What is heartworm disease?
Heartworm disease is a serious—but preventable—disease that is spread by mosquitoes and can affect any dog.
Heartworms are tiny, parasitic worms that live inside a dog's body. While many parasitic worms live inside a dog's intestinal tract, heartworms live in a dog's heart and lungs. The presence of these worms causes inflammation that can lead to serious damage.
If the disease isn't treated, the number of heartworms can grow. The mass of worms can even grow large enough to block blood flow, which can be life-threatening, and cause other heart or lung problems. That's why heartworm disease may be considered a medical emergency in advanced stages.
How do dogs get heartworm disease?
Heartworms are spread to dogs by mosquitoes. Here's how:
- When a mosquito bites an animal that has heartworm disease—such as another pet dog, a wolf, a fox, or a coyote—they can pick up microscopic baby heartworms.
- If that mosquito bites a dog, it can pass the heartworms on to that dog.
- In this dog (the new "host"), the heartworms grow into adults that can reach 12 inches. As the heartworms mature, they migrate to the heart and lungs.
All dogs exposed to mosquitoes are at risk of getting heartworm disease. Since mosquitoes are found in most parts of the world, including in your backyard and sometimes inside your home, any dog can be bitten and develop heartworm disease.
Thankfully, heartworm is preventable, and working with your vet on a preventive plan can protect your dog.
How common is heartworm disease?
Heartworm disease has been diagnosed in dogs in all 50 states, according to the American Heartworm Society. However, the disease is most common in areas where mosquitoes are more prevalent, such as the warm, humid southern states. Hundreds of thousands of dogs are diagnosed each year. Prevalence maps published by the Companion Animal Parasite Council (CAPC) show that more than 188,000 dogs tested positive for heartworm in 2025. Since not all dogs are tested, the actual number of infected dogs may be much higher.
Many factors affect mosquito populations and the animals they infect with heartworms, so the risk of this disease is constantly changing. For example, according to the American Heartworm Society:
- Wildlife carrying heartworms may move into different areas, bringing the parasite with them.
- Infected dogs may be moved from high-risk areas to lower-risk areas.
- Pet owners may also travel with their dogs, taking them to areas where heartworm disease is more prevalent.
For these reasons, even dogs in relatively low-risk areas need year-round prevention.
What are the symptoms of heartworm disease?
Since heartworms live in the heart and lungs, the symptoms they cause stem from damage to these organs. However, signs can be subtle. Here’s what to look for:
Early signs that can be easy to miss
Dogs in the early stages of heartworm disease regularly show no signs of illness. It takes about six months for the immature worms to grow into adults, and infection often goes unnoticed during this phase. Once adult heartworms are present, it can take many more months, or even years, for symptoms to appear.
The first signs of heartworm disease may include:
- Low energy or tiring more easily
- Decreased appetite
- Coughing
- Weight loss
Signs of advanced heartworm disease
As heartworm disease progresses, damage to the heart and lungs can lead to more serious symptoms. Signs of advanced heartworm disease include:
- Weight loss
- Severe coughing
- Difficulty breathing
- Swollen abdomen
- Pale or bluish gums
- Collapse
If your dog shows signs of early heartworm disease or isn't acting like themselves, it’s a good idea to schedule an appointment with your vet. If your dog shows any signs of advanced heartworm disease, they should be seen urgently. Your vet will partner with you in making a treatment plan if your dog has heartworm disease.
How do vets test for heartworm?
Although your vet will perform a thorough physical exam, they can't tell from the outside alone whether a dog has heartworm disease. Heartworm testing is done with a simple blood test that detects the presence of adult heartworms (and is often combined with testing for diseases spread by ticks) in a dog's body.
The American Heartworm Society recommends:
- Testing dogs suspected of having heartworm disease.
- Yearly heartworm testing for seemingly healthy dogs, even those on prevention medication. This is typically started when a dog reaches 7 months of age at a routine wellness exam.
