How to Prevent Resistant Heartworm in Dogs
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 strains.
Does this mean that you should stop using ivermectin to protect your dogs?
What are the best methods to prevent resistant heartworm?
1. 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 The researchers found out that the efficacy rate for ivermectin was about 95%, but in unpublished results it was 100% effective after three consecutive doses were given.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. 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. You can continue to give ivermectin for its effectiveness against dogs infected heartworms. 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, 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 should not count on ivermectin as your heartworm preventative. 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.
Is there one best way to prevent heartworm disease?
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 decision.
(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 dose.)
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.
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 available.
What is the best way to keep your dog free of heartworm disease?
More About Heartworm
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- Does Your Dog Really Need Heartworm Testing?
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- Safe Heartworm Treatment for Your Dog
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