxygen until use.
