Heartworm treatment while possibly having Heartworm

Hi All, I had a Border Collie Pure Bread Male he was 14 years and 8 months old, He was fine on the week end, he had normal issues that a Border Collie of that age would have but still seemed very happy with life, he was my shadow, he was suffering some incontinence issues, on Wednesday I noticed he had peed again on the floor but this time while he slept. he moved to another spot and did the same and then when I got up to start work, i noticed he had peed on the leather lounge where he was laying. he got up like normal and stood at the door, peed again and i opened it to let him out. (he had issues peeing like this before so didn't come across as an urgent issue.) He went out as normal and I went about my normal day where I prepare for work. normally he barks at the door to let me know he wants to come in (however this has not really happened as much of late because I put a electronic fob on his collar to open the dog door and he was starting to figure out how to use it) anyway my partner asked where he was at about 3pm and i said i was not sure so i went out looking for him. I found him laying there in thick slimy yellow stuff so made an urgent call to the vet took him there immediately He died within the hour. I checked the security footage today (that was really hard to do) and he sat down there and had the issue only about 20 min before i went to check on him. before that he was wandering around and following my other border collie around who likes the outdoors. In any case the vet said when i went there that he doesn't look like he will make it and they suspect he has internal bleeding from a cancer or something that has ruptured due to his gums being so white when he presented. Now that has had me thinking. he was normally on comfortis plus for his flee and heart worm tablets however the last couple of batches i have not received and each time i looked they where out of stock, so its been about 5 months that has lapsed and when i went to try to get it again this week it was unavailable from my usual place i buy it from so i switched to nextguard. My question I have is, could i have caused his death? my fear is that he was infected with a heart worm and by putting him on this treatment could it have killed the heart worm causing him internal bleeding? If it was how long would it have taken for that to have happened? I gave him the treatment on Saturday morning and he died on Wednesday afternoon. He was tested about 12 months ago for heart worm and found to be in the clear at that point.

Hello - and I'm so sorry to hear that you lost your dog.  The stages of grief do involve playing events back in your head and even torturing yourself with them; 'bargaining' that 'if only I had done things differently it would all be different,' or looking for sticks to beat yourself with.  Please be aware that your mind can be extremely cruel at times like these - procrastinating by telling ourselves horrid stories can prevent us from actually having to face the loss - so paradoxically, it can all be down to your brain trying to protect itself. I'm not sure what the orange stuff was or where it had come from in this instance, but I assume from the context that it was jaundiced urine (your vet will comment on that).  Jaundice can occur when the liver is overloaded e.g. in liver failure, or with internal bleeding, where too many red blood cells are getting broken down to be fully processed by the liver.  Internal bleeding would fit with white gums.  Causes of red cell breakdown then, include trauma, cancer, autoimmune disease (e.g. haemolytic anaemia - sometimes associated with collies), heartworm, toxic plants and Babesia (which is more prevalent in some parts of the world than others, specifically the Northeast and Upper Midwest America, especially in coastal area).  To answer your questions about heartworm, it is rare in dogs under a year old because of the time it takes the parasite to spread in the body and mature.  You tell me that there has only been a five month lapse in treatment.  Now, I know nothing about the type of treatment that you were giving or what life-cycle stage it was targeting.  But it sounds likely that there may have been too small a gap there. However, please do check with your vet.