Heartworm Disease in Dogs

Although not contagious, Heartworm disease is a serious and potentially fatal disease. It can lead to severe lung disease, heart failure, other organ damage and death in pets, especially dogs. The disease is caused by a parasitic worm called Dirofilaria immitis and is spread through the bite of a mosquito.

Meet Bourbon a 4 year old, 175 pound, male, mastiff mix. He was adopted from a shelter in 2013 and has been in perfect health.

Bourbon initially presented to have a growth removed from his shoulder. Routine pre-surgical blood work was preformed and he tested positive for heartworm disease. We usually retest to be sure we haven’t had a false positive test, which is rare. He was still positive.


Heartworm larva are injected into a dog as a mosquito feeds. The mosquito picks up the larva by feeding on a heartworm positive dog, who all have larva circulating in the blood.The mosquito is a required host in the life cycle of this parasite.


We follow the general guidelines of heartworm treatment set up by the American Heartworm Society.The first step was to start Bourbon on a monthly heartworm preventative. We started him on Triheart,which kills the initial larval stage of the worm. It can take 6-7 months for a larva to become an adult worm so we usually will have a dog on monthly preventative for 4-6 months prior to attempting to kill the adult worms. This allows any larva to fully mature so we are less likely to need to repeat the adult worm killing treatment.


The second step is to start Bourbon on an antibiotic called doxycycline for a month prior to giving the adult worm killing treatment. This medication kills a bacteria which live in the adult heartworm. The bacteria help the worm survive and reproduce, and by killing them we achieve a better kill rate with the adult killing treatment. Bourbon is so big he needs 16 pills per day!


The final step in Bourbon’s treatment is killing the adult heartworms. We do this by giving him an injection of a medication called Immiticide. This medication is fairly expensive, especially for such a big boy, and has to be given deeply into the muscles along the spine. Immiticide can cause some intense inflammation in the surrounding tissue and can result in a very painful response. We usually use steroids to help reduce the inflammation, but many dogs are still uncomfortable after the injection and need to be observed in the clinic for 24 hours after the injection.


We try to have a “slow kill” of the adult heartworm so for the next month Bourbon will be kept as completely restricted in activity as possible. This means that he will need to be kept in a confined space, like a crate, for the majority of the next couple of months. If the worms die too quickly they travel down the bloodstream to the lungs and act almost like a clot leading to potentially severe and sometimes life threating side effects.


One month later we will give Bourbon 2 more immiticide injections 24 hours apart and he will again need to be very restricted in activity for another month or longer depending on clinical signs. We will test Bourbon for heartworm 6 months after treatment is finished to be sure all the adult worms were killed.

The moral of Bourbon’s story is that monthly use of heartworm preventative has the power to stop a very serious and potentially life threating condition before it even starts. As you can see the treatment is intensive, painful, expensive, and involves months of confinement all of which I hope provide motivation for monthly use of prevention.