Heartworm disease is caused by the internal parasite Dirofilaria immitis, better known as the heartworm. These parasites are long thin worms that live in the right side of the heart and the pulmonary arteries, the blood vessels that carry blood from the right side of the heart to the lungs. Dogs become infected when they are bitten by an infected mosquito that is carrying immature worms called larvae. Once they enter the dog, the larvae move through the tissues of the body, eventually entering the bloodstream and migrating to the right side of the heart. Within the heart, the worms complete their maturation, mate, and produce immature worms called microfilariae, which circulate in the bloodstream. When the infected dog is bitten by another mosquito, the microfilariae will pass from the dog to the mosquito. It takes about 5 – 6½ months for the heartworm life cycle to be completed.


In the early stages of disease, dogs often have no clinical signs, especially if they only have a small number of worms in their heart. As the disease progresses, clinical signs become more noticeable and include reluctance to exercise, rapid fatigue with exercise, coughing or collapse. In advanced disease, dogs develop congestive heart failure. Dogs in congestive heart failure lose weight, have a poor body condition, breathe rapidly or with difficulty, and develop a build-up of fluid in the abdomen.


Heartworm is usually diagnosed with a simple blood test. In some infected dogs, the blood tests are negative even though there are heartworms in the heart. If a dog is suspected of having an ‘occult’ (hidden) heartworm infection, your veterinarian may either recommend x-rays to look for changes that indicate the presence of heartworm, or an ultrasound to examine the heart structure and function directly.


The only product currently available for the treatment of adult heartworms is melarsomine dihydrochloride. The treatment can be done in 2 doses over 24 hours interval, and a intramuscular injection is injected deeply in the epaxial (lower back) muscles.

After treatment, the patient must be strictly confined for one month following the final treatment. No walks, no running around. The dog must live the indoor life. The reason for this is that embolism to some degree is inevitable and it is important to minimize embolism-related problems. Exercise increases heart rate and oxygen demand and we need the heart to rest during this recovery period.

Melarsomine treatment is expensive and often out of reach for rescue groups, shelters, and many individuals. If the dog is stable and the only option is to simply leave the dog on an ivermectin based preventive. This option has led to a great deal of misconception about the ability of ivermectin to kill adult heartworms. Let us lay the rumors to rest now:

  • Ivermectin does not kill adult heartworms.
  • Ivermectin does shorten the lifespan of adult heartworms.
  • Ivermectin does sterilize adult heartworms.
  • Ivermectin does kill microfilaria (keeping the dog from being a source of contagion)
  • Ivermectin does kill L3 and L4 larvae (preventing new infections).

This means that if one opts to treat a heartworm positive dog with an ivermectin heartworm preventive only, one can expect the dog to remain heartworm positive for as long as 2 years and the heartworm disease will be progressing during that 2 years. This is not good for the dog but certainly beats getting no treatment of any kind.

There is another aspect to this treatment which is important to mention and that is the development of resistant heartworm strains. Using heartworm preventives long term in dogs with adult heartworms, leads to the production of heartworm preventive resistant microfiliaria. In other words, this “slow kill” method of treating heartworm infection leads to strains of heartworm that cannot be stopped by any of the heartworm preventives presently available on the market.