Why test every year? Because:
- Even though tests only detect adult worms, yearly screening helps identify infections before dogs develop noticeable signs or more severe symptoms.
- A single missed or delayed dose of heartworm prevention medication can leave a dog open to infection.
Annual testing helps catch infections early—before serious damage occurs—and helps confirm your prevention plan is working.
Treating heartworm disease
If your dog tests positive for heartworm disease, they will need specific treatment to kill the heartworms before they can cause more damage. Heartworm treatment is more involved than prevention and includes:
- A series of injections over several months
- Medications to reduce inflammation
- Strict activity restriction during recovery
During treatment, it's very important to keep your dog calm and limit activity. As worms die, they can cause complications, so limiting activity helps reduce risk. Your vet can guide you on how long and how firmly your dog should be restricted based on your dog’s case. They may also recommend you:
- Keep walks short and leashed.
- Avoid running or playing.
- Monitor your dog closely for any changes.
It can feel stressful, but your vet will guide you through each step.
How long can dogs live after heartworm treatment?
Many dogs return to their regular activity and enjoy a normal life after treatment. However, in some dogs, heartworms may have already caused irreversible damage to the heart and lungs, leading to long-term complications. This is more likely when the disease is more advanced.
That's why yearly screening and early detection are so critical. The earlier heartworm disease is found and treated, the less likely dogs are to develop long-term damage. Ultimately, prevention is the best strategy to protect your dog from serious, lasting health issues.
How can you prevent heartworm disease in dogs?
Preventing heartworm disease is easier and safer than treating an infection. Consistent prevention avoids the risk of long-term damage caused by adult heartworms. Because all dogs—including indoor dogs—are at risk for heartworm disease, the American Heartworm Society recommends year-round prevention for every dog.
Heartworm prevention medications come in many forms, including:
- Monthly medications applied to a dog's skin
- Monthly oral medications
- Longer-acting options, such as 6- and 12-month injections given by your vet
During your dog's annual wellness visit, your vet can help you choose the option that works best for you and your pet.
Final thoughts
Heartworm disease can sound scary, and in many ways, it is. But it's also one of the most preventable conditions your dog could face. The most important things you can do are:
- Keep your dog on year-round heartworm prevention.
- Stay consistent with annual testing.
- Talk to your vet if you ever miss a dose or have concerns.
By taking these steps, you're not just checking a box—you're actively protecting your dog's heart, lungs, and long-term health. And that peace of mind? That's something every pet parent deserves.
Frequently asked questions
Frequently asked questions about this topic
No. The good news is that dogs can only get heartworms from a mosquito bite. The parasite cannot be passed from one dog to another. However, having a heartworm-positive dog nearby can make it more likely that a mosquito will pass heartworms from the infected dog to your dog.
If you accidentally skip a dose of your dog's heartworm prevention, you should call your vet for advice. Depending on how long your dog went without protection, your vet may recommend adding a heartworm test in about six months to make sure they didn't become infected.
Yes. Having heartworm disease does not provide your dog with immunity against future infections. They will still need year-round heartworm prevention to avoid becoming reinfected.
Even if you give heartworm prevention regularly, annual testing is still recommended. A delayed or missed dose—even once—can leave a dog at risk for heartworm disease. In some cases, a full dose may not be received; for example, a chewable may be spit out, or a topical product may be inadvertently washed off. Yearly testing helps catch infection before dogs develop noticeable signs and confirms your dog's prevention plan is working as intended.
No, heartworm prevention doses should not be split. Each dog needs a full dose based on their weight to be properly protected against heartworm disease. Splitting doses can lead to underdosing and reduced protection. If you have questions about dosing for multiple dogs, contact your vet.
Yes. While dogs are the primary host for heartworms, other animals can be affected. Cats can get heartworm disease, though it often looks different than in dogs and may be harder to diagnose. Ferrets are also susceptible and can develop serious illness. Wildlife such as coyotes and foxes can also carry heartworms, helping maintain the spread of disease in the environment.
