Heartworm Treatment Review

I've certainly talked a lot about heartworms in the past, and there is a ton of information out there on the web, and here, for that matter - my long seminar, problems with treatment, and more questions.

Still, a reader emails us with questions about the dog she adopted. 

The dog tested negative for heartworms at the shelter when acquired last year, but shows up positive (positive = bad) this year at check-up time.  She gave lots of background and had lots of questions, but it mostly boiled down to this one question:

"Any suggestions to maximize the chance we have a happy outcome?"  And here's my answer.

Under the circumstances you described, with a negative test last year, and a positive test this year, one would expect the dog to have only a small burden of adult heartworms.  This is not guaranteed, and would depend on the level of mosquito exposure and the dog's individual resistance to the parasites.

It is certainly important to continue giving your heartworm preventive medicine regularly.  

If you want to do "everything", then you could certainly ask for the doxycycline.  Doxycycline inhibits Wohlbachia, a microbe that is beneficial to the heartworm.

Giving this medicine for 4 weeks prior to the Immiticide injections (to kill the heartworms) causes the worms to become weaker (thus more susceptible to being killed by the medicine) and actually physically smaller (so less junk to clog up the blood vessels, and less for the white blood cells to have to clear out).

Treating the adult heartworms in two stages is considered to be safer than killing them all at once.  If you have only a few worms, this would not be a big benefit.  On the other hand, if you have a lot of worms, then killing half now and half later would be easier on the dog when the dead worms go downstream and lodge.

The two-stage treatment consists of giving only a single injection of Immiticide (which will kill the weaker worms), then four more weeks of doxycycline.  When the body has gotten rid of the first batch, one gives the two injections, 24 hours apart, and this kills the rest of the worms.

Two injections given 24 hours apart will kill all of the worms in most dogs, but not all.  This is true of the 3-injection, 2-stage protocol also.

Giving the corticosteroids (I use prednisone) immediately following the Immiticide is not a universal practice.  However, since I heard Dr. Tom Nelson (past president of the American Heartworm Society) recommend it, I have thought that it makes sense.  You know that inflammation will occur, so why wait until it gets bad enough to make the dog sick?  (Which is what I did "forever" until three years ago.)

Now we give a strong anti-inflammatory dose of prednisone daily for the first seven days, then half that once daily for 7 days, then continue that lower dose once every 48 hours for 14 days.  This has resulted in fewer night calls, because fewer dogs develop complications (that we can see).  In fact very few dogs at all have developed outward complications.

Again, that is not "standard" across the profession, but I really think it is helping our patients.

Unfortunately, despite our best efforts,  there are no guarantees.  No matter what we do, sometimes a dead worm will lodge in such a way that a large vessel ruptures and the bleeding in the lungs is fatal.  This is very rare, but it does happen.

Keeping the dogs vigorous physical activity at a minimum is an important part of the aftercare.  Changes in your dog's attitude can reflect a problem coming on, so let your veterinarian know sooner, rather than later.  Get the dog checked BEFORE it becomes an emergency.

Most complications are minor and treatable if they do occur. 

Six weeks after the final injection of Immiticide, we recheck the patient (if not before), and treat for microfilariae (the baby heartworms) if there were any present.  At this point, we should be "out of the woods".  It is very rare for complications to occur after this time.  Four months following treatment, a blood test is done to be sure that all the adult heartworms are gone.

Leaving the dog untreated is no solution, as the worms can cause a similar problem even if you have not yet treated the dog with Immiticide to kill them. 

We treated a dog this week who was known to be heartworm positive.  They had put postponed treating her.  Sunday evening she had severe pulmonary hemorrhage, and coughed up a LOT of blood.  She's on doxycycline and prednisone and cage rest now.  I hope we will be successful in getting her through the whole process, but we've got a long  haul ahead of us.

Comments

[Kellie Olinsky]

I have a 12-15 mouth old Bullmastiff that I just rescued with B2-Class 3 heartworm. I do trust my vet. But I was just confussed because I was told to give him interceptor 3 days after the first tx with immiticide it was one dose and then in 35 days the next 2 injections will be given, He is on prednisone.I am suppose to give the interceptor on Fri. 9-17-2010 Just scared because I have always been told that you do not give interceptor to dogs with heartworm.

[Doc]

Hello, Kellie,

Sorry to be so late with a reply. I was out of town for several days.

The first heartworm preventive medicine was diethylcarbamazine, also known as D.E.C. This preventive had to be given daily. If given to a dog with heartworms, there was about a one in three chance that the dog would die - Russian Roulette with two bullets.

The monthly preventives have carried warnings not to give to dogs unless they are tested negative for heartworms. Now that these drugs have been around for quite a while, we find that the risk is not as high as previously thought.

Selamectin (Revolution) is actually FDA approved as safe to be given to dogs that already have heartworms.

The big boys in heartworm research tell us that ivermectin (Heartgard) is
safe to use in heartworm-infected dogs, and actually recommend it when starting a dog's treatment regimen. Several have told me that they don't think that milbemycin (Interceptor) is as safe.

That being said, I have had numerous patients who were taking Interceptor and got heartworms anyhow (missed dose or whatever) continue taking the medicine through their heartworm treatment with no trouble.

It sounds like your veterinarian has had a similar experience, and isn't worried about it. While the "authorities" recommend a different medicine, the risk with the Interceptor appears to be low. In my own practice, if the dog has not been on a heartworm preventive, we are starting them with Heartgard in cases like this.

If you have misgivings, it is better to discuss them with your veterinarian directly.

Good luck.