More Heartworm Questions

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".

The Question:
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.

My Answer:

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.

Comments

[Lynda]

Dr. Mobley, since you stated this:

With the dramatic increase in the number of dogs who get small numbers of heartworms despite taking monthly ivermectin(Heartgard 30, etc.)

Are you recommending something other than Heartgard for your patients? Have you seen an increase in the number of dogs infected while on Interceptor?

I remember you writing on your blog previously about dogs on preventative becoming infected and I asked my vet at my dog's yearly checkup and she said she hasn't heard of that happening in my area. Is this becoming more widespread across the country?

[Doc]

Hello, Linda,

This problem appears to be mainly in the Mississippi valley region, south of Jackson, Missouri. The farther south you go, the worse it is. It does not appear to be a problem in other mosquito infested areas, other coastal areas, lake areas, etc.

The reason for this is unclear, though several explanations have been proposed, most notably more tropical storms in the Gulf, leading to more flooding, leading to more mosquitoes, leading to more heartworms.

It is difficult to accept this explanation when you live in the Mississippi Delta on reclaimed swampland where rice is farmed. We have always had and continue to have a jillion mosquitoes.

As to Heartgard versus Interceptor, the percentage of failure rate is about the same with all three drugs that we use: ivermectin (Heartgard-30), milbemycin oxime (Interceptor and Sentinel), and selamectin (Revolution). Other drugs and products on the market are related compounds: avermectins.

If I found a preventive that was clearly better, you can bet I would be switching everyone to it. Unfortunately, this is not the case. No new drugs in the pipeline, either.

I'm attending another seminar on this on April 21st, and will post afterward.

Thanks for reading and writing.

[kathleen]

I'll check back to see what you learn at your seminar. I give my three dogs Heartguard. I certainly want to protect them from heartworms and would like to know the best way to do that.

[Cat Products]

Hi, Great site and a lot of useful information

[Twila]

Have 2 labs both tested positive to heartworms.
1-lab vet treated, new blood work still positive.
2- lab pulling more blood work and sending to different lab. Interceptor given first of month always.
Interceptor gives us year supplies due to dogs being treated and still contacting heartworms.
Are other people having this problem with dogs.

[Doc]

Check the series of six posts beginning with "Are Heartworms getting worse?"

http://www.yourpetsbestfriend.com/your_pets_best_friend/2009/05/are-heartworms-getting-worse-a-seminar-part-1-of-6.html

[Jason]

Dear Dr. Mobley,

I found your blog through google and was very impressed. I have a few brief questions and I'd greatly appreciate your thoughts.

Recently my wife and I found a Pit Bull mixed dog in our neighborhood. We tried like crazy to find the owners, but had no luck. We are not willing to see the dog put to sleep, and thus she is now ours. I took her to our vet yesterday and he told us she had heartworms (Spot Test) and that treatment is extremely likely to be successful (assuming class 1). I knew little about heartworms at the time, and thus asked just simple questions. I'll see him in two weeks, but after some research, I have many thoughts and few answers.

We are not wealthy by any means. She is young and has no cough nor breathing troubles after activity. Your article on the "slow kill" treatment was pretty scathing, but in this case does it at least seem reasonable? I was thinking weekly ivermectin with regular doxcycline for 6 months. My hope is that the adult heartworms' lives will shortened, the extended time span of treatment will only result in limited cardiac damage, and that embolism problems will be minimal.

Also, some vet sites mention that ivermectin based preventative products are better for dogs w/ heartworms than milbemycin products, due to the milbemycin's rapid killing of the larvae. Do you agree? Some also mention that ivermectin products are more effective on shortening adult heartworms lives. I'm confused and not sure what is true and what is just internet recklessness. Regardless of whether we treat with immiticide or try the "slow kill," it seems the dog needs to be on some preventive heartworm pill to make sure new larvae heartworms are forming.

I'm sure you're busy, but hopefully you'll have a chance to offer your assistance. Thank you

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That Heartworm disease can affect people o chemotherapy ?

[Doc]

Hello,

Heartworm disease does not generally affect people in any way, other than how they feel about their dogs.

The heartworm larvae enter people all the time via mosquito bites, but they rarely live and leave the spot where they enter. There are rare reports of finding an adult heartworm in a person's abdominal cavity, or some other weird location.

Sometimes the migrating microscopic baby heartworm will reach the lungs, and the body forms a little knot around it. This can show up on a chest X-ray, leading to a lung biopsy to find out whether you have TB or cancer or what.

Generally speaking heartworms do not affect people.

[dolores Hein]

what if I miss a pill for heartworms.

[Doc]

Hello, Dolores,

You should give the pill as soon as you realize that you missed one. Do not stop giving the pills in a situation like this.

If the dog has had mosquito exposure during this time, then it may possibly develop heartworms. You will not be able to detect this with a blood test until at least six months after the missed pill.

So, get back on schedule, keep giving the pills, and get a blood test in six months.

Thanks for reading and writing.

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Thanks for writing

[catherine]

To ensure that your dog is free from heartworm parasites, preventive medication should be given especially in the early months of the dog to avoid such problem on the dog’s health the same applies on the cats.Visit http://dogs-corner.blogspot.com/2009/09/dealing-with-your-dog-heartworm.html

[Roberto]

Hi Doc!
I see you told Dolores to if one if I misses a pill for heartworms..that they should give the pill as soon as you realize that you missed one...now I was wondering once you give the missed pill to your dog..do you give him the rest of the pills counting from when you actually gave him the missed pill or do you keep counting form when you were suppose to have given him that pill..hope this makes sense :) ?!

[Doc]

Hello, Roberto,

You could start counting 30 days or four weeks or one month from the time that you finally remember to give the pill.

For most people (like me), if I am late giving it on the first of the month, I probably will do worse trying to do it on the seventh the next month or the fifteen the next month. I'm afraid I would just be worse about forgetting.

I recommend setting a date that's easy to remember, like the first, or when you pay the rent, or payday or something.

The medicine is really safe. Thus, if you were two weeks late one month, giving it on the fifteenth instead of the first, it would be perfectly safe to give it on the first of the next month. Two weeks apart is no problem. Two DAYS apart would be safe, as far as that goes.

Thanks for reading and writing.

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[Mychol]

Hey Doc

I was giving my dogs their monthly heartworm medicine. I had already fed my 8 month old puppy, Panda, and when I was taking the heartworm out for my 2 1/2 old dog, Bear, Panda ate it! I was wondering if anything bad would happen to her after eating 2 heartworms.
Thanks for your time.

[Doc]

Hello, Mike,

The medicine is super safe. The active drug ingredient is present in very small amounts. While I wouldn't recommend it on a routine basis, she could probably eat a handful of monthly doses with no ill effects.

Not to worry.