In The News: Heartworm drug resistance

It’s Real!


Leading authorities in pet parasites now concur that resistance to the class of drugs used in heartworm preventive drugs is real.

“We now have proof there is resistance to the macrocyclic lactone (ML) class,” said Dr. Byron Blagburn, an Auburn University parasitologist who did much of the research on strains of heartworm found in the Mississippi Delta, where loss of efficacy to preventive drugs was first suspected. “It’s accepted by the entire industry now,” Blagburn said.

The question of whether drug-resistant strains of heartworm are emerging in the United States has been studied for years. Until now, however, the experts have been careful to avoid using the so-called R word, “resistant,” to describe the strains that appear NOT to respond as expected to macrocyclic lactones drugs.

The macrocyclic lactone class of drugs in question comprises avermectin drugs (ivermectin and selamectin) and milbemycins (milbemycin oxime and moxidectin).

Blagburn credits the evolution in understanding of heartworm susceptibility and identification of the resistant strains to “an inter-collegiate, inter-laboratory, collaborative process.”

The consensus came from a recent update by the Companion Animal Parasite Council (CAPC) of its heartworm prevention guidelines. The new text on the CAPC website states: “Recent work has shown that there are isolates of heartworms that are capable of developing to adults in dogs receiving routine prophylaxis with any of the available macrocyclic lactones.”

The findings are saying that: “a dog faithfully given heartworm-prevention medication may still become infested with the parasitic worm Dirofilaria immitis, which inhabits the arteries of the lungs and sometimes a portion of the heart, causing serious and potentially fatal disease.

Macroyclic lactone drugs are found in all commercial heartworm preventives, including:

Heartgard (ivermectin)
Tri-Heart Plus (ivermectin)
Sentinel (milbemycin oxime)
Revolution (selamectin)
Advantage Multi (moxidectin)
ProHeart6 (moxidectin)
Trifexis (milbemycin oxime).

Heartgard and TriHeart Plus


  • Heartgard and TriHeartPlus both contain the active medication ivermectin. Adverse reactions include ataxia (coordination problems), staggering walk, diarrhea, vomiting, lethargy, depression, anorexia, mydriasis (extended time with dilated pupils), hypersalivation and convulsions. As noted in the prescribing information for both medications, there are specific breeds (like Australian Shepherds and Collies) which are prone to increased sensitivity.


Interceptor and Sentinel

  • Interceptor contains the active ingredient milbemycin oxime. Users reported adverse reactions like vomiting, anorexia, lethargy, depression, weakness, ataxia, hypersalivation and convulsions
  • Senitinel contains a combination of milbemycin oxime and lufenuron. Adverse reactions were reported like pruritus (severe itching), (hives), anorexia, diarrhea, vomiting, depression, lethargy, hypersalivation, convulsions, ataxia and weakness.

REVOLUTION and Proheart6

  • REVOLUTION® (selamectin), Topical Parasiticide For Dogs and Cats: pre-approval reactions of vomiting, loose stool or diarrhea with or without blood, anorexia, lethargy, salivation, tachypnea, and muscle tremors. Post-approval experience included the above plus pruritis, urticaria, erythema, ataxia, fever, and rare reports of death and seizures in dogs.
  • Proheart 6 :  severe allergic reactions (anaphylaxis): facial swelling, itching, difficulty breathing, collapse;  lethargy (sluggishness); not eating or losing interest in food; any change in activity level; seizures; vomiting and/or diarrhea (with and without blood); weight loss; pale gums, increased thirst or urination, weakness, bleeding, bruising; rare instances of death. This product was voluntarily withdrawn from the market in 2004 because of deaths but has been reintroduced with a new name.

So,  now I ask you this question:   Why are you still using these TOXIC pesticides to prevent or kill heartworms and other parasites?

The fact is that none of these pesticides (yes I said pesticides) prevent your dog from being bitten by a mosquito carrying the parasites in the first place.   The chemicals are supposed to kill the worm in its early stages of development so it can not migrate to the heart and there grow and multiply.  Now it has been found that these toxic chemicals are no longer killing the microfilaria – they are become immune to the very drug designed to kill them.

Are you aware that there are safe and effective ways to protect your pets from heartworms?  And wonderful thing – the heartworms are not able to build up a resistance to these natural remedies!

geraniumEucalyptusI use Young Living Therapeutic grade essential oils to repel/deter mosquitoes.  Read more HERE


Want more information on natural heartworm prevention and treatment?  Click Here

For more information on medical/therapeutic grade essential oils for you and your pets Click Here

For more on natural flea and tick prevention and treatment, Click Here

Dr. Jeannie Thomason



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