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At What Age Can Dogs Get Heartworms?



Reviewed By: Dr. Joel Robertson

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Heartworms are a serious threat to dogs in many parts of the world. These parasites can cause significant harm to a dog’s health, and in severe cases, heartworms can be fatal.

As a responsible pet owner, it’s crucial to understand heartworm disease and take the necessary preventative measures.

Heartworm prevention is essential for dogs of all ages, but at what age are they susceptible to contracting heartworms? The answer is that dogs of any age can get heartworms, but most dogs become infected at a young age.

Key Takeaways:

  • Dogs can potentially contract heartworms at any age, but it’s typically rare in puppies under a year old as the heartworm larvae need about 5-7 months to mature into adults.
  • Preventing heartworm disease is crucial for all dogs.
  • Understanding heartworm disease and its transmission can help pet owners take the necessary precautions.

At What Age Can Dogs Get Heartworms?

At What Age Can Dogs Get Heartworms

Dogs that are older than 5 months can get heartworm because it takes 5-7 months for the heartworm larvae to mature into adults.

Puppies can begin heartworm-preventative medication from as young as 6-8 weeks old.

If a puppy is older than 6 months, it’s important that they are tested for heartworms before starting prevention.

Most veterinarians test puppies for the first time anywhere between 6-10 months of age.

The American Heartworm Society recommends starting a puppy on heartworm prevention no later than eight weeks of age.

Understanding Heartworm Disease in Dogs

Heartworm disease in dogs is a serious and potentially fatal infection caused by a parasitic worm called Dirofilaria immitis. The disease is transmitted through the bite of an infected mosquito and can affect dogs of all breeds and ages.

Once inside a dog’s body, heartworms mature and grow into long, thin worms that reside in the heart, lungs, and associated blood vessels. Adult heartworms can grow up to 12 inches in length and cause severe damage to these vital organs.

Heartworm Lifecycle in Dogs
The lifecycle of heartworms begins when an infected mosquito bites a dog and injects immature heartworms, called microfilariae, into the dog’s bloodstream
The microfilariae mature into larvae and travel to the heart and lungs, where they continue to grow and develop into adult worms
Adult heartworms release microfilariae into the bloodstream, which can be picked up by mosquitoes and transmitted to other dogs

The underlying damage caused by heartworm disease can lead to pulmonary hypertension, heart failure, and even death if left untreated. Due to the severity of the disease, prevention and early detection are critical to ensuring a dog’s health and well-being.

Age of Susceptibility to Heartworms in Dogs

Heartworm disease is a potentially fatal condition that can affect dogs of all ages, but puppies are particularly vulnerable. While dogs of any age can contract heartworm disease, they are most susceptible at 6-7 months of age. At this age, many puppies have not yet started heartworm-preventative medication and are still developing their immune systems.

It’s important for dog owners to understand the developmental stages of heartworms to identify when their pets are at the highest risk of infection. Heartworm larvae grow and mature inside an infected mosquito and are then transmitted to dogs through a mosquito bite. Inside the dog’s body, heartworm larvae develop into adults and reproduce, causing damage to the heart, lungs, and other organs.

As such, it’s critical to start heartworm-preventative medication at the appropriate age to minimize the risk of heartworm infection. The American Heartworm Society recommends starting heartworm prevention in puppies at 6-8 weeks of age and continuing treatment year-round.

Even if a dog has already started their preventative medication after 6-7 months of age, it’s essential to remain diligent with the administration to prevent heartworm disease.

Early Detection and Diagnosis of Heartworm Disease

Regular heartworm testing for dogs is essential, especially during the susceptible age range. Testing should be conducted at least once a year and more frequently if recommended by your veterinarian. Early detection of heartworm infection is critical, as the disease can progress quickly and become more challenging to treat.

There are two types of heartworm tests available: antigen tests and microfilaria tests. Antigen tests detect the presence of adult heartworms in a dog’s bloodstream, while microfilaria tests detect the presence of heartworm larvae in a dog’s blood.

Your veterinarian may recommend a combination of both antigen and microfilaria tests to ensure accurate results. It is crucial to follow your veterinarian’s testing guidelines and schedule, as missing a test can delay diagnosis and treatment.

Remember, heartworm disease is much easier to prevent than to treat. Ensuring that your dog is on a consistent heartworm prevention regimen and receiving regular veterinary care is crucial in preventing the spread of this potentially fatal disease.

