Update on Doxycycline and Heartworm Disease
A safer and more effective alternative to to slow-kill method of heartworm treatment.
News item written by Mary Straus, published in the Whole Dog Journal, August 2009.
In August 2006, we reported on the finding that heartworms harbor a symbiotic organism called Wolbachia, and that treatment with doxycycline to destroy the Wolbachia could reduce the chance of adverse reactions during heartworm treatment.
According to studies published in late 2008, treatment with a combination of weekly ivermectin (Heartgard) and intermittent doxycycline helps rid the body of adult worms, reduces the effects of the worms, and stops the heartworms from being infectious to other dogs. The effects of the combined treatment are more pronounced than with either drug alone, making the treatment a more effective and safer alternative or adjunct to Immiticide.
“Preliminary observations suggest that administration of doxycycline and ivermectin for several months prior to (or without) melarsomine [Immiticide] will eliminate adult heartworms with less potential for severe thromboembolism than melarsomine alone,” one study concluded.
This study followed dogs treated with weekly ivermectin at standard monthly heartworm preventative doses (6 mcg/kg), and with doxycycline at 10 mg/kg daily for weeks 1-6, 10-11, 16-17, 22-25, and 28-33 (the study lasted 36 weeks). Antigen test scores gradually decreased, reflecting a reduction in the number of adult worms (antigen test scores did not change for dogs treated with either ivermectin or doxycycline alone).
After 36 weeks, adult worms had been reduced by 78.3 percent in dogs treated with both medications. In addition, all microfilariae were gone by week 9, meaning that the adult worms had ceased reproducing. Mosquitoes that fed on the dogs treated with doxycycline produced heartworm larvae that were not infective to other dogs.
An Italian study found that dogs treated with the same combination of doxycycline and ivermectin had less perivascular inflammation. Dogs who also received Immiticide showed no thrombi in their lungs (pulmonary thromboembolism, or blood clots in the lungs, are the major life-threatening adverse effect of worm death). A third study with the same protocols concluded, “Results indicate that the combination of these two drugs causes adult worm death.”
These studies clearly indicate that treatment with a combination of weekly ivermectin and daily doxycycline given intermittently will sterilize the heartworms, prevent the dog from being infectious to other dogs, speed up the death of the worms prior to (or in place of) Immiticide treatment, limit inflammation and damage caused by the worms' presence, and reduce the chance of serious adverse reaction from Immiticide treatment.
Weekly doses of the Heartgard should be safe for all but those dogs with the mutant MDR1 gene that affects collies and related breeds, making them more sensitive to ivermectin and a number of other drugs.
More information:
- Important New Information Regarding Heartworm Treatment and Doxycycline
- Heartworm and Wolbachia: therapeutic implications
- Wolbachia and its influence on the pathology and immunology of Dirofilaria immitis infection
- Combined ivermectin and doxycycline treatment has microfilaricidal and adulticidal activity against Dirofilaria immitis in experimentally infected dogs
- Testing for MDR1 gene, Washington State University Veterinary Clinical Pharmacology Lab, (509) 335-3745
- American Heartworm Society Current Canine Guidelines: Adjunct Therapy
If you have any questions or comments, please contact me. My name is Mary Straus and you can email me at either or
