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May 14, 2009


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Thanks so much for this really detailed and informative series about heartworm!

I recently adopted a shelter dog who is heartworm-positive. She is somewhere between 5 and 9 years old (we think around 6-7) and in good health other than her infection (stage 2). She is on a 2 week round of doxycycline to prepare for her 2-shot treatment at the shelter in a few days. Lucy is an incredibly sweet dog and we are worried sick about the treatment.

My question for you is this (if you don't mind weighing in!). The shelter prescribed the 2 week course of doxy (it sounded to me like if we had not adopted her right away, they would have skipped the doxy and started her on the treatment right away, probably for their convenience) but as far as I know did not give her any ivermectin. Does it make sense for me to push this with the shelter vets and ask for another 2 weeks of doxy + ivermectin (weekly?) before treatment? I know they won't pay for the 3 treatment protocol for cost reasons, but I might be able to convince them to provide 2 more weeks of meds.

Is there any reason you can think of for going ahead and doing the treatment vs. waiting and medicating? Are there any significant dangers for Lucy in waiting the two weeks w/ heartworms?

I'd appreciate any thoughts you might have on the issue. Once again, great blog series!


Hello, Rachel,

Four years ago, your dog wouldn't have received ivermectin or doxycycline. Now it's pretty standard for dogs where we suspect a bigger worm burden. If I have a dog who was taking preventive, but got the heartworms anyway, I don't bother with giving the pretreatment of doxy and ivermectin.
When I feel it is necessary, four weeks of doxycycline is the recommended time, but two weeks should help some.
The ivermectin may not be given at higher doses if you have a Collie-type dog (Collie, Border Collie, Sheltie, Australian Shepherd, Blue Heeler, etc.). These dogs DO tolerate the dose in Heartgard.
The authorities I have heard recommend starting these dogs on Heartgard during the treatment process, not waiting until it is over. You don't want the dogs to acquire more worms from current mosquito exposure.

As far as danger from waiting, if two weeks are going to put her over the edge, then you were in deep trouble already. The doxycyline is more likely to help than the delay is to hurt.

Dogs that are clinically ill already may benefit from treatment with prednisone to open up the pulmonary arteries a little.

Good luck.


Thank you for all the information regarding heartworm prevention and treatment. There is a lot of misunderstanding.

We are an animal welfare group who took in a dog which tested positive for heartworms. The dog has received ivermectin weekly + doxycyline daily for two weeks, paused a week, and will receive another week of doxycycline.

A few studies conclude doxycyline + ivermectin kill adult heartworm:

"Results indicate that the combination of these two drugs causes adult worm death."


There is also internet talk of a "slow kill" method.

Do you have any advice for animal welfare groups or others with a budget that would not involve Immiticide? There are also such frequent shortages of Immiticide, it appears to be an annual event.

Thank you.

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