Symptoms of Heartworm Disease in Dogs

Heartworm disease can be silent for months, making early detection challenging. However, as the infection progresses, certain symptoms become more pronounced, indicating advanced stages of the disease.

Early symptoms of heartworm disease in dogs may include:

  • Mild cough
  • Lethargy or decreased activity level
  • Decreased appetite
  • Weight loss
  • Reluctance to exercise

As the disease progresses, heartworms begin to damage the heart and lungs, leading to more severe symptoms such as:

  • Severe coughing
  • Difficulty breathing
  • Bulging chest
  • Collapsing, fainting, or convulsions
  • Heart failure

If your dog displays any of these symptoms, it is important to seek veterinary care immediately. Your veterinarian can perform a heartworm test to confirm or rule out the presence of heartworms in your dog and recommend appropriate treatment options.

Tip: Annual heartworm testing is recommended for all dogs, especially those in areas with higher rates of the disease, to ensure early detection and prompt treatment.

Prevention of Heartworm Disease in Dogs

Preventing heartworm disease is crucial to safeguard your furry friend’s health and well-being. There are several heartworm preventive medications available, including chewable tablets, topical solutions, and injectables. These medications work by killing the heartworm larvae before they can develop into adult worms in the dog’s bloodstream.

Heartworm preventive medications are highly effective when administered properly, according to the recommended dosing schedules. It is crucial to start administering preventive medication to puppies at six to eight weeks of age or as soon as possible after adoption or rescue.

Puppies must receive monthly preventive medication until they reach six months of age, after which they can transition to an annual preventive medication regimen.

Heartworm Preventive Medication Options for Dogs

Preventive Medication TypeBrand Name
Chewable tabletHeartgard Plus, Interceptor Plus, NexGard
Topical solutionRevolution, Advantage Multi
InjectableProHeart 6, ProHeart 12

It is crucial to consult with your veterinarian to determine the most suitable preventive medication for your dog, based on their age, health status, and lifestyle. Administer preventive medication as directed, and do not miss a dose. If you miss a dose, contact your veterinarian immediately for proper guidance.

Additionally, it is essential to practice environmental control strategies to minimize your dog’s exposure to mosquitoes, the primary carriers of heartworm larvae. These measures include removing stagnant water sources, using mosquito repellents, and keeping your dog indoors during peak mosquito activity times, such as dusk and dawn.

By taking proactive preventive measures, you can protect your furry friend from heartworm disease and ensure their health and happiness.

Treatment Options for Heartworm Disease in Dogs

If your dog has been diagnosed with heartworm disease, prompt treatment is crucial to improve their prognosis. The treatment process can be complex and prolonged and requires veterinary guidance throughout. The treatment plan typically consists of three stages:

  1. Killing the adult heartworms
  2. Killing the microscopic larvae that are circulating in the bloodstream
  3. Rebuilding the dog’s immune system and overall health

Each stage may involve a different medication or combination of medications, including injections and oral medications. Complications may arise during treatment, such as blood clots or inflammation, so close monitoring and follow-up by a veterinarian are necessary.

It’s important to note that treating heartworm disease can be expensive, so pet owners should be prepared for the financial commitment. Some pet insurance policies may cover heartworm treatment, so it’s worth checking with your provider.

Prevention of heartworm disease through regular testing and administration of preventive medications is more cost-effective than treating the disease. However, if your dog does become infected, prompt and proper treatment can improve their prognosis and quality of life.

Additional Measures for Heartworm Prevention

In addition to preventive medications, there are other measures dog owners can take to protect their furry friends from heartworm disease.

Mosquito control: Since heartworms are transmitted through the bite of infected mosquitoes, minimizing your dog’s exposure to mosquitoes can help reduce the risk of infection. Avoiding outdoor activities during peak mosquito hours and using mosquito repellent on your dog can be helpful strategies.

Annual vet visits: Regular veterinary check-ups can ensure that your dog is healthy and can detect potential heartworm infections early before the condition worsens.

Environmental control: Keep your dog’s environment clean and free of standing water, which can serve as breeding ground for mosquitoes. Trim tall grass and bushes around your home, and clean up any leaf litter or debris that could hold moisture.

By implementing these additional measures and adhering to preventive medications, you can keep your dog safe from heartworm disease.

