Heartworm in Dogs: Symptoms, Diagnosis & Prevention