Its been a long time since we have had a dog with heartworm disease, and so I have fallen behind on research. However, if I recall, the average lifespan of an adult heartworm is about 5 yrs. The slow kill method kills the microfilaria and keeps the dog free of “new young worms’ but it does nothing to kill the adults. Without Immitricide, the present adult worms will continue to cause damage to the heart and circulatory system until the natural expiration of its life cycle. I would completely expect the dog to show positive until that last worm is dead on an antigen test.
By that same token, I would expect any of the monthly preventives to accomplish the same thing, over time, and of course with varying degrees of speed and success depending on the medication choice used.
Old treatment was “arsenic” based and is no longer considered the gold standard in heartworm treatment. The caparsolate used was nasty stuff. These days its Immitricide and is safer than the old school arsenic. For me and my dogs, unless there is a medical reason known ahead of time that would contradict the Immitricide treatment, it is what I have and would continue to go with. The thought of continuing damage for 5 years is not something that I personally could do.
