Are Heartworms getting worse? A Seminar: Part 5 of 6

Immiticide Treatment to Kill Adult Worms

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.

Comments

[Jessica]

My dog has been on preventive interceptor since I adopted him at age 2. When I took him to the vet when I adopted him he was negative. He now tested positive 2 years later even though he has been on interceptor since negative test. My vet is recommending a slow kill method using only heartguard and not using the arsenic shots. How do you feel about the slow method and not using the shots?

[Doc]

Hello, Jessica,

Problems with the heartworm treatment are related to the death of the worms, and their subsequent shift in position downstream to a smaller section of pulmonary artery. The difficulties are NOT due to the medication itself.

The problem with "slow kill" is that it can take as long as two years, when it works.

There is no way to predict when the worms will die, so you will not be as alert for the side-effects produced by dead worms as you would be if you had the Immiticide treatment.

With the arsenical injection, you know that the worms will be dying and shifting position in the next few days. You can be alert for signs of complications, plus you can keep the dog quiet and restrict activity that would make those complications worse.

You are not going to do that for one to two years.

This is certainly a judgment call and something you need to discuss further with your veterinarian.

Thanks for reading and writing.

[Doc]

Hello again,Jessica,

With the slow kill method you just don't have any idea when the worms will die, other than it will be sometime one or two years from now. Maybe they'll die a week apart, maybe a month apart, maybe they will all hold hands and drink the Kool-Aid together. {http://en.wikipedia.org/wiki/Jim_Jones#Deaths_in_Jonestown}

It is best to keep the dog's activity level as low as possible while the dead worms are being gradually eaten away by the microscopic white blood cells. This is a process that takes several weeks. Until the dead worms are gone, they are clogging the blood vessels. This causes back-pressure behind them, and a high-pressure jet downstream. Increasing the heart's output (like when you run) further increases both pressures. This makes it more likely that the blood vessel would break and bleed into the lungs.

You cannot keep a dog still for six weeks, much less two years. We just do the best we can - no Frisbee, no going to the country to run, just staying as laid-back as possible.

The reason that restricted activity is not recommended for the slow-kill is that it would be impossible, or highly impractical. Same reason you don't "need" seat-belts on school buses.

Certainly, when fewer worms hit the downstream vessels at any given time you are less likely to experience ill effects from the event. Thirty are worse than ten are worse than one. SO, if only one at a time dies, then even if the blood vessel ruptures, maybe it will be a minor hemorrhage.

We want to deal with smaller numbers of smaller worms. That's why we pre-treat with doxycycline to shrink and weaken the worms. That's why we treat in two stages (single Immiticide injection, wait one month, then two Immiticide injections, 24 hours apart): kill half now, and half later.

I do not believe that I can say that there is a hard and fast "right vs. wrong" answer. I treat lots of dogs for heartworms, but I don't treat YOUR dog, in YOUR part of the country.

Your veterinarian is taking responsibility for doing what he/she thinks is best for your dog. I am in no position to criticize that. I can offer general information, and information about what I usually do here. Each case is an individual, and is evaluated individually. There is no standard formula that I "ALWAYS" use.

I understand that you are confused in this area. Any time that you see fifteen different ways to do something, it seems that there must not be one "best way". When even the real experts (of which I am not one) have differences of opinion, it's a little hard to know what's best.

If your dog is young and strong and otherwise healthy, you probably have plenty of time for the slow-kill method.

Again, you and your veterinarian who actually sees your dog need to make the decision together.