Strategies for Managing Heartworm Strains That Resist Standard Prevention

Understanding Heartworm Resistance

In past writings, I have suggested using generic ivermectin, originally formulated for cattle or sheep, as a heartworm preventative for dogs. However, evidence indicates that Heartgard, a preventative made by Merial, does not achieve complete effectiveness in every region. Resistant heartworm strains, known as MP3, have been identified in the southern United States, with one strain in Georgia and another in the Mississippi Valley.

Following a 2010 joint meeting of the American Heartworm Society and the Companion Animal Parasite Council, researchers at Auburn University's College of Veterinary Medicine conducted a study. They evaluated four commercial heartworm preventatives and discovered that only Advantage Multi provided full protection against the emerging strains.

Given this, should you abandon ivermectin for your dog's protection?

Approaches to Prevent Resistant Heartworm in Dogs

1. Administer ivermectin as a heartworm preventative throughout the year.
2. Use a higher dosage of ivermectin.
3. Give ivermectin on a twice-monthly schedule.
4. Keep using ivermectin for its ability to combat existing heartworm infections.
5. In certain situations, ivermectin should not be your primary heartworm preventative.

Heartworm disease can strike any dog at any age.

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1. Administer Ivermectin Year-Round

Providing ivermectin as a heartworm preventative all year is an option. Research indicated an efficacy rate of approximately 95%, but unpublished data showed complete effectiveness after three consecutive doses. Year-round administration likely eliminates all microfilaria.

2. Consider a Higher Ivermectin Dose

Increasing the ivermectin dose is possible. I will adjust the dosages in my heartworm prevention article to match roughly two Heartgard chewables. This amount remains low and safe. Dogs receiving treatment for demodectic mange get a higher dose over an extended period. (Avoid using Comfortis flea control, as it is a different insecticide that may harm your dog. Chemical prevention should target heartworm, while flea elimination should be done without chemicals.)

3. Give Ivermectin Every Two Weeks

Administering the preventative every 15 days rather than monthly enhances effectiveness and kills more microfilaria. However, if your dog contracts a resistant strain, this approach likely will not suffice.

4. Keep Using Ivermectin for Its Ongoing Effectiveness

Ivermectin is nearly 100% effective when dogs already have heartworm and it is given continuously for 31 months—it acts as a slow treatment; your dog is unlikely to die from a dead mass of worms lodging in his lungs. (Sudden worm death is a serious side-effect of the rapid-kill method.)

5. In Some Cases, Do Not Rely on Ivermectin

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

Other alternative heartworm preventatives, such as milbemycin, may also work against ivermectin-resistant infections.

Resistant heartworm disease can strike any dog.

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Optimal Strategies for Preventing Heartworm Disease in Dogs

A three-year supply of ivermectin (in sheep drench formula) given year-round at a higher dose remains cheaper than a six-month supply of other commercial preventatives. If you prefer annual testing and year-round commercial prevention, that is a valid choice—it is your money and your decision.

(The only issue might be with the dewormer pyrantel pamoate in Heartgard Plus, which can cause vomiting in some dogs if the dose is too high.)

If you keep your dog on the Merial product so the company will cover heartworm treatment, that offers little comfort as he endures a long needle in his back, cold arsenic injected deeply, or weeks in a cage after treatment.

Guard Against Mosquitoes

The best protection is to prevent mosquito bites entirely. That may be impossible, but you can reduce risks by keeping her indoors when mosquitoes are heavy (early morning or dusk in my area), installing screens on windows and doors, and using yellow bulbs for outdoor lights.

I live where several people die monthly from Zika and dengue, so our health department weekly eliminates standing water sources—birdbaths, old tires, and bottles can breed mosquitoes. Your dog remains at risk, so keeping him on a preventative is wise, but infection chances are very slight.

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

Further Reading on 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.