Dr. Mark is a veterinarian. He works mostly with dogs and exotic animals.

Your dog should stay heartworm free and able to do

Your dog should stay heartworm free and able to do everything!

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What Is Resistant Heartworm?

In several articles I have published here, I have recommended the purchase of
generic ivermectin (for use in cattle or sheep) to use for heartworm
prevention in dogs. Recent reports have shown that the Merial heartworm
preventative, Heartgard, is not 100% effective in all areas. Some resistant
strains of heartworm (MP3) have been reported in the southern US. One strain
was found in Georgia, another in the Mississippi valley.

A meeting of the American Heartworm Society and Companion Animal Parasite
Council in 2010 led to some new research by Auburn University´s College of
Veterinary Medicine. They tested four commercial heartworm preventatives and
found that only the Advantage Multi was 100% effective against the new

Does this mean that you should stop using ivermectin to protect your dogs?

Tips for Preventing Resistant Heartworm in Dogs

  1. You can give ivermectin as a heartworm preventative year round.
  2. You can give ivermectin at a higher dose.
  3. You can give ivermectin twice a month.
  4. You can continue to give ivermectin for its effectiveness against dogs infected heartworms.
  5. In some cases, you should not count on ivermectin as your heartworm preventative.

Heartworm disease can strike any dog at any age.

Heartworm disease can strike any dog at any age.

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1. Give Ivermectin as a Heartworm Preventative Year Round

You can give ivermectin as a heartworm preventative year round. The
researchers found out that the efficacy rate for ivermectin was about 95%, but
in unpublished results it was 100% effective after three consecutive doses
were given. Giving heartworm preventative year round is probably effective
enough to kill all of the microfilaria.

2. You Can Give Ivermectin at a Higher Dose

You can give ivermectin at a higher dose. I will revise the dosages in my
heartworm prevention article so that they will be equal to about two Heartgard
chewables. This dose is still low and quite safe. A dog being treated for
demodectic mange receives a higher dose for a long period of time. (You should
not use comfortis flea control, however. It is another type of insecticide and
it may affect your dog. Heartworm should be prevented chemically but you
should get rid of fleas without chemicals.)

3. You Can Give Ivermectin Twice a Month

Giving heartworm preventative every 15 days, instead of every month, is more
effective and will kill more of the microfilaria. If your dog becomes infected
with a resistant strain, this probably will not help, however.

4. Continue With Ivermectin for Its Effectiveness

It is almost 100% effective when given continuously for 31 months—the drug can
be used as a type of slow treatment; your dog is unlikely to die from having a
dead mass of worms die and lodge in his lungs. It’s a serious side-effect of
the rapid-kill method. Revolution may kill some adults, but less effectively,
and neither Interceptor nor Advantage are effective in killing adults.

5. In Some Cases You Shouldn’t Count on Ivermectin

If you live in the Mississippi Valley, from Missouri to Louisiana, current
recommendations are that you purchase Advantage Multi from your veterinarian.
This product is more expensive, and I cannot guarantee that it is worth the
additional expense.

Resistant heartworm disease can strike any dog.

Resistant heartworm disease can strike any dog.

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What Is the Best Way to Prevent Heartworm Disease in Dogs?

A three-year supply of ivermectin (in the sheep drench formula) given year-
round and at a higher dose is still less expensive than a six-month supply of
other commercial heartworm preventatives. If you want to continue having your
dog tested every year, and continue using the commercial heartworm prevention
year-round, there is nothing wrong with that—it is your money and your

(The only problem might be with the dewormer, pyrantel pamoate, which is
present in Heartgard plus. It can make some dogs vomit if given in too high a

If you are keeping your dog on the Merial product so that the company will pay
for your dog´s heartworm treatment, that is going to be little comfort to him
as he feels the long needle shoved into his back, feels the cold arsenic
injected deeply into his body, or is incarcerated in a cage or crate for weeks
after the treatment.

Scroll to Continue

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Watch Out for Mosquitos

The best way to protect your dog from heartworm infection is to make sure she
is never bitten by a mosquito. That may be impossible, but you can lessen the
risks by keeping her inside during the time when mosquitoes are heavy around
your house (around here it is early morning or dusk), putting mosquito screens
up on all your windows and doors, and using yellow bulbs on all of your
outside light fixtures.

I live in an area where several people die each month from zika and dengue so
our health department comes around each week to discover and eliminate any
sources of standing water—birdbaths, old tires, and bottles can all become
mosquito breeding grounds. Your dog will still be at risk, so it is a good
idea to keep him on a preventative, but the chances of her becoming infected
are very slight.

This situation may change. I will add any updates as new material becomes


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More About Heartworm

  • Buy Heartworm Prevention Cheap
    Heartworm disease is an internal parasite that can cause death in dogs and
    cats. This article gives dog owners a less expensive option for buying
    heartworm preventative.

  • Does Your Dog Really Need Heartworm Testing?
    This common test is not really necessary every year. Find out how to prevent
    the disease, how to avoid testing, and inexpensive alternatives to protect
    your dog.

  • Safe Herbal Heartworm Treatment
    Among the alternatives for heartworm treatment, herbal therapy is available.
    It may not be as good as slow kill treatment but it is a lot safer and less
    painful than the Immiticide injections. This article will discuss what is
    involved in the therapy

  • Safe Heartworm Treatment for Your Dog
    Safe heartworm treatment for your dog is possible but it is not always the
    best option. This article will give you details on how you can treat your dog
    safely if he contracts this disease.

