In re failure to clear dogs of worms with the Immiticide treatment, there are several considerations that are important, particularly in dogs that have not been receiving preventive medicines. I have written previously about problems with the treatment.
Immiticide does NOT kill immature adult worms, or “juvenile worms”. These are small worms, less than one inch long, in the “tissue phase” or the pulmonary circulation (arteries in the lungs). These are worms 60 to 120 days post infection. They are long past being susceptible to preventive medicines and the Immiticide doesn’t kill them either. You would need to treat the dog at least four months after the mosquito season (peak July and August) to be sure these juveniles would be old enough to be killed by the medicine. That would mean treating in January or February in dogs that have not been taking preventive medicine. [This is not as relevant to the dog that has been taking year-round preventive, but has developed a few heartworms in spite of that.]
In dogs who have not been taking year-round preventive, treating at the wrong time guarantees that juvenile heartworms will survive. The dog WILL have adult heartworms within a few months, despite the treatment.
Effectiveness of different treatment regimens
When we talk about percentages of worms killed, we are talking about the number of worms that were experimentally implanted in test subjects. We are not talking about the worms in a single dog. In these tests, some dogs were totally cleared of worms and some were not. In other words, if ten dogs were infected with ten worms each (100 worms), and ten worms survived, then we say 90% of worms were killed. This is true whether the ten surviving worms were in one dog, two dogs, or divided equally between the ten dogs (and they weren’t).
Two stage Heartworm treatment:
A single dose of Immiticide kills male worms. All females survive. Statistically, infections are 50/50 male/female. This is why you consider that you are killing “half the worms” with a single injection.
A treatment of two injections (24 hours apart) kills 90% of worms. This has been the “standard” treatment. This will totally clear some dogs (if they don’t have tough worms) and will leave a few worms present in others.
Giving the two doses of Immiticide more than 24 hours apart decreases its effectiveness. Giving the doses less than 24 hours apart increases the effectiveness, but also the toxicity of the drug.
Three injections (a single injection, followed four weeks later by the two injections 24 hours apart) kill 98% of worms. In the past, this has been used primarily for dogs where larger worm burdens were suspected. [The dog only had to deal with the breakup of half the dead worms at a time – less stress on his system.] This will totally clear most dogs, but some dogs will still have worms present.
Two injections followed two months later by two more injections kills an even higher percentage of worms, but you cannot ever guarantee 100% kill. This regimen is not in widespread use (but has been studied in the laboratory setting).
In the past, I have been treating dogs who are apparent "preventive failure cases" with two injections, 24 hours apart, based on the near certainty that they have very low worm burdens. I am currently re-evaluating my position here, and considering going to treating all patients with the two-stage, 3-injection regimen. While it should give me more "complete kill", it also extends the patient's exercise restriction by four weeks, and adds to the expense. This is not a clear-cut decision for these dogs.