In recent years there's been an increasing prevalence of heartworm infections in both dogs and cats. The belief that “heartworm isn’t really a problem in my area” is no longer a safe or reliable assumption. Diagnoses have been made in every state of the United States including places that were once thought to be at very low risk such as northern Idaho, Nevada, Washington, and Wyoming¹.

Infections with this parasite – worms that can reach up to a foot in length – can lead to heart failure, lung disease, damage to other organs of the body, and can be fatal. The good news is that it is nearly 100% preventable; the American Heartworm Society recommends administration of heartworm preventive year-round regardless of geographic location².

Heartworm infections are not transmissible from pet to pet — mosquitoes are the only way it spreads. Any pet who spends time around mosquitoes — indoors or out — has some level of potential exposure. To appreciate how prevention works, it helps to understand the transmission and lifecycle of the parasite:

1. Adult female heartworms can live in dogs, foxes, coyotes and wolves. These adult females produce “baby worms” or microfilaria that, then, circulate in the animal’s bloodstream.
   
   
2. When a mosquito comes along, bites an infected animal and takes a blood meal, it picks up these microfilaria. Over a period of 10 to 14 days, these microfilaria go on to develop inside the mosquito into infective stage larvae.
   
   
3. When the infected mosquito then bites another animal (e.g. dog, cat, ferret, or other wild animal), it deposits those infective larvae onto that animal’s skin. It only takes one bite from an infected mosquito to pass heartworm larvae to a pet! Those larvae then make their way into the animal’s body through the mosquito bite wound.
   
   
4. Approximately 6 months after entering that new host, those larvae complete their development and mature into adult heartworms.

Heartworm preventives work by eliminating the parasite's larval stages before they mature into an adult. That window of time is very narrow — in as few as 51 days, immature larvae can molt into an adult worm. Preventives are not effective against adult worms and, therefore, need to be given on schedule — whether that's monthly for oral (chewable pill) or topical, spot-on products or twice yearly for the injectable option.

In spite of rising heartworm incidence rates and the presence of heartworm in all 50 of the United States, data reported by Banfield Pet Hospital³ from a 2023 survey found that:

• 21% of dog and cat owners did not think that mosquitos in their area carried the parasite, and nearly 40% didn’t believe that their pet was at risk of getting heartworms
• 41% of pet owners surveyed believed that heartworm was only a risk during part of the year
• 51% of pet owners said they skip year-round prevention with only 18% using heartworm prevention for their pets during the spring and summer
• 30% of dog and cat owners indicated that they did not give heartworm prevention to their pet at all

These are sobering statistics and may be one reason why we have more than a million pets infected with heartworm in the United States.

In addition to the year-round administration of preventive, the American Heartworm Society recommends testing dogs for heartworm every 12 months, even if they are on a preventive. No single prevention method is 100% foolproof. Furthermore, owners may inadvertently miss a dose or be late in administering a monthly treatment. Giving a preventive to a dog that has heartworm can be dangerous, and catching an infection early (through annual testing) can make a significant difference in how things play out for that patient.

If prevention does lapse and a dog tests positive for heartworm, treatment is possible. However, treatment is neither quick nor simple. There are no approved treatments for heartworm in cats. As with most diseases, prevention is easier, safer and less expensive than treatment when it comes to heartworm infections. For the health of their pet, owners should talk to their veterinarian about annual heartworm testing and year-round prevention options that best suit their individual situation.

Resources:

1. The American Heartworm Society - 2025 Heartworm Incidence Survey (released in April 2026).

2. The American Heartworm Society - Heartworm Prevention for Dogs

3. New pet owner survey suggests common myths fueling significant gaps in prevention of deadly parasite for U.S. dogs and cats. PR Newswire. April 2024.