This article is accurate and true to the best of the author’s knowledge. It
is not meant to substitute for diagnosis, prognosis, treatment, prescription,
or formal and individualized advice from a veterinary medical professional.
Animals exhibiting signs and symptoms of distress should be seen by a
veterinarian immediately.


Charlene Latimer on June 23, 2017:

Thank you for this valuable info.

Bob Bamberg on September 24, 2012:

I vote for piperazine and parantyl pamoate as the likely p-word dewormers in
Rachel’s barn.

Dr Mark (author) from The Atlantic Rain Forest, Brazil on September 24,

Rachel I would not even worry about it in Pennsylvania. The MP3 strain (the
resistant heartworm) was found one time in Georgia and never seen again. The
only time it has been seen consistently is in the Mississippi valley, from St.
Louis to Louisiana. The other heartworm preventatives you could buy are
expensive and will need a prescription. It may be a problem 10 or 15 years
from now, but at the moment your area is fine.

Rachel Koski Nielsen from Pennsylvania to Minnesota on September 24,

Hi DrMark, great hub as always. I’m concerned about the ivermectin-resistant
heartworm strains. The large animal vet we used to use here at the farm
recommended that we rotate dewormers for the horses, cattle and sheep every
three months. I can’t remember the active ingredients in the other 2 dewormers
(I can go out to the barn and look if you want, both started with a “P” and
were long words, lol) but I was wondering if there are other heartworm
preventatives besides ivermectin that would be safe for dogs?

Bob Bamberg on September 23, 2012:

I should set up a mosquito dunk kiosk on your beach! Bet I could make a few
bucks. 🙂

Dr Mark (author) from The Atlantic Rain Forest, Brazil on September 23,

I think the mosquito dunks are just overlooked. The government makes a lot of
recommendations that don’t help much (like making sure that you do not
overwater your house plants) but I would think they would think of everything,
since so many people die every month (from dengue).

We have the generic ivermectin available here too, and the prices have fallen
a lot in the last few years!

Bob Bamberg on September 23, 2012:

Re: the BT mosquito dunks not being recommended where you are… are they not
recommended as an unintentional oversight, or are they recommended against? I
can’t think of any reason why they should be prohibited, but if you guys have
one, I’d really be interested in learning about it.

I carried the Ivermectin Paste for the horse community, but never could sell
any of the drench. I think there was a price aversion, although I never
thought about bringing it in after the patent expired.

When that happened, Zimecterin went from $14.99 a tube down to $9.99, and then
all the other mfgrs started producing their versions of Ivermectin so now it’s
down to 6 or 7 bucks.

Dr Mark (author) from The Atlantic Rain Forest, Brazil on September 23,

Thanks for the input on the mosquito dunks. That isn’t something recommended
around here, although it should be since we have such a huge mosquito problem.

The only problem with the ivermectin paste is the strength, it is 1.87%, so a
lot more than the sheep drench product. Still, as you point out with the
“beagle guys”, there is a good margin of safety, so it is unlikely to cause

Thanks for your comments. I am glad always glad to read your interesting

Bob Bamberg on September 23, 2012:

Good hub, Doc. I echo your economic points about Ivermectin. At my feed and
grain store, we sold the paste for $7.99 a tube. The syringe is graduated for
a 1,000 lb animal, so dosing is tricky but I knew of several people,
especially the “beagle guys” who used it and I never heard of any problems. Of
course I also don’t know how effective their dosing was, either.

But that one tube provides a large number of dosages. Even if one couldn’t use
it up by its expiration date, they’d still be saving money.

Through you, may I suggest to Jaye and others who have a lot of standing
water, the use of mosquito dunks. There are various brands but they all work
the same way…BT (bacillus thurengiensis).

It biologically controls mosquito larvae in standing water, but is harmless to
all other life forms. You can even use it in bird baths and garden ponds
without harming the birds, fish and other aquatic life in garden ponds. You’ll
find the product at any feed and grain, nursery or garden supply outlets.

I believe it’s meaningful that alternative methods be offered from credible
sources, which is why I think your hubs are so important. Keep ’em coming!
Voted up, useful and interesting. Regards, Bob

Dr Mark (author) from The Atlantic Rain Forest, Brazil on September 22,

I think the people in Georgia and Alabama don’t have to worry much but you
definitely need to be concerned. The new strain might be rare but dogs in your
area definitely need the extra protection.

Does your vet want to give your dog the Proheart injection, knowing of her
other health issues? The Advantage Multi might be a better idea. (They both
use moxidectin.) The Proheart injection caused a lot of health problems, and
some dogs even died, so it was pulled from the market. (This was back around
2004, but I am sure you can still find plenty of info on the internet about
it. Be sure to read about this before making your decision.)

Jaye Denman from Deep South, USA on September 22, 2012:

I’m so glad you wrote about this. I read a report about a study by a
veterinary school, and the results showed only the active ingredient
moxidectin (found in Advantage Plus monthly and Proheart 6-month injection)
was truly reliable in the prevention of resistant heartworms. Ivermectin did
not protect against them. This really concerned me because I live in an area
that is heavily infested with mosquitoes. To make matters worse, there is a
city drainage ditch nearby, and the city water & works department does nothing
about standing water.

My dog gets her annual heartworm test next month and her first Proheart
injection afterward. No more Heartgard.