After my last post, one reader asked me if I would post the questions and answers. I replied (pretty much without thinking) that they are posted as comments on the original post. So how would you know where to look, gentle reader? I'm going to start posting the ones that have more general interest. I'm taking more time to write some of them than I do to write a regular blog entry. This one, for instance, popped up as a comment on "Problems with Heartworm Treatment".
I have enjoyed reading your thoughtful responses to so many questions. Here is mine asked with much respect.
What are your thoughts on the "slow kill" method? A rescue I foster for
recommends Doxy for a month (although I found a study that suggests 2 weeks
is better) then Ivermectin every 2 weeks for up to a year until a negative result [blood test free of heartworms]
is met. The reasons are primarily to make the treatment less harsh and of
course financial [less dollars].
My second question is what Heartworm treatment is recommended for a lethal white [gene]
Aussie with heartworms. I believe they are in the Collie family and cannot have ivermectin.
The doxycycline is intended to kill a microorganism called Wohlbachia that infests the heartworm itself. Many researchers believe that a great deal of the reaction we see to the breakup of the dead worms is related to the presence of this organism. The doxycyline pre-treatment is not a universally adopted practice, but is inexpensive and does no harm (unless the delay allows more heart and artery disease to develop in dogs who have clinical signs of disease, versus just a "positive blood test").
Even when the Wohlbachia is gone, you still have chunks of dead worm to deal with, and they WILL cause inflammation in the pulmonary arteries. The inflammation causes swelling of the blood vessel, which restricts its inner opening. This makes circulation in the lung tissue poor, and increases the pressure on the compromised artery, and increases the back-pressure the hear must pump against.
This brings us to your question about using long-term ivermectin to kill the worms, and whether this is somehow "less harsh". Dead worms are dead worms, no matter what kills them. When they die and drift downstream, it doesn't matter what drug killed them or how long it took. They will still lodge and plug things up and cause inflammation.
With the dramatic increase in the number of dogs who get small numbers of heartworms despite taking monthly ivermectin (Heartgard 30, etc.) I have serious doubts that taking ivermectin every two weeks is going to kill the heartworms. Previous studies documenting this noted that it took as long as two years for the worms to die with this treatment (with no treatment at all, the worms would die of old age in about four years). These studies were made before we began to see the apparent failure of the drug to prevent heartworms 100% (in the past 3 years). Since they are getting new worms while taking the ivermectin, it's hard for me to have confidence in the drug getting rid of the old ones.
Even if we accept that giving ivermectin twice monthly would eventually kill the heartworms (and I am not confident of this), we have to ask if this is really a good thing. When we give the standard treatment of the Immiticide injections, we know that the worms will die within the next few days. Thus we are alert for signs of complications. We also are now giving the dogs prednisone after their treatment in order to minimize the inflammation experienced when the dead worms move.
The bottom line is that we know when the worms will be dying and moving, so we can be on the alert and detect complications rapidly, and deal with them. We know that the dog's activity should be restricted while the dead worms are clogging up the works, and we can do this for the next five or six weeks.
If the worms were going to die at some unspecified time in the indefinite future, how in the dickens are you going to be on constant "red alert" for the next two years? Are you going to crate the dog for two years? Give him prednisone for two years? I submit that you will not be doing any of these things. Therefore, when the worms DO die, you are more likely to experience complications.
Since complications are related to how many dead worms are breaking up and hitting the smaller pulmonary arteries, it would make sense that the dog would do better if the worms died one at a time over a long period of time. There is NO evidence to say that this is what happens with the ivermectin (if indeed, anything is happening with the ivermectin). Even if you could prove that "in six months, the worms began to die one at a time, regular as clockwork, one-tenth of the worms per month for ten months" (and this is absurd), would you crate the dog for ten months, starting six months from now?
The sad fact of the business is that the Immiticide treatment is the only thing that we know is effective. We can reduce the risk by giving a half-treatment, and then waiting a month to give the full treatment. This also makes it more likely that ALL of the worms will be killed. (Unfortunately, there is no such thing as 100%, "always" or "never" when we're talking about this).
This (Immiticide treatment) would be the appropriate treatment for the Aussie. While they cannot tolerate high doses of ivermectin, they tolerate the preventive dose in Heartgard 30. You could certainly use Interceptor as your preventive instead, but it's not going to be effective in ridding the dog of adult heartworms.
It is certainly cheaper to give Ivermectin twice monthly for years than to do the Immiticide treatment. On the other hand, giving nothing is even cheaper. Doesn't work, but it's cheap.
Sorry that I don't have a quick fix for you.
Thanks for reading and writing,
Everett Mobley, D.V.M.