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Heartworms in Dogs: Signs, Causes & Treatments (Vet Answer)

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By Dr. Iulia Mihai

Adult heartworm dirofilaria removed from a dog in a kidney dish

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Dr. Iulia Mihai

DVM MSc (Veterinarian)

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Dirofilariasis in dogs—or heartworm disease, as this condition is also known—can endanger your pet’s life if left untreated. This condition is transmitted by mosquitoes, which inoculate the larval stages of this parasite when they feed on an animal’s blood. Although they are called heartworms, the parasites only enter the heart in the final stages of the condition, when they are pushed from the main pulmonary artery (where they reside) retrograde into the heart (into the right ventricle and then into the right atrium).

In this article, you will learn about the particularities of this disease, how dogs get heartworms, and tips to prevent it.

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What Is Heartworm Disease in Dogs?

Heartworm disease, or dirofilariasis, is caused by a roundworm, Dirofilaria immitis. Mosquitoes transmit this parasite when they feed on blood.

The severity of this disease in dogs is directly determined by the number of heartworms present in the body and the time spent there. Massive changes will occur in the heart and lungs if the number of parasites is high or the disease is not treated. In turn, this can lead to life-threatening respiratory problems.

The most common clinical signs of heartworm disease in dogs are a dry cough and intolerance to exercise—things that may indicate that a dog has heart problems. In the advanced stages of the disease, fatal heart and organ failure can also occur.

Therefore, this condition should not be underestimated and should definitely be treated by a vet.

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What Are the Signs of Heartworms in Dogs?

Dirofilariasis is a disease that first settles in the pulmonary arteries, where it causes inflammation, lung hypertension, and vascular dysfunction. As it progresses, it reaches the chambers of the heart. Heartworm disease is a condition in which the most common clinical signs of the disease in dogs are:

  • Dry cough
  • Lack of resistance to effort
  • Matted fur
  • Weight and muscle mass loss
  • Seizures (sometimes, in severe cases)

Clinical signs will vary depending on the severity of the condition. Dirofilariasis has four classes of severity:

  • Class 1 — Dogs are usually asymptomatic (no clinical signs) or show mild clinical signs, such as a rare cough.
  • Class 2 — Dogs show dry cough and intolerance to exertion.
  • Class 3 — Dogs lose weight, have matted fur, lose muscle mass, and have difficulty breathing due to fluid accumulation in the abdominal cavity.
  • Class 4 — This is the last stage of the disease, and the condition is impossible to treat. Dogs are infested with a huge number of parasites that are blocking the blood flow to the heart.

In severe cases of the disease, heart failure leads acutely to serious circulatory problems (vena cava syndrome). The affected dog often dies within 24–72 hours. These sudden clinical signs may manifest as follows:

  • Acute lethargy and weakness
  • Pale mucous membranes (anemia)
  • Liver failure
  • Kidney failure
  • Weak pulse
  • Rapid heart rate (tachycardia)
  • Increased breathing rate (tachypnea)
  • Bloody urine (hemoglobinuria)
maltese sick dog
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What Are the Causes of Heartworms in Dogs?

These parasites are transmitted through the bite of a mosquito that carries the infective L3 larvae. The larvae then migrate through the dog’s body to the heart and the blood vessels of the lungs, a process that takes about 6 months. Dogs that live in environments where they are exposed to mosquito bites present the highest risk of developing the disease.

Dirofilaria immitis is the roundworm responsible for this condition. Its life cycle begins when a mosquito feeds on blood infected with D. immitis microfilariae from a sick dog. In the mosquito’s body, the microfilariae develop into infective L3 larvae. These larvae can complete their life cycle only when they reach a dog’s body. Otherwise, they can stay in this stage for up to 2 years.

When a mosquito that carries the L3 larvae bites a dog to feed, it transfers these larvae into the dog’s blood through its saliva. The L3 larvae migrate in the bloodstream and move to the pulmonary arteries and later to the heart. They reach the adult stage in 6–7 months, at which time, they also become sexually mature (i.e., they can reproduce). Once the adults (male and female) reproduce, the microfilariae will appear, which will be present in the blood of the affected dog.

