heartworm in dogs – symptoms and treatment

Heart Worm in Dogs: Symptoms & Treatment

Heartworm disease in dogs is a highly preventable disease. However, because of insufficient knowledge about this canine disease, many dogs are at risk of pulmonary and cardiac complications that ultimately lead to the dog’s demise. Increasing your knowledge about heartworm in dogs especially its signs and symptoms and how it is managed should help you realize the importance of taking more concrete measures to prevent this dreaded disease from infecting your beloved pooch.

What Causes Heartworm Disease in Dogs?

Heartworm disease in dogs is caused by the roundworm parasite known as Dirofilaria immitis. While the parasite makes dogs its host, our canine friends are not the only ones affected since mosquitoes are the only vectors that have been identified in the transmission of the disease from one animal to another. Regardless, our hounds are the principal hosts of Dirofilaria immitis parasites.

heart worm dog

The process begins when a mosquito bites a dog that has the parasite in its blood. The larvae of Dirofilaria immitis known as microfilariae are drawn together with the blood by the mosquito. Microfilariae further develop inside the mosquito until it reaches the 3rd stage of larval development. Since the parasite also develops inside the mosquito, these insects are also considered as intermediate hosts. When this very same mosquito lands on and bites another dog that is otherwise healthy, it injects microfilariae into the subcutaneous tissues of the dog where they stay for about 2 weeks, turning into stage 4 microfilariae.

About 1.5 to 2 months after the initial mosquito bite, the microfilariae creep towards the muscles of the abdomen and the chest, further developing into the 5thand final stage of larval development. Between 2.5 and 4 months after infection, the 5th-generation microfilariae enter the bloodstream and pass through the chambers of the right side of the heart before finally resting in the pulmonary artery. Within the next 3 to 4 months, Dirofilaria immitis adults grow in size. Females are known to grow about 7 centimeters longer than adult males with the average female length at 30 centimeters.

Seven months into the infection the parasite would have already mated and begin reproducing a new generation of microfilariae. These can stay inside the dog’s blood for 2 years just waiting for a mosquito to bring them to another host.

You May Also Like: Mosquito Repellent for Dogs

What are the Clinical Manifestations of Canine Heartworm Disease?

The signs and symptoms of heartworm disease in dogs are closely related to the different stages in the parasite’s complex lifecycle. As such, experts typically categorize the clinical manifestations into 4 stages.

  • Stage 1

You won’t see any clinical manifestations during this stage. Perhaps the only symptom you might see is a slight cough which you can easily dismiss as nothing more than minor throat irritation. Your dog will also appear quite happy and active. The laboratory tests will turn out negative, too.

  • Stage 2

In this stage your dog will be showing signs of mild respiratory distress accompanied by  much because of easy fatigability. Lab tests should already give you a heads-up on the status of your pet.

  • Stage 3

The manifestations in this stage grow in severity and extent. They will already show moderate respiratory distress as the growing parasite is slowly obstructing the flow of blood to the lungs. There will be incessant coughing to go along with more pronounced difficulty breathing. Your dog may no longer want to play or exercise as it becomes fearful it might not be able to breathe at all. It is also possible that your pet will be coughing up blood. X-ray scans can help provide a picture of the extent of the parasitic growth.

  • Stage 4

The symptoms of heartworm disease in this stage become increasingly more severe. In addition to the aforementioned manifestations in the earlier stages of the disease but in substantially more severe forms, your dog will also present with abnormal lung sounds, the presence of abnormal heart sounds like murmurs and gallops, and enlargement of the liver. These are all indicative of heart failure in dogs. If left untreated it can lead to cardiovascular collapse. If your dog reaches this stage without receiving appropriate treatment, there’s a strong chance your pet will lose its life.

How is Canine Heartworm Disease Treated?

The treatment for canine heartworm disease should be started as soon as a definitive diagnosis is made. It is important to keep in mind that as the disease progresses the more intensive, riskier, and more invasive the treatments. That is why it is imperative that early diagnosis is accomplished to initiate the treatment as early as possible. Here is how the disease can be treated:

  • Immiticide injections

An arsenic-based compound is injected into your dog’s bloodstream in an effort to kill the parasites. Immiticide is better-tolerated and is more clinically effective than its predecessor, Caparsolate. However, it is important that the dog rest for as long as a few months, to allow its body to absorb the dead heartworms. If your dog goes right back to its active lifestyle soon after treatment, there’s a possibility that the dead heartworms will get dislodged. This can send the remnants to the pulmonary tree, blocking smaller blood vessels resulting in respiratory failure. In many cases, this almost always spells certain death for the dog. The injection is repeated after several weeks until laboratory tests indicate there are no more heartworms in the blood.

  • Surgery

In severe cases of heartworm disease where the heart itself is already involved, surgery may be required to physically remove the bolus of worms.

  • Long-term Ivermectin therapy

Ivermectin administered every month has also been used in some cases. The dose of Ivermectin for heartworm disease is 3 times the normal dose for Ivermectin preventatives. Unfortunately, this treatment doesn’t kill adult heartworms until about 18 months of continuous treatment. The increased dose and chronic use of Ivermectin are simply not worth it because of potential complications.

dog heart worm

Managing heartworm disease in dogs is a very tricky endeavor. You may want to use Ivermectin but its safety implications are simply too great. Immiticide promises excellent outcomes but at the expense of a thousand dollars. That’s why preventing it with monthly preventatives is still the best way to go.

Related Post: Best Dog Pill Shooter


  1. Cardiology Team Saves Street Dog From Deadly Heartworm Condition, Cornell University College of Veterinary Medicine
  2. Why Animal Humane Society Transports And Treats Heartworm Positive Dogs, Animal Humane Society


Leave a reply

Please enter your name here
Please enter your comment!