How Dogs Get Heartworm [2 Possible Ways]

How dogs get heartworm
How dogs get heartworm

If you’re a dog owner, one of the most important things you should know is how to keep your furry friend healthy and happy.

One common health issue that dogs can face is heartworm. But how do dogs get heartworm?

In this article, we’ll explore the causes and symptoms of heartworm, and give you some tips on how to prevent this potentially deadly disease from affecting your pet.

How Dogs Get Heartworm

Heartworm - How Dogs Get Heartworm
Heartworm – How Dogs Get Heartworm

Heartworms are transmitted to dogs through two major routes: through an infected mosquito bite or the consumption of Dirofilaria immitis-infected meat or food.

Heartworm infection causes severe lung illness, heart failure, other organ damage, and death in dogs.

Dogs become infected with heartworm when they are bitten by mosquitoes carrying the parasite.

First discovered in the United States in 1944, heartworm has now spread to most of North America and parts of Europe.

A dog’s risk of contracting heartworm disease depends on where you live and what type of mosquito carriers are prevalent in your area.

Learn more about different ways dogs get worms.

Signs of Heartworm in Dogs

Signs of Heartworm in Dogs
Signs of Heartworm in Dogs

Here are some common symptoms or signs of heartworms in dogs:

  1. Cough: A persistent cough, especially after exercise or exertion, is one of the most common signs of heartworm disease in dogs.
  2. Shortness of breath: As the disease progresses, dogs may experience shortness of breath or difficulty breathing.
  3. Fatigue: Dogs with heartworm disease may tire easily and show signs of fatigue or lethargy.
  4. Loss of appetite: Dogs may lose their appetite as the disease progresses, leading to weight loss and malnutrition.
  5. Weight loss: As the disease progresses, dogs may lose weight despite eating the same amount of food.
  6. Swollen abdomen: In severe cases, dogs may develop a swollen abdomen due to fluid accumulation in the abdomen.
  7. Reduced exercise tolerance: Dogs with heartworm disease may show reduced exercise tolerance, become easily exhausted, or show reluctance to exercise.
  8. Vomiting: Dogs may vomit or have diarrhea as a result of heartworm disease.
  9. Changes in behavior: Dogs with heartworm disease may show changes in behavior such as depression, irritability, or restlessness.

Learn more about the common symptoms of worms in dogs.

The Four Stages Of Heartworms In Dogs

Here are the five basic stages of heartworm in dogs:

Stage one

The initial stage of heartworm infection in dogs is usually symptom-free.

Heartworms are present and settle inside the heart at this time.

However, the illness has not advanced to the point where the heartworms have formed a new generation of microfilariae and the dog’s body has produced antigens in sufficient amounts to be detected.

Stage two

Heartworms in dogs at this stage are characterized by mild symptoms such as resistance to activity and a persistent cough.

At this stage, the heartworms have been resident in the body long enough to produce antibodies and possibly microfilariae.

During this time, blood tests may be used to identify heartworm disease.

Stage three

In the third stage, the spread of the disease symptoms will be obvious and have a significant influence on the dog’s health.

Dogs will experience much coughing and exhaustion will persist after any activity, and dogs may be hesitant to exercise at all.

The dog may also have respiratory problems, most of the time the dog may also cough up blood at this time.

On X-rays, the heartworms are visible at this stage, and the worms in the heart and major vessels are also visible.

Stage four

Heartworm disease symptoms in dogs at this stage are quite evident.

These symptoms are accompanied by long-term health consequences for the dog.

The dog will be in a critical condition and will be hesitant to exercise, will be weary afterward, and will cough.

The dog will have difficulties breathing as well as abnormal noises in the dog’s heart and lungs, as well as an enlarged liver, which may indicate the disease’s impact during testing.

Even with therapy, there is a substantial chance of long-term debilitation and mortality at this stage of the disease.

Final stages

The final stages of heartworms in dogs are the adult stages of the heartworm which take 6 months and the heartworm can live for 5 to 6 years of the lifecycle.

During the final stages, there will be abnormal sounds in the dog’s heart and lungs, as well as an enlarged liver, which might indicate the disease’s impact.

Even with treatment, there is a significant risk of long-term disability and death at this stage of the disease.

Learn more about live worms in your dog poop after deworming.

Type of mosquitoes transmit Heartworm to dogs

The mosquitoes Aedes, Anopheles, and Mansonia are all capable of spreading or transmitting heartworm to dogs.

Mosquitoes transmit heartworm by biting an infected host and then passing the parasite to another host during a blood meal.

How mosquitoes transmit Heartworm to dogs

Heartworm is a parasite that can be transmitted to dogs through mosquito bites.

When an infected mosquito bites a dog, the mosquito leaves a wound on the skin of the dog and then passes on microfilaria by creating a path for infective larvae to enter their new host and continue their lifecycle in the dog’s bloodstream.

The worm’s larvae are spread through the blood flow and grow into mature worms inside the dog’s heart and lungs.

The most common way a dog comes in contact with heartworm is by being bitten by an infected mosquito.

It can also be transmitted if the dog eats an animal that already has the worm, or if it gets infected by its mother who had been bitten before giving birth to her pups.

Types of heartworm carried by mosquitoes

Despite having a similar name, heartworm disease is not found in just dogs. There are two types of heartworm disease in dogs:


‘Heartworm-positive’ refers to the dog’s body being infected with heartworm parasites, which can be picked up through mosquito bites.

