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Heartworm in Dogs

Heartworms, the treacherous mosquito-transmitted parasites, find dogs as their primary hosts, causing rather serious disease. The disease has been known in the USA for over 150 years, causing infections mostly in dogs. In the coastal countries, the number of mosquitoes is greater, thus the occurrence of the disease is more common.

Heartworms can cause severe damage to the heart, lungs and their associated blood vessels (cardiopulmonary system); consequently, it can become a life-threatening condition. Frequent testing and prevention of the disease is essential, since the treatment can be tedious and long, followed with risk.

What Causes Heartworm in Dogs?

Heartworm disease is a disease caused by Dirofilaria immitis, a parasitic roundworm that invades the heart and lungs, including their associated blood vessels. It can persist in a dog’s body for up to seven years, causing life-threatening damage to the cardiopulmonary system, thus decreasing the overall pet’s wellbeing.1

How Do Dogs Get Heartworm?

Heartworm disease is a mosquito-borne disease. So there is a wide range of mosquito species that could be involved, depending on the region (urban, rural) or the landscape elements.2

Dog looking at mosquito infected with a heartwormMosquitos act as intermediate hosts; they are where the heartworm reaches its infective stage in about two weeks. After the dog has been bitten, the heartworm develops itself on that spot, primarily under the skin.

Later on, the heartworm enters the bloodstream, travels to the right side of the heart as well as the pulmonary arteries, causing damage and eventually heart failure, and pulmonary complications can occur. This is why this parasite is called “heartworm.” 3

So to be more specific, the infectious larvae matures into another stage 3-12 days after the infection. Then after 50-70 days, they are ready to invade the pulmonary arteries and the right ventricle. The microfilariae can live up to two years in the bloodstream, whereas adult worms can get to over seven years old. Mosquitos may then take it further into other dogs.4

Can Humans Get Heartworms From Dogs?

Humans cannot get heartworms directly from dogs and their body liquids. The infection takes place only when you get bit by a mosquito carrying the larvae. After the larvae mature into adult worms, they mostly die very soon, causing inflammation in the pulmonary arteries, as a reaction from the body because it wants to get rid of them.

The diagnosis is called pulmonary dirofilariasis. Mostly, people do not show any particular symptoms. But when they do, they can expect a cough, which can also include blood, fever, pain in the chest and even water in the lungs. In case you notice these symptoms, you should immediately seek medical help.

Heartworm cycle

Your doctor will probably suggest an x-ray. On the x-ray, the lesions usually look like coins on the chest. Besides x-rays, a CT scan is one of the diagnostic methods your doctor could perform.5

Treatment for removing heartworms directly from the blood is not necessary, since the lifespan of heartworms is not long. What is treated are the granulomas, which are possibly the result of tissue buildup in the arteries from the dead heartworms. Surgery may be suggested to prevent further disease. 6

Heartworm Symptoms in Dogs

Symptoms of heartworm in dogs depend mostly on the number of worms present in the dog’s bloodstream, the duration of the infection, and the immune reaction to the presence of the heartworms. A dog’s physical activity can also play a significant role in the disease and its severity, because the heavier blood flow induced by exercise causes things to develop quicker.

Dogs who are prone to a more sedentary and calm lifestyle, like bulldogs, bichons, chow chows or dogs with a low worm burden, do not develop such distinctive symptoms.

There are four stages of this heartworm disease. The higher the stage, the worse the manifestation of the disease. The symptoms in each of the stages are as follows:

  • First stage: Mostly no symptoms, since the heartworms are just settling into the heart, but sometimes an occasional cough can be noticed.
  • Second stage: Moderate symptoms with an occasional cough and fatigue after light exercise. This is when the heartworm can be detected via blood tests.
  • Third stage: This is when the disease becomes evident and severe symptoms occur. A persistent cough is present and heavy breathing, together with an overall sick appearance and malaise. In this stage, changes can be seen on chest x-rays.
  • Fourth stage: This is the last and most severe stage, also known as the caval syndrome. The number of worms is big enough to block the blood flowing back to the heart. When in this stage, surgical intervention is necessary and the only way to remove the heartworms from the body. This is indeed a life-threatening stage, so there is a great percentage of deadly outcomes.

Other symptoms include loss of appetite, weight loss, lack of energy, swollen chest and allergic reactions.

When Should Dogs Be Tested for Heartworms?

The time of year when tested, as well as the number of tests that the dog should get, will depend on various determinants:

  • Age of the dog when it started prevention or if it started the prevention at all
  • Duration of the heartworm season in the particular region
  • If the dog traveled to a region with a higher prevalence of heartworm disease

Your dog should also be tested before starting prevention if it’s older than seven months. Especially when you take into account that your pet can seem perfectly normal for a very long time period before showing any symptoms, which after they do start appearing, means it is already a more serious state.

