Why You Won’t See Your Dog’s Heartworm Symptoms Until It’s Advanced

Heartworm or dirofilariasis is a serious and fatal condition typically found in dogs, cats, wolves, and ferrets.

The blood-borne parasitic roundworm called Dirofilaria immitis is responsible for the cause of this disease. The bite of a mosquito transmits it.

Although this invisible disease causes heart failure, severe lung disease, and organ damage, it still goes unnoticed, and here’s why. 

The Life Cycle of Heartworms in Dogs

The life cycle begins when the female mosquito bites an infected dog. This mosquito becomes infected with microfilariae. 

Over the next two weeks, the microfilariae become infective larvae under suitable environmental conditions. They stay in the mosquito’s gut. 

The infected larvae enter the mouthpart of the mosquito. When it bites a dog, the bite creates a wound, and larvae enter the dog’s bloodstream.

Since the dog is a definitive host, these larvae complete their maturation inside the body. It takes about six months for the infected larvae to grow into adult heartworms. 

Once the larvae have matured, they mate and produce offspring inside the dog’s body to complete their life cycle. Inside the dog’s body, the lifespan of heartworms is about five to seven years. 

The female infectious entities grow as long as 12 inches, and male heartworms grow about 7 inches long. They look similar to a strand of noodle.

What Do Heartworms Do to the Dog?

An infected dog can have about one to 250 heartworms. It is called the worm burden. These spaghetti-like strands can grow up to 10 inches in length and cause blockage of arteries.

Heartworms can block the blood flow within the heart, which causes heart failure. Besides this, they also live in associated blood vessels and lungs and clog the pulmonary artery.

Once they clog the main blood vessels, the blood supply reduces. As a result of reduced blood flow, the kidneys, liver, and lungs are affected. Decreased levels of oxygen cause organ malfunction, which poses the risk of death.

What Are the Symptoms of Heartworms in Dogs?

To our dismay, heartworms do not have obvious symptoms. The severity of the disease depends on the worm’s burden and the timeframe of infection. 

In the early stages of the disease, there are no heartworm symptoms in dogs.

The clinical symptoms appear once the disease has affected and spread to the heart and lungs. Before that, the dog appears healthy, and this is why you won’t see heartworm symptoms in dogs until they are advanced.

Symptoms start to develop once the disease persists for longer. As the worm burden increases, the clinical signs become more visible. 

Heartworm symptoms in dogs include:

  • Mild persistent cough
  • Decreased appetite
  • Weight loss
  • Tiredness after moderate activity
  • Labored breathing
  • Swollen belly due to excess fluid
  • Caval syndrome
  • Lethargy 

The most important thing that dog owners should remember is that before the symptoms even start to show, the disease has caused damage. Sometimes, symptoms do not appear even when the disease has spread to other organs.

If a heartworm is left untreated, it can damage the dog’s vital organs such as the heart, liver, kidneys, and lungs, which ultimately causes death.

Can a Dog Fully Recover From Heartworms?

Heartworm is a progressive disease, but it is also successfully treated if detected earlier.

The first step is to test the dog, which requires a few simple blood tests. The test recognizes the heartworm proteins. 

As the treatment for heartworm is expensive, it’s necessary to know the requirements.

When the dog tests positive for an antigen test, an additional blood test must confirm the diagnosis.

Once the diagnosis is confirmed, the second step is to stabilize your dog. After that, the veterinarian will treat your dog with injectable medications. It includes Melarsomine dihydrochloride, which is injected into the muscles to kill heartworms.

The treatment also involves a lot of other blood tests, x-rays, and radiography. Your dog may need hospitalization for three to four days. 

The treatment can also cause side effects and serious complications, such as the appearance of blood clots. The veterinarian conducts another test approximately six months after the treatment. This test confirms the complete removal of heartworms.

The treatment may cost you a fortune; however, the good news is that it can save your dog’s life. 

How to Prevent Your Dog From Getting Heartworm Disease? 

Preventive methods and medication are crucial for your pet’s well-being, and they are relatively affordable than the treatment. That is why when we talk about heartworms, prevention is indeed better than cure.

Here are some tips that can help you prevent your dog from getting heartworms. 

  • Get your dog tested for heartworms annually
  • Use preventative heartworm medicine
  • Give your dog home-cooked meals or organic dog food
  • Take your dog for regular checkups
  • Avoid exposure to mosquitoes

Other than this, you can also consult with your vet to get preventative medication, including chewables or topical cream to prevent mosquito bites.  

Can Heartworms Transmit From One Dog to Another?

The female mosquito is the intermediate host that is responsible for transmission.

Since heartworm is not a contagious disease, it does not spread from dog to dog. Your dog is only at risk during the mosquito season.

If you protect your dogs from mosquitoes and keep them inside, they are at a lower risk for heartworm. 

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Can Humans Get Heartworms From Dogs?

Humans can get heartworms if bitten by an infected mosquito. However, the larvae never fully mature in the human body as it is not their definitive host.

Rarely, when humans do get heartworms, the body responds by inflammation and tries to kill heartworms. It can cause a fever and a few other symptoms. The condition is called pulmonary dirofilariasis


Heartworm can go unnoticed very often; therefore, your dog is in dire need of regular checkups.

A negative test can indicate that preventative measures work and can keep your dog safe and healthy. This disease is fatal, and it is better to take as many precautions and preventive measures as possible. 

With timely treatment, you can save your dog from this life-taking disease.

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