Dogs can get heartworm disease from one of two places: by being bitten by a mosquito, or when they’ve eaten an infected piece of meat.

Dogs are vaccinated against heartworm, but in countries where the risk of bites is high, the virus often evades these vaccinations

‘Heartworm-positive’ animals are put to sleep because of their poor prognosis, and it is thought that between 40 and 70 percent of dogs euthanized in the US for heartworm have only been infected with the heartworm-negative species.


‘Heartworm-negative’ refers to a dog being infected with a different species of heartworm, but not having any symptoms.

In other words, heartworm-positive dogs are the most likely to die of heartworm, but with the rarest type of heartworm-negative dog in the UK, it’s likely that most of them will still develop heartworm.

Unfortunately, most dogs in the UK have been vaccinated against heartworm, which means they can’t get the virus anyway.

But if they’re bitten by a mosquito, have eaten contaminated meat containing the virus, or maybe even because of a stray dog’s heartworm infection, they can get the virus.

As a result of eating an infected meal, dogs can start to show signs of the disease within two weeks.

Dogs can’t contract the same kind of heartworm that affects humans in the US (the disease which causes ‘Africa Fever’) but a dog can have a heartworm parasite in his or her heart and get heartworm disease as a result.

Animals who survive heartworm disease are often left incontinent, as the disease causes the dog’s heart muscles to get flabby.

They can also develop a wasting disease, which means that their muscles get ‘shrunken’, and eventually, the heart will shut down, causing it to stop pumping blood.

Preventing Heartworms in Dogs

Heartworm disease is a serious and potentially deadly disease in dogs, and preventing it is much easier than treating it.

Here are some common ways to prevent a dog from getting heartworms:

  1. Monthly Heartworm Preventative Medication: This is the most common method for preventing heartworm disease in dogs. Heartworm-preventative medications come in different forms, including pills, topical solutions, and injections. These medications work by killing immature worms before they have a chance to grow and start causing damage to the heart and lungs.
  2. Annual Heartworm Testing: Even if your dog is taking a monthly preventative medication, it is still important to have annual heartworm testing done to make sure that the medication is working and to catch any infections early.
  3. Mosquito Control: Since heartworms are transmitted by mosquitoes, controlling your dog’s exposure to mosquitoes is essential in preventing heartworm disease. Use mosquito nets, fans, and bug repellents, and avoid taking your dog out during peak mosquito hours, which are typically at dawn and dusk.
  4. Keep Your Dog on a Leash: Keeping your dog on a leash during walks can help prevent your dog from wandering off into mosquito-infested areas.
  5. Avoid Known Heartworm-Infested Areas: If you live in an area where heartworms are common or if you are traveling to an area with a high prevalence of heartworm disease, take extra precautions to protect your dog, such as staying indoors during peak mosquito hours or keeping your dog on a preventative medication.
  6. Keep Your Dog’s Environment Clean: Removing standing water around your home, cleaning up pet waste promptly, and keeping your yard tidy can help reduce the number of mosquitoes in your dog’s environment.
  7. Keep Your Dog Healthy: A healthy immune system is better equipped to fight off infections, so keeping your dog in good health is an important part of heartworm prevention.
  8. Regular Veterinary Care: Regular exams with your veterinarian can help catch heartworm infections early, and they can give you advice on how to effectively prevent heartworm disease.
  9. Spay or Neuter Your Dog: Studies have shown that heartworm incidence is higher in intact dogs, so spaying or neutering your dog can help reduce the risk of heartworm disease.

Learn more about ways to improve your dog’s health.


Do dogs poop out heartworms?

No, dogs cannot pass heartworms through feces therefore, heartworms cannot be passed directly from one host to another, it requires mosquitoes to transfer the heartworm larva.

How long will it take a dog to start showing symptoms of heartworms

On average a dog will show symptoms of heartworm disease after 2 weeks of being infected by an infected mosquito or after consumption of heartworm larva by eating infected meat.

How long will it take for a heartworm larva to grow to adult stages

When a healthy dog is bitten by an infected mosquito and the larva is transmitted to the dog, it takes 6 months for the larva to develop into an adult.

During this period a lot of internal organs are damaged.

What locations have the highest risk of heartworm in the U.S.?

Heartworm is very rare in the Phoenix area but is more common in Central Arizona, U.S., where the mosquito population is high.

Heartworm is a type of mosquito-borne infection that spreads to your pet from an infected mosquito bite.

For instance, dogs living in warmer climates like southern Florida have a higher risk than those living in more northern states like Canada or New York.

There are many reasons your dog may have gotten heartworm, but the most common is when you’re in a community where there are infected mosquitoes.

Heartworm is a fatal disease, and it kills dogs. The most common cause of death is fluid in the lungs.

That’s why preventing a dog’s exposure to infected mosquitoes is so important.

Learn more about how long it takes worms to leave a dog after deworming.


In conclusion, heartworm is a serious and potentially fatal disease that can affect dogs of all ages and breeds.

It is caused by a parasitic worm that is transmitted through the bite of an infected mosquito.

While heartworm is preventable, it is important to be aware of the risks and take steps to protect your furry friend.

Regular check-ups with your veterinarian, preventative medication, and mosquito control measures can all help to keep your dog safe and healthy.


By Aiguo Kai

Aiguo Kai is one of the authors and editors of Pet Creeks, he is focused on teaching and providing useful information about pets. Especially dogs, he enjoys writing and sharing his 10 years of experiences with dogs so far.