I adopted my dog from a shelter 2 years ago. She was estimated to be 1 year old at the time. She is a mix of Chihuahua and Terrier. She weighs approx. 11 pounds and is 12 inches tall. She tested positive for heartworms but I elected to adopt her anyway.
I had her treated with Immiticide. She was then put on Heartguard and am giving it to her monthly per my Vets instructions. After 2 years she is still testing positive for heartworms (adults) but has been negative for the babies for over a year. The vet has said that she has not previously seen a dog still testing positive for adult heartworms after two years.
The vet has discussed two choices: 1) Second round of treatment of Immiticide, or 2) Continue giving the Heartguard monthly and hope that the adult worms present will die. The vet was not clear on what was the better alternative.
I ask, “What would you do if it were your dog?" and the response was "I guess I would treat it with the Immiticide", but she wasn’t overly convincing.
Overall, Sadie is in good health. She is not experiencing any symptoms consistent with advanced heartworm disease. I am not certain I want to subject Sadie to this procedure unless it is the better alternative. I have read your web pages on this topic and appreciate your thoroughness. I would be interested in your opinion in this matter.
Generally speaking, the best person to advise you is the doctor who is seeing your dog.
That being said, there are two scenarios that come to mind.
First, we have to remember that the Heartgard (and other monthly preventives) work to kill the lifestage of the parasite that exists from mosquito bite up to about five or six weeks afterward.
Second, it takes around six months from the time of the mosquito bite for the adult worms to be developed in the heart. It is only at this stage that they can be detected with the blood test. This is also the stage when the Immiticide works.
SO, there is roughly a four-month window where the parasites have matured past the point where the monthly preventive can kill them, but they haven't reached the point where they can be detected with a blood test, or killed with Immiticide.
If your dog was treated with Immiticide in October, for instance, and started on Heartgard at that time, baby heartworms acquired in June, July, August and September will go ahead and mature, and be showing up in the next spring's blood test. They are too mature for the preventive, and not mature enough for the Immiticide to get them.
The Immiticide may have killed all the adult worms present at time of treatment, but it didn't keep the partially mature ones from growing up. Thus you have a positive blood test again, even though you were faithful with your preventive.
The other scenario is the dog where the worms just don't all die with the treatment. Some dogs (rare) will never get a totally clean blood test.
My bias would generally be to re-treat with Immiticide in early spring (April is not too late, and don't stop your preventive medicine). Any previous parasite exposure will then be in the life-cycle stage where they can be killed.
In the case of a dog who had already undergone Immiticide treatment, and started on monthly preventive right away, we would expect very few worms to be present, and we would expect a good result from the treatment.
I hope this is helpful to you.