It’s recommended to owners to do annual testing. But you should, of course, consult your veterinarian.

How Is a Dog Tested for Heartworms? (Diagnosis)

There are several kinds of tests, but the most used is the antigen blood test. Adult female worms induce antigen production, which are hereby being detected. But you should be cautious since in the early stages of the infection, antigens cannot be detected, so there is a possibility that a false negative can occur.

Additional tests provided are the microfilarial concentration tests, echocardiography and chest x-rays.

Dog Heartworm Knott testIf your dog tests positive for heartworms, you can also expect further examinations from the veterinarian who wants to check the stage of the infection and the damage that has been already done (liver and kidney function).

Heartworms can also be diagnosed with a Knott test, where they are being identified in a blood smear. This test is quite insensitive, though. But it can help with the determination of Dirofilaria Immitis from other microfilariae.

For a successful diagnosis, it is crucial to know and understand the worms’ lifecycle as well as the diagnostic limitations and issues.7

Treatment of Heartworm in Dogs

If your dog was at last diagnosed with heartworms, proper treatment will be conducted. You should have in mind that sometimes, it’s a long process and comes with a risk. Since dead or dying adult worms are being flushed after treatment, they can clog pulmonary arteries, causing obstruction of the blood flow to the lungs.8

The protocol of treatment is mostly personalized and according to the overall condition of a dog and the stage of the infection. The whole treatment could last for a few months, so be prepared.

One of the first things that are suggested is the reduction of exercise, by limiting their space for movement and shorter walks. With this, the blood flow is not so turbulent. So the worms that are now dead together with their fragments do not travel all at once through the bloodstream causing a clog in the arteries.

To stabilize the state and minimize the inflammation and reaction to the worms that are going to be killed throughout the treatment, most veterinarians will reach out for prednisone and doxycycline. Following that, a preventative drug will be given to kill off juvenile worms.

What usually follows is a melarsomine (Immiticide) injection, which kills adult worms. Thirty days later, the dog gets another shot, and one day after, a third melarsomine shot is given. Exercise should be restricted the whole time.

Three to five months after the treatment, you should get a check-up for juvenile heartworms and six months after for adult worms.

If the infection was critical from the start, dogs may also need antibiotics, pain relievers, and diuretics to remove the fluid accumulated in the lungs, drugs to support heart function and a proper diet.

ACE-inhibitors, beta-blockers, glycosides, and low-salt diets are used when the severity of the disease leads to heart failure and as such, they need to be given for the rest of the dog’s life.

Home Remedies for Heartworm in Dogs

It should be emphasized at the beginning that home remedies and other natural products are NOT FDA approved, which means there is no scientific proof that this kind of treatment is truly effective. This is when one should be very cautious.

Since the usual treatment for heartworms can be very tedious and complicated with strong medication, there are some home remedies that are recommended to help you with protecting your dog from these little intruders. Even in this case, it is better to consult your veterinarian.

Turmeric root, mugwort, wormwood, black walnut and clove flower buds can be used as a repellent for mosquitoes (used in sprayers) by preventing the bite in the first place.

One should be very careful even with these, since black walnuts have been recorded to induce tremors and seizures, while wormwood can be damaging for liver and kidneys.

It is NOT recommended for you to take this alternative, but if you decide to do it, you should endeavor to pay even more attention to your pet’s diet, as one of the main steps for ensuring a stronger immune system. Be sure to give your dog enough water and high-quality food. Also, testing your animal should be more frequent, such as every three to four months.

How to Prevent Heartworm in Dogs

Before starting with the prevention, the first thing you should do is make sure your dog is not already infected in the first place, especially if it’s a stray dog or a dog you got from a shelter.

After you finally confirm that your pet is heartworm-free, there are various preventatives approved by the FDA. Your veterinarian will choose the best one, depending on the dog’s overall health, lifestyle, and environment.

Dog eating a chewableThe most common products for heartworm prevention are pills, topical spot-ons or injectable medication. Some of them are given once a month, others every 6-12 months. While some of them are designated just to prevent heartworms, others represent a combination for several different parasites (ticks, fleas, mites and heartworms). That again depends on the region, climate and the prevalence of those parasites. Seek the advice of your veterinarian before starting any treatment. .

Other measures can be taken, including preventing the mosquito bite in the first place. This is accomplished by avoiding standing water and using mosquito repellents.