The Role of Dog Owners in Heartworm Prevention

While a veterinarian plays an important role in preventing heartworm disease, dog owners also have a crucial responsibility in keeping their furry friends safe. Here are some guidelines to help prevent heartworm disease:

  • Administer preventive medication: Regular administration of preventive medication is essential to protect dogs from heartworm infection. Speak with your veterinarian to determine the most effective medication for your dog and adhere to the recommended dosing schedule.
  • Schedule annual veterinary visits: Regular check-ups with a veterinarian can help detect heartworm disease early and provide an opportunity to discuss appropriate preventive measures.
  • Practice mosquito control: Mosquitoes are the primary mode of heartworm transmission, so it’s important to minimize exposure to mosquitoes whenever possible. This can be achieved by limiting outdoor activities during peak mosquito hours and using mosquito-repellent products.

By following these guidelines, dog owners can take an active role in preventing heartworm disease and keeping their furry companions healthy for years to come.


Q: At What Age Can Dogs Get Heartworms?

A: Dogs can become infected with heartworms at any age, including puppies as young as 6-8 weeks old. It is important to start heartworm prevention as early as possible to protect your dog from this potentially life-threatening disease.

Q: How does heartworm disease develop in dogs?

A: Heartworm disease develops when a dog is bitten by an infected mosquito. The mosquito transmits heartworm larvae into the dog’s bloodstream, where they mature into adult worms and migrate to the heart and lungs. The lifecycle of heartworms within a dog’s body can take several months and can cause significant damage if left untreated.

Q: When are dogs most susceptible to heartworms?

A: Dogs are most susceptible to heartworms during the developmental stages of the larvae. This is usually between 6-7 months after being bitten by an infected mosquito. It is crucial to begin heartworm prevention before this age to prevent infection.

Q: How can heartworm disease be detected in dogs?

A: Regular heartworm testing is essential for early detection of the disease. There are different types of heartworm tests available, including antigen tests and microfilaria tests. Consult with your veterinarian to determine the appropriate testing schedule for your dog.

Q: What are the symptoms of heartworm disease in dogs?

A: The symptoms of heartworm disease can vary depending on the severity of the infection. Common early symptoms include coughing, fatigue, and decreased appetite. In advanced stages, dogs may experience difficulty breathing, weight loss, and even heart failure. If you notice any of these symptoms, seek veterinary care immediately.

Q: How can heartworm disease in dogs be prevented?

A: Heartworm disease in dogs can be prevented through the use of preventive medications. There are various options available, including monthly chewables, topical treatments, and injectables. It is important to choose a preventive medication that is suitable for your dog’s breed, age, and weight. Consult with your veterinarian to determine the best prevention plan.

Q: What are the treatment options for heartworm disease in dogs?

A: Treatment for heartworm disease in dogs can be complex and costly. It typically involves a series of injections to kill the adult worms and may require additional medications to address any complications. It is important to seek veterinary guidance for proper diagnosis and treatment.

Q: Are there additional measures to prevent heartworm disease?

A: In addition to preventive medications, dog owners can take additional measures to minimize the risk of heartworm disease. This includes minimizing exposure to mosquitoes by keeping dogs indoors during peak mosquito activity, using mosquito repellents, and addressing any standing water sources in the surrounding area.

Q: What is the role of dog owners in heartworm prevention?

A: Dog owners play a crucial role in preventing heartworm disease. This includes administering preventive medications as prescribed, scheduling regular veterinary check-ups, and practicing mosquito control measures. By being proactive and responsible, dog owners can help keep their pets safe from heartworm disease.


Heartworm disease is a serious condition that can be fatal if left untreated. As discussed in this article, dogs of all ages can develop heartworm disease, but they are most susceptible during certain developmental stages.

Therefore, it is crucial for dog owners to start preventative measures at the appropriate age and adhere to recommended guidelines.

Regular check-ups with a veterinarian and heartworm testing are essential to keep dogs safe and detect any potential infections early on. Along with heartworm preventative medications, environmental control strategies, such as minimizing exposure to mosquitoes, can further reduce the risk of heartworm disease.

Ultimately, the responsibility of preventing heartworm disease falls on dog owners. By following the proper guidelines and prioritizing their canine companion’s health, dog owners can ensure their furry friend’s well-being and prolong their life.

Please take the time and leave a comment below if this article helped you, or you have any additional questions.

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