Don't Let Heartworm Become A Heartbreak!

Guest post by Lorie Huston, DVM

Keeping your pet healthy requires regular preventive health care. Timely administration of heartworm preventive medication is one of the preventive measures that all dog owners should be practicing.

What are heartworms?

Heartworms are mosquito-borne parasites that live in the heart and blood vessels of an infected dog. These worms cause damage, often significant damage, to the cardiovascular structures and can lead to serious illness in the infected dog. In severe cases, heartworms can be fatal. Their scientific name is Dirofilaria immitus.

What are the symptoms of heartworm disease?

Heartworms can damage both the pulmonary arteries and the heart. Symptoms seen may include:
  • coughing
  • exercise intolerance
  • hemorrhage from the nose
  • pneumonia
  • right-sided heart failure
  • cardiac arrhythmia, which may lead to sudden death in some cases
A particularly devastating form of heartworm disease is known as caval syndrome. In caval syndrome, there are extremely large numbers of adult worms (approximately 100 or more) and the right heart chambers are too small to contain all of them, allowing the worms to overflow into the vena cava, one of the large vessels that leaves the right side of the heart. Collapse, shock and the destruction of red blood cells are the symptoms associated with caval syndrome and death may occur within one to two days without veterinary intervention. Survival of the dog requires surgical removal of the worms from the heart and blood vessels. If enough worms are removed, blood flow is resolved and the dog has a chance of survival. Without surgery, the chance of survival is small.

How can heartworms be prevented?

Numerous medications are available that prevent heartworm infections from occurring in a susceptible dog. Most of these medications are either tablets or topical medications that must be applied monthly. Though the risk of side effects can occur with any medication, these drugs are generally safe and are very effective in preventing heartworm infections if used correctly.

Alternatively, there is an injection that can be administered every 6 months which is also capable of preventing heartworm.

Why prevent heartworms rather than treat the infection if it occurs?
  • Heartworms can cause significant damage to the circulatory system before they are detected and this damage is likely to be irreversible.
  • Heartworm treatment (treating an existing infection) carries the risk of complications, even though the medications used today are safer than those used in the past.
  • Strict cage rest is required for long periods of time while the dog undergoes treatment, sometimes as long as 6-8 weeks or longer.
  • Heartworm treatment is not guaranteed to kill all of the infecting heartworms.
Isn’t administration of a monthly preventive effective in killing the worms in an infected dog?

Monthly preventive medications only address the immature form of the heartworm. They do not kill the adult worms directly. With time, the adult worms will age and die of natural causes. Therefore, monthly heartworm preventive medications can eventually be effective in eliminating heartworm infections. However, elimination may take up to 2 years to occur. Meanwhile, the worms remain in the heart and blood vessels causing additional damage and disease.

In some instances, such as when a dog is debilitated or suffering from concurrent disease, he may not be considered a good candidate for heartworm treatment with a medication that kills adult worms. In these cases, treatment with a monthly preventive may be the optimal form of treatment. However, in most cases where the dog is otherwise healthy and deemed a fit candidate for treatment, the drug of choice is Immiticide® (melarsomine dihydrochloride).

Immiticide® is the only drug available that will kill adult heartworms. The major risk with Immiticide® injection is embolization of dying worms to the lungs, which can cause serious side effects for the dog being treated. Strict cage rest is necessary to minimize these complications. Before treatment with Immiticide®, a monthly preventive medication is usually administered to remove the immature heartworms before addressing the adults.


  1. Super useful info. I found your blog through the blog hop. Thanks for this info!

  2. Hi Carol! I'm glad you found the information helpful. Knowledge is our strongest armor.

  3. Note that "Monthly preventive medications only address the immature form of the heartworm". This is true. Preventative medications do prevent heartworm, but are actually treating immature stages of heartworm. Heartworm 'preventatives' are immature-heartworm poisons. There is no true preventative, only products that interrupt the development of heartworm in these immature stages by destroying them at an early stage.

    If there was a true preventative, I might be more flexible. But I really do object to the year-long treatment of heartworm. This is surely overkill.

    I do not recommend daily, monthly or yearly treatment for heartworm when climatic conditions are not appropriate for heartworm development. Heartworm needs temperatures above 18C (day and night) for one month to develop. If the temperature is higher, 27C (day and night), then it only takes 10 days for heartworm to develop. If either of these situations occurs, you should treat for heartworm. If the temperature has been under 14C at any point for longer than a few hours, then there is no need to treat for heartworm, as this drop in temperature prevents heartworm development.

    This information has been obtained from the American Heartworm Society.

  4. Heartworm is a truly awful disease to treat. The rescue I used to foster with has taken in 5 HW+ dogs in the two years I was with them. I fostered one of them myself. Reading about it and actually seeing it were two completely different things - we were really concerned about this dog's health during treatment. Thankfully, he's made a 100% recovery. Prevention is definitely preferred over treatment!

  5. Hi Ashley, yes, it is a bad one. I agree that theory can't compare to real life experience. Would you like to share the story(ies) on my blog?

  6. Hi Jana,

    I just saw your reply to my comment now. I'd be happy to share my experience.

  7. Hi Ashley, awesome, please shoot to my email.


Post a Comment