A dog with a bug on its nose

Even with our mild Texas winters, we can feel the spring fever blooming all over Austin. People are out and about, pets are enjoying the warm sunshine, and everyone is ready for play and fun. But one of the things that no one finds fun is the sudden explosion of mosquitoes!

Annoying for us, mosquitoes carry a parasite that can cause serious and even deadly disease for our pets. Heartworm disease may be something you’ve heard of, but may not know much about. Brodie Animal Hospital explains, here.

What is Heartworm?

Heartworm is a roundworm, Dirofilaria immitis, that is transmitted to pets by the bite of a mosquito. The mature heartworm lives in the heart and pulmonary arteries of cats, dogs, and wildlife, and causes permanent and life threatening damage to your pet’s heart and other organs.

The lifecycle of the heartworm is long and complex. The basic steps to infection are:

  • A mosquito bites an infected animal (pet or wildlife) and takes up the heartworm larvae, or microfilariae, into its body
  • The microfilariae develop inside the mosquito’s body over 10-30 days
  • The mosquito bites a new pet and injects the microfilariae into the healthy pet’s body.
  • Over the course of several weeks, the microfilariae develop further in the bloodstream before making their way to the pet’s heart and pulmonary arteries. There, they develop into adult heartworms capable of reproduction.
  • After about 6-7 months, the adult heartworms reproduce and send new baby microfilariae into the pet’s bloodstream, which can now be taken up by a mosquito and transmitted to a new pet.

Signs of Heartworm Disease

Early signs of heartworm may be similar to those of other diseases, and may be subtle at first.

Signs of heartworm in dogs:

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

Signs of heartworm disease in cats:

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


Because heartworm signs are often subtle at first, diagnosis is rarely made based solely on clinical signs.

Dogs are ideal heartworm hosts, and may have upwards of 30 adult worms when infected. In dogs, diagnosis is typically made with a combination of diagnostic testing.

Cats are resistant hosts for heartworm and diagnosis in cats may be challenging or nearly impossible. They usually only have 1 -3 adult worms present when infected, but even immature worms can cause serious complications in cats, especially HARD (heartworm associated respiratory disease).

Treatment of Heartworm Disease

In dogs, treatment consists of several powerful injections that kill the adult heartworms over the course of several weeks. Most dogs must be hospitalized and monitored for complications post injection, and all dogs should also be on exercise restriction for the duration of treatment to mitigate these risks.

Sadly, the same injectable drug is not approved for use in cats due to the risks to the cat’s life. Cats can have a strong reaction in their lungs as the worms are killed off by the medication that can cause an acute respiratory emergency and sudden death. Some choose to treat the cat anyway in the hopes that the reaction doesn’t occur. Others choose to treat respiratory symptoms of heartworm disease with oxygen therapy, corticosteroids, and other medications and treat acute respiratory events as they occur. An emergency and sudden death is always a risk with either approach.

A Spark of Hope

You may be feeling a bit bleak, but take heart (pun intended!). Heartworm prevention is safe, effective, and much less expensive than treatment of the disease or its complications.

Once you and your veterinarian have chosen a medication, it’s important to give it on a regular and year round schedule. A mosquito can bite at any time, and can even survive indoors over winter!

Schedule an appointment if you need help getting your pet back on track with heartworm disease prevention. If you have any questions or concerns, please don’t hesitate to reach out to us.