A dog with a bug on its nose

The winter thaw is on the horizon, and with that comes a whole host of creepy, crawly, and voraciously hungry bugs. Although we may all be looking forward to balmier weather, the mosquitoes that come with spring and summer are definitely not welcome! Not only are they annoying you, but they could pose a serious and deadly risk to your pets.

With their bite, mosquitoes can transmit a roundworm called heartworm that can cause shortness of breath, lethargy, collapse, exercise intolerance and even death in dogs and cats. Heartworm is found in all 50 states and year round – including Maine! So we thought it a prudent time to have a heart to heart about this health concern for our pets.

The Life Cycle of the Heartworm

The adult heartworm lives in your pet’s heart and pulmonary arteries, and causes permanent and life threatening damage to your pet’s heart and other organs. The heartworm life cycle is long and complex, giving good reason for a multi pronged approach to prevention.

  • A mosquito bites an infected animal (dog, cat, or wildlife) and takes up the heartworm larvae, or microfilariae into its body.
  • The microfilariae develop in the mosquito’s body 10 to 30 days
  • The mosquito bites a pet, injecting the microfilariae into the pet’s bloodstream. here , the microfilariae develop further over the course of several weeks. They finally make their way to the pet’s heart and pulmonary arteries, where they mature into foot long adult heartworms, capable of reproduction
  • After 6-7 months of infection, the adult heartworms release new microfilariae into the pet’s blood stread, and the process begins again.

The Signs of Heartworm

Signs and symptoms of heartworm may be subtle at first, and differ between dogs and cats. Signs are similar to many other diseases, and diagnosis is rarely made solely on the basis of clinical signs.

Signs in cats:

  • Wheezing
  • Sudden onset cough
  • Vomiting
  • Rapid breathing
  • Weight loss
  • Sudden death

Signs in dogs:

  • Lethargy
  • Exercise intolerance
  • Soft, dry cough
  • Weight loss
  • Rapid breathing
  • Bulging chest
  • Collapse


Diagnosis of heartworm is made through blood tests. At Androscoggin Animal Hospital, we recommend annual testing for heartworms. We now also have the ability to not only test for the presence of adult heartworms, but also for the presence of microfilariae in the blood. As a result, we can detect heartworm infection earlier, and thereby prevent extensive organ damage. Early detection of disease saves your pet discomfort and pain, and can potentially save you money in the long run.

The Heart of the Problem: Treatment

Cats are resistant hosts for heartworm, and typically only have 1-6 adult heartworms present. However, the cat’s body mounts an immune response that kills the heartworms, but also poses a great risk. As the heartworms die, they break into pieces, which can get stuck in the pulmonary arteries, causing lack of oxygen and sudden death. If we don’t know that the cat is at risk, we can’t monitor and mitigate problems. Sadly, there is no current treatment for heartworm in cats.

In dogs, adult heartworms living in the organs and arteries typically number in the 30’s. Treatment is given by a series of injections, which kills the adult heartworms over the course of several weeks. Since dead and dying heartworms can cause blockage of the pulmonary arteries, heartworm treatment always poses a serious risk of respiratory emergency. For this reason, dogs must be hospitalized for several hours after injections, and must be on exercise restriction for the duration of treatment.

The Best News: Heartworm Prevention

The best news about heartworm disease is that it’s preventable.

We carry several options in monthly oral heartworm preventives that are safe, effective, and far less costly than treatment. Give us a call to learn more about starting or continuing your dog and cat on heartworm prevention.

Preventives must be given on a strict schedule. It takes about 51 days for the microfilariae to mature into adult heartworms, and interrupting this process on a continuous basis is key to prevention. As we mentioned, testing for the presence of heartworms and microfilariae is an important component to prevention and early detection as well.

Talk to us about the best options for your pet at their next annual preventive care visit. Together we can give them a lifetime of good health!