Adult worms can live 5–7 years, and females can produce millions of microfilariae during this time.

How Is Heartworm Disease Diagnosed?

To diagnose heartworm disease, several test methods are used.

veterinarian conducting a blood test in miniature schnauzer dog
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1. Blood Tests

When heartworm disease is suspected, the veterinarian will recommend blood tests (blood biochemistry, complete blood count, and rapid antigen tests, e.g., 4DX test) and observe the microfilariae in the peripheral blood under a microscope). Antigen tests can detect live adult Dirofilaria females that are sexually mature.

A PCR test can also be recommended. This test enables the detection of DNA material, helping to distinguish the respective species of heartworm. This test is more sensitive for detecting heartworm larvae.

2. Echocardiography

The diagnosis is made with certainty when heartworms can be seen in the arteries or the heart on echocardiography. In addition, this test can accurately determine the extent of changes in the heart.

3. X-rays

Lung X-rays can help diagnose the changes that already exist in the vessels and lung tissues.

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How Is Heartworm Disease Treated?

Heartworms in dogs are only treated after the diagnosis is confirmed through blood tests. The treatment recommended by the veterinarian will target the adult worms but also the microfilariae. In addition, the vet may simultaneously treat the clinical signs and hospitalize your dog during the treatment. In this way, they can act promptly in case of complications, such as allergic reactions to dying microfilariae.

Certain bacteria live in heartworms (Wolbachia). These bacteria help the worms multiply. For this reason, the vet will recommend a long-term antibiotic treatment to “sterilize” the heartworms so they can stop producing microfilariae.

In severe cases, when there is a massive infestation, the veterinarian can recommend a surgical intervention to remove these parasites. When there are many heartworms gathered in one place, they can form a ball and block the blood flow. In addition, when they die, they can cause a blockage.

sick border collie dog at the vet clinic
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How Do I Care for a Dog With Heartworms?

First, follow the veterinarian’s instructions and treatment. Second, make sure your dog gets plenty of rest from the time of diagnosis. They should only be walked on a leash and should not be allowed to play with other dogs. This way, you will reduce the progression of the permanent changes in the lungs.

When dogs play, walk for too long, or have intense physical activity, their heart rate increases, which will lead to an increase in blood flow to the lungs. Heartworms that migrate or die will be “washed” and transported through the blood vessels, which can lead to pulmonary embolism, which will block blood flow and put your dog’s life in danger.

To ensure that your four-legged friend does not get bored, replace long walks and intense play with mental activities, such as dog puzzles, learning a new trick, etc.

To prevent heartworm disease, it is necessary and vital to apply antiparasitic products to your dog regularly (to prevent mosquito bites). You can also avoid walking your dog around bodies of water like lakes in the evening and at night.

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Frequently Asked Questions (FAQs)

How Long Can a Dog Live With Heartworms?

Depending on the severity of the condition, a dog diagnosed positively with Dirofilaria can live from a few weeks to a few months (6–7 months) if organ damage has occurred. The veterinarian will make the best recommendations to treat your dog. In mild cases, the condition can be cured, and your dog can live a long, healthy life.

Do Heartworms Show in Dog Poop?

No, heartworms won’t show in your dog’s feces because they don’t live and thrive in the digestive tract. Heartworms (Dirofilaria immitis) are roundworms that live in the circulatory system, more precisely in the pulmonary arteries and the heart.

Can People Get Heartworm Disease From Dogs?

No, people cannot get heartworm disease from dogs. Heartworms are only transmitted by mosquitoes that carry the infective L3 larvae. The transmission occurs when the infected mosquito feeds on an animal’s blood.

weimaraner dog check by vet
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Dirofilariasis or heartworm disease is a severe condition in dogs transmitted by mosquitoes. In the early stages of the disease, dogs can be asymptomatic or present only a mild cough from time to time. In severe cases, the affected dogs will lose weight, their muscle mass will decrease, and they will have matted fur, respiratory problems, anemia, liver and kidney failure, tachycardia, and tachypnea. If organ damage occurs, even if therapy is instituted, the affected dog will survive from a few weeks to months. To prevent the disease, it is recommended to use external antiparasitic products that can keep mosquitoes away.

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