Heartworm Disease – What you need to know

With the emergence of summer here in the U.S., our pets are at higher risk for some contagious diseases. The one we will talk about today is heartworm disease.

Heartworm disease is an infection in the blood and heart with an actual worm, called Dirofilaria immitis, or heartworms. Heartworms can affect many organs of the body and even be fatal if not treated early. You may have your dog or cat on heartworm prevention (you should have them on prevention! More on this later in the post), but do not know exactly what heartworms are or what they can cause in your pets.

Heartworms start as microscopic larvae called microfilaria. Mosquitoes carry the microfilaria internally and when they bite a dog or cat, they pass along the microfilaria to infect the host. These “baby worms” go through several phases and grow into adult worms that produce more microfilaria. This perpetuates the life cycle of heartworms in the dog or cat’s body.

It takes about 6 months from infection by a mosquito bite until a patient can show positive on a heartworm test. We won’t go into a lot of detail about heartworm testing as it does vary between dogs and cats. However, dogs are commonly tested at your veterinarian with just a few drops of blood, and the results are usually available within 10 minutes. Cats can also be tested similarly, but we tend to send their blood sample to an outside lab for more extensive testing. Heartworm disease in cats is not as straightforward to diagnose as compared to dogs.

A good amount of patients that we diagnose with heartworm disease do not show any clinical signs. If a pet is showing clinical signs of heartworm disease, the infection is likely fairly advanced into late stages. Clinical signs in dogs include coughing, weight loss, exercise intolerance (unable to perform normal physical activities they once did), increased heart rate, and respiratory rate to name the most common. Clinical signs in cats include vomiting, diarrhea, coughing, lethargy, weight loss, difficulty breathing, and sometimes neurological changes like seizures, trouble walking/weak in the back end (ataxia), and blindness.

You may be wondering if cats and dogs can get heartworm disease – can people get it, too? Technically speaking, people can contract heartworms through a mosquito bite, but people are considered “dead-end hosts”. This means our bodies are not appropriate environments for the baby worms (microfilaria) to mature and grow into adult worms. The microfilaria end up dying and our body gets rid of it on its own – usually with no changes that we notice!

Now back to the pets! Due to mosquitoes being the carrier of heartworms, pets in certain areas in the U.S. are more at risk for contracting heartworm disease. Pets in the southeast US, especially the southern Mississippi River valley, are at an increased risk given the warmer, damp climate. The Companion Animal Parasite Council has a great website where you can view a map showing the prevalence of heartworm disease (canine and feline separately) in the U.S. (pro tip: this website has similar maps and loads of pet owner information on different parasites in cats and dogs).

Although not as common, there have been some cases of resistant heartworms, specifically in the Mississippi River valley, as previously mentioned. A resistance heartworm infection is a pretty rare occurrence, however, this is another reason to be consistent with heartworm prevention and testing, even if your pet is on year-round prevention.

In the northern U.S., we think because the winters are harsh enough that mosquitoes are not a concern. There are many cold days in the winter, however, there can be warmer days in the fall through spring, which are warm enough for mosquitoes to be present. For this reason, it is recommended that pets received heartworm prevention year-round, despite the region of the country you are located.

This disease is 100% preventable! No pet or pet owner should have to suffer from a heartworm disease diagnosis as there are many safe, prescription heartworm preventions (oral and topical) on the market. To determine the best prevention for your baby, please discuss it with your veterinarian and keep your pets safe and healthy!