Heartworms pose a serious health risk and can be life-threatening for dogs, where even light infections can cause severe damage to the heart, lungs and their associated blood vessels. This roundworm can cause symptoms like cough, loss of appetite and lethargy. Taking into account the human cases as well, we should be very serious and cautious with this disease.

Do your part, test your dog and carry out all the preventive measures suggested. Use mosquito repellents to avoid the bite in the first place, and follow this with preventatives prescribed by veterinarians, which can be pills, topical spot-ons or injectable medication. Some of these treatments are given once a month, others every 6-12 months. In the end, prevention is the best treatment.

Diagnostic methods are usually in the form of antigen blood tests or blood smears. But for a successful diagnosis, it is important to know the worms’ life cycle as well as the diagnostic limitations and issues.

Frequently Asked Questions (FAQs) About Heartworm in Dogs

What Are the First Signs of Heartworm in Dogs?

Mostly, there are no visible signs of heartworm in dogs, long after the actual infestation. But when they do appear, you will most likely notice a persistent dry cough, loss of appetite, loss of weight and apathy. This means the disease has already progressed, so you should seek the help of a vet immediately. These symptoms can indicate a whole list of other diseases that should be determined, so a proper therapy should be conducted.

How Long Can a Dog Live With Heartworms?

Left untreated, the disease is fatal. The lifespan depends on the size of the dog, its general health condition and the number of worms present. But even when the treatment is carried out, depending on the stage of the infection, the pet can be left with the damage the worm has already made to the heart and lungs, which has to be monitored with treatment.

What do Heartworms Look Like?

Since the heartworm belongs to nematode (roundworms) phylum, they look like 'cooked spaghetti.'

How do You Know if Your Dog Has Heartworms?

Mostly there are no noticeable symptoms, especially in the beginning of the infection. It may take a long time for first symptoms to appear, but if they do, usually the dog has a persistent cough, loss of appetite, and is generally apathetic. These are warning signs, and the dog should be immediately taken to the vet. Regular testing is the best way to prevent this.

How Much is a Heartworm Test in the USA?

If you suspect your dog has heartworms, or if you just want to do an annual check-up, you should expect a price range from $45-$50. Whereas the treatment can be from $400-$1000.

How Common is Heartworm in Dogs?

The incidence of heartworm disease in dogs is increasing, with the coastal regions being more affected, taking into account climate and the vectors. One out of 200 dogs is getting infected with heartworms.

Are Heartworms Contagious in Dogs?

Heartworms are NOT contagious; only mosquitoes can infect the animal throughout the bite. Mosquitoes are the ones responsible for transmitting the parasites from one dog to another.

How to Get Rid of Heartworm in Dogs?

After confirmed diagnosis, the vet will treat your dog with suitable medications according to the state of infection. And in extreme cases, the vet may suggest surgery to physically remove the worms. To learn more about the treatment, please read the 'Treatment of Heartworm in Dogs' section.

What do Heartworm Look Like in Dogs?

Heartworms have an impact on several organs from heart, lungs, and kidneys to the liver, and the clinical signs vary as well. But mostly the disease is reflected on the heart and lungs with their accompanying symptoms.

Can Heartworm in Dogs Be Cured?

Dogs with heartworms can be cured if diagnosed on time and with the appropriate therapy. This is why you should use preventatives and do regular testing.

Can Cats Get Heartworms?

Cats cannot get parasites directly from dogs; the only infection path is through mosquito bites. As in humans, cats are not heartworms’ final destination. This is why the disease manifests itself often very differently. The cat gets infected either with just a few or no adult worms because of their petite body structure. Therefore, the infection can be left unnoticed. But the damage can be done, nevertheless, even by immature worms; cats can develop heartworm associated respiratory disease (HARD). The symptoms can range from very mild to extremely serious and heavy. When developed, the symptoms are usually a cough, a lack of appetite, asthma-like attacks, vomiting and weight loss. They can even get seizures or faint, and like dogs, they get an accumulation of fluid in the abdomen. The problem is that sometimes they do not develop any of the symptoms, but suddenly just collapse or even die. It is important to consider the fact that there is no medication that can be used for the treatment of heartworms in cats because the ones designated for dogs are ineffective. So the only way is to prevent the infection in the first place.

Dr. Iman Krijestorac (DVM)

Dr. Iman is a young, ambitious veterinarian working in a small animal vet station while volunteering in the local dog shelter. Zoonotic and infectious diseases are one of her major interests, together with dog nutrition and diet for which she attended several seminars and conferences. She has always had a heart for big dogs, she has an old Appenzeller Sennenhund going by the name "Yoda".

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