A sad dog o the couch.

The winter thaw is on the horizon, and in our neck of the woods, that means we start thinking about longer days, gardening, and….The coming mosquito season! Although steamy days and nights might bring with them summer fun, the mosquitoes that also come with warm weather are definitely not welcome. Not only are they annoying for you, but they could also pose a serious health threat to your pet.

Here are some of the basics of heartworm prevention from your friends at Oakhurst Veterinary Hospital.

What Is Heartworm?

You may know that heartworm disease is caused by a mosquito-borne parasite, the Dirofilaria immitis. This common roundworm can be transmitted to dogs and cats by over 30 species of mosquito. The heartworm lives in the heart and pulmonary arteries, and causes permanent and life-threatening damage to your pet’s heart, as well as to other organs.

How Does My Pet Get Heartworm?

The heartworm life cycle is long and complicated. The basic steps for infection are as follows:

  • A mosquito bites an infected animal (cat, dog, or wild species) and takes up the heartworm larvae, or microfilariae, into its body.
  • The microfilariae develop in the mosquito’s body for 10-30 days.
  • The mosquito bites a pet, injecting the microfilariae into the pet’s bloodstream. The microfilariae further develop over the course of several weeks, finally making their way to the heart and pulmonary arteries, where they mature into foot long adult worms, capable of reproduction.
  • After about 6-7 months of infection, the adult worms reproduce and release new microfilariae into the pet’s blood stream. The pet can now spread the disease by a new mosquito bite.

Signs and Symptoms

The signs and symptoms of heartworm disease may be subtle at first, and they differ in dogs and cats. Common signs may look like other diseases, and diagnosis is rarely made based solely on clinical signs.

In cats, indicators can be:

In dogs, common signs are:

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


Cats are resistant hosts for heartworm, and they typically only have 1-6 adult worms if infected, compared to 30 or more in dogs. This makes diagnosis virtually impossible in cats as there are no tests with definitive results. Meanwhile for dogs diagnosis can be made with a combination of tests that may include:

  • Blood antibody test
  • Blood antigen test
  • Chest X-rays
  • Cardiac ultrasound or echocardiogram

Once diagnosis is made, treatment can begin.


There are differences between the treatment for dogs and for cats. In dogs, there is a drug that can be given by injection, which will kill the adult heartworms over the course of several weeks. Because the medication kills the worms and the dead and dying worms can block pulmonary arteries, it is recommended that dogs being treated are on exercise restriction.

Sadly, there is no approved drug for the treatment of heartworms in cats. Some choose to treat the cat with the dog-approved drug, and hope that there is no reaction. Sudden death may be caused by the dying worms initiating a reaction in the cat’s lungs. Another thought is that the dead and dying worms may migrate through the pulmonary arteries, blocking oxygen and blood flow to the heart.

Treating the cat’s respiratory illness caused by heartworm is another option, with the hope that the cat outlives the worms. Adult heartworms live 2-3 years in the cat (compared to 5-7 years in dogs), and a respiratory emergency can occur at any time during that period.

Heartworm Prevention

You can see that compared to the illness, permanent organ damage, and risk of death, prevention is truly the best measure. Luckily, heartworm prevention is easy, safe, effective, and inexpensive, compared to the cost of diagnosis, treatment, and disease management.

All preventives kill the heartworm microfilariae circulating in the bloodstream, as well as other parasitic worms. It is imperative that monthly preventives be given on a strict schedule. The microfilariae take about 51 days to mature into adult heartworms, and so interrupting that process on a consistent basis is key to prevention. With iCal or other electronic calendaring system, heartworm prevention is easy to remember for everyone!

There are several heartworm preventives that can be given monthly as a chew or a topical, and also an injectable option which can be given every 6 months. Talk to us about the best options for your pet, and we can easily get them started on a routine that will protect them for life.