Merial Runs Out of Immiticide for Heartworm Treatment

Here’s what owners of heartworm-positive dogs can do until the only approved heartworm treatment is available again.


The supply of Immiticide (melarsomine dihydrochloride), the only drug approved to treat heartworm infections in dogs, has been low since December 2009, but now it’s completely gone. Merial confirmed that it was officially out of the drug on August 9, 2011. The current situation is due to a new and separate manufacturing challenge related to technical issues with the company who provides the finished product, according to a Merial spokesperson. Merial said the company is working hard to make the drug available again, but cannot speculate when that might happen.


With Immiticide unavailable, the American Heartworm Society (AHS) has issued guidelines for treating heartworm-positive dogs to try to mitigate the damage that heartworms cause while in the body and the danger they present when they die. Briefly, here is what the AHS advises:

1) Verify all positive antigen tests with a second antigen test from a different manufacturer.

2) Give monthly heartworm preventive medication to prevent further infection. If the dog tests positive for microfilariae, pretreat with corticosteroids prior to giving the first dose of heartworm preventive medication and keep the dog under veterinary observation for at least 8 hours afterwards, due to the risk of anaphylactic shock from the rapid die-off of the microfilariae.

3) Treat dogs with doxycycline at the rate of 10 milligrams per kilogram of body weight twice a day for four weeks (give half as much if a dog cannot tolerate the higher dose). Repeat this dose quarterly (one month on, two months off) for as long as the dog is infected with heartworms. This will reduce the possibility of the infection being passed to other dogs through mosquitoes, shorten the lifespan of the adult worms, and lessen the chance of adverse side effects from worm death.

4) Restrict all activity and limit all exercise, as the danger from adult heartworms increases with physical activity.

5) Dogs with symptoms from the heartworms should receive medical treatment to alleviate respiratory distress. Surgical removal of the worms should be considered if cardiac function is affected.

6) When Immiticide becomes available, proceed with treatment if the dog still tests positive for heartworms.

I’d like to offer a few additional suggestions, based on what I’ve learned about this disease over the years.

While the AHS does not suggest any particular heartworm preventive medication, I recommend using Heartgard (ivermectin) or generic equivalent. Of the four drugs currently used to prevent heartworms, ivermectin has the strongest effect against adult worms. It also has a weaker effect against microfilariae, and so is unlikely to cause an adverse reaction, making pretreatment and observation less critical.

I also suggest giving weekly heartworm-preventive doses of ivermectin to most infected dogs. Be sure to use Heartgard, not Heartgard Plus or any other combination product, when giving weekly. Studies have shown that weekly ivermectin used along with pulsed doxycycline can reduce the number of adult heartworms by more than 78 percent over 36 weeks.

Do not combine Comfortis (spinosad), a newer flea-control product, with weekly doses of ivermectin, as it increases the risks of neurological side effects. Ivermectin should not be given more often than monthly to dogs with the MDR1 gene mutation making them more susceptible to certain drugs. Commonly affected breeds include the Australian Shepherd (standard and mini), Collie, Long-Haired Whippet, and Silken Windhound. See for a list of affected breeds, and an inexpensive test to identify affected dogs.

As we discussed in “Update on Doxycycline and Heartworm Disease” (WDJ August 2009) and “Shortage of Immiticide for Heartworm Treatment” (April 2010), doxycycline is given to kill wolbachia, a symbiotic parasite that lives within heartworms. Destroying wolbachia weakens heartworms, makes them unable to reproduce viable offspring, reduces their adverse effects on the body, and decreases the likelihood of adverse reaction from their death. Doxycycline must be pulsed (given periodically) to keep wolbachia from repopulating. Studies that combined doxycycline with ivermectin for 36 weeks gave doxycycline during weeks 1 to 6, 10 to 11, 16 to 17, 22 to 25, and 28 to 33, but the schedule suggested by the AHS may be equally effective. Giving doxycycline with food can reduce gastric upset.

If Immiticide becomes available while your dog still tests positive for heartworms, you’ll have to decide whether treatment is warranted. Remember that the worms are doing damage as long as they are in the body, and danger of pulmonary embolism from their death also remains as long as any worms are present. Dogs showing symptoms of heartworm infection, those with heavy infections, and those who have been infected for a long time are most likely to benefit from Immiticide treatment, as are very active dogs, since the need for activity restriction would be shorter. Young heartworms are killed more quickly by the combination of ivermectin and doxycycline, so if the infection was caught early, later treatment with Immiticide is less likely to be needed. Keep in mind that dogs may test positive for heartworms for up to six months following the death of all worms.

If Immiticide treatment is not done, continue to give doxycycline quarterly until your dog tests negative for heartworms. Give ivermectin weekly to monthly while your dog remains infected.

For those considering natural heartworm treatment, remember that natural does not necessarily mean either safe or effective. These products are untested and usually contain toxic ingredients, such as wormwood (Artemisia absinthinium) and black walnut (Juglans nigra). The biggest danger of heartworm treatment is from the death of the worms, which can cause pulmonary embolism no matter how the worms die. There’s no reason to believe that alternative treatments are safer than conventional therapy, particularly if you do not give doxycycline as well.

– Mary Straus

For more information
The American Heartworm Society: Guidance for Heartworm Disease Management During the Adulticide Unavailability

Heartworm Disease in Dogs: Prevention and Treatment

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Mary Straus has been a regular contributor to Whole Dog Journal since 2006. Mary first became interested in dog training and behavior in the 1980s. In 1997, Mary attended a seminar on wolf behavior at Wolf Park in Indiana. There, she was introduced to clicker training for the first time, and began to consider the question of how we feed our dogs after watching the wolves eat whole deer carcasses. Mary maintains and operates her own site,, which offers information and research on canine nutrition and health. has been created to help make people more "aware" of how to make the best decisions for their dogs. It's designed for people who like to ask questions and understand the reasoning behind decisions, rather than just being told what to do.  Mary has spent years doing research for people whose dogs have health problems, or who just want to learn how to feed them a better diet. Over this time, she has learned a great deal about dog nutrition and health, including the role of diet, supplements and nutraceuticals.  In 2007, she was asked by The Ivy Group to contribute to The Healthy Dog Cookbook. She previously also wrote a column for Dog World.