Heartworm in Dog: Causes, Symptoms, Cure & Prevention

Heartworm in dog, also called dirofilariasis, is a really serious and sometimes deadly sickness. It happens when a parasite, called Dirofilaria immitis, gets into a dog’s bloodstream.

These parasites grow up and make their home in a dog’s heart, the big blood vessel close to it (called the pulmonary artery), and other nearby big blood vessels. Sometimes, you might even find them in other parts of the dog’s blood system. The lady parasites can get pretty big, about 6 to 14 inches long and really skinny, just 1/8 of an inch wide. The guys are smaller, about half the size of the ladies. If a dog gets infected, it could have as many as 300 of these worms inside it!

These parasites can live for a long time, about five to seven years. During this time, the lady worms make a lot of babies, called microfilaria. These tiny babies mainly hang out in the small blood vessels in the bloodstream.

Life Cycle of Heartworm

The heartworm in dog life cycle is a bit tricky; it needs a mosquito to help it out before it can finish growing inside a dog. Around 30 kinds of mosquitoes can pass on heartworms.

The cycle begins when a lady mosquito bites a dog that has heartworms and slurps up some baby heartworms during her meal. These baby heartworms grow up inside the mosquito for about 10 to 14 days, then move into its mouthparts. Now, they’re ready to infect another dog. When the mosquito bites another dog, these baby heartworms get into the dog’s body.

Once inside the dog, these baby heartworms travel through the blood to the heart and nearby blood vessels. There, they grow up into adult heartworms, mate, and make more baby heartworms in about 6 to 7 months. And the cycle continues


Heartworm in dog can be found all around the world. In the United States, most cases are in the southeastern part, but it’s been seen in all 50 states. In Canada, it’s a problem in areas where there are lots of mosquitoes, like along rivers and coastlines in some provinces such as British Columbia, Manitoba, Ontario, Quebec, and New Brunswick.

Things like what kind of mosquitoes there are, the weather, and if there are animals that can carry the disease can affect how common heartworms in dog. The risk is highest when mosquitoes are active, which usually happens when it’s warmer than 50°F (10°C).

How Heartworm Spread

Heartworm in dog doesn’t pass directly from one dog to another. Instead, it needs mosquitoes to help it spread. So, when mosquitoes are active, usually during mosquito season, the disease spreads. In the United States, this season can last all year in many places. The more infected dogs there are and the longer mosquito season lasts, the more heartworm disease there is in an area.

What do heartworms do to dogs?

It can take a few years before dogs show signs of being sick. Any dog, no matter the age, breed, or sex, can get heartworm disease. But it’s not common in dogs under one year old because it takes about 5 to 7 months for the tiny heartworm babies to grow into adult heartworms. By the time dogs start showing signs of being sick, the disease is usually pretty far along.

Adult heartworms cause heartworm disease by messing with the heart and blood vessels leading to the lungs. They make the blood vessels narrow and scarred, which makes it hard for blood to flow. This makes the heart have to work harder to pump blood to the lungs for oxygen.

Heartworms can also mess with the heart valves, making the heart work even harder. This can lead to heart failure. They can also affect the kidneys or liver by damaging other blood vessels in the body. How sick a dog gets from heartworms depends on where the worms are, how long they’ve been there, and how much damage they’ve done to the heart, lungs, liver, and kidneys. Even just a few worms can make a dog really sick.

The main signs of heartworm disease are coughing, shortness of breath, weakness, being tired all the time, and not being able to exercise like before. Some dogs might even faint or get confused after exercise. Your vet might hear strange sounds in the dog’s chest with a stethoscope.

In severe cases, fluid buildup can make the belly and legs swell. Dogs might also lose weight, look sick, and have anemia. Really sick dogs might suddenly die during exercise or when they’re excited.

Tiny heartworm babies, called microfilariae, travel around the body but mostly stay in small blood vessels. Since they’re about the same size as these vessels, they can block blood flow. When blood can’t get to where it’s needed, tissues don’t get enough oxygen and nutrients. Microfilariae mostly hurt the lungs and liver. Lung damage causes coughing, while liver damage leads to jaundice, anemia, and weakness. The kidneys might also be affected, letting toxins build up in the body.

How is heartworm in dog diagnosed?

Most of the time, a simple blood test can tell if a dog has heartworm disease. But sometimes, more tests are needed to make sure it’s safe to treat the disease. Before starting treatment, vets might do one or more of these tests:

  1. Antigen Test: This blood test looks for certain substances that adult heartworms produce. It’s called an ELISA test.
  2. Chest X-rays: These are pictures of the dog’s chest. They help the vet see how much damage the heart and lungs have before treatment.
  3. Heart Ultrasound: This test, also called an echocardiogram, uses sound waves to make pictures of the heart. It helps the vet see if the heart is enlarged, how well it’s working, and if there are worms in it.
  4. Bloodwork: This checks the levels of different things in the blood, like red and white blood cells. It helps the vet see if the heartworms have damaged any organs.

These tests help the vet figure out the best way to treat the dog’s heartworm disease safely.

How is heartworms disease treated?

Treating heartworm disease in dogs comes with some risks, but it’s rare for dogs to die from it.

When dogs are diagnosed with heartworms, they often already have pretty bad damage to their heart, lungs, blood vessels, kidneys, and liver. In some rare cases, the damage is so severe that it’s safer to focus on keeping the dog comfortable instead of trying to kill the heartworms. Dogs in this condition might only live for a few weeks or months. Your vet will guide you on the best approach for your dog if they have advanced heartworm disease.

To kill the adult heartworms, dogs get a series of shots with a drug called melarsomine (brands: Immiticide® or Diroban®). This drug is given over a few months. Dogs might also take antibiotics like doxycycline to fight off any bacteria that live with the heartworms.

After treatment, dogs need to rest a lot. As the adult heartworms die, they break apart and can cause problems in the lungs. So, it’s super important that dogs don’t do any exercise for about a month after treatment. If they get too active, it can be really dangerous.

The first week after treatment is the most critical because that’s when the worms are dying. Some dogs might cough a lot for about two months after treatment, especially if they had a lot of heartworms. If the coughing is really bad, let your vet know so they can help.

If your dog has any bad reactions after treatment, like not eating, having trouble breathing, coughing up blood, or feeling sick, get help from your vet right away. They might need medicine, rest, or fluids to feel better.

Along with killing the adult heartworms, dogs also get medicine to kill the baby heartworms. Sometimes, dogs need to stay at the vet for a bit after getting this medicine to make sure they’re okay. After treatment, dogs start taking medicine to prevent heartworms from coming back.

There are different ways to treat heartworms, so your vet will choose the best one for your dog’s situation.

Do dogs need more treatment?

Yep, if a dog has really bad heartworms disease, they might need more than just the regular treatment for heartworms. They could need antibiotics to fight infections, anti-inflammatory drugs to bring down swelling, and painkillers to help them feel better.

Sometimes, they might need special diets or diuretics, which are meds that help get rid of extra fluid in the lungs. And before treating the heartworms, some dogs might need meds to make their heart work better.

Even after the heartworms are gone, some dogs might need lifelong treatment for heart problems. This could mean taking diuretics, heart meds like ACE-inhibitors or beta-blockers, or eating special diets with less salt.

Prevention of Heartworm in Dogs

The best way to prevent your dog from getting heartworms is by giving them heartworm prevention medicine.

After a dog has been treated for heartworms, it’s really important to start giving them this medicine to stop them from getting heartworms again.

There are lots of safe and affordable options for heartworms prevention. Your vet can help you pick the one that’s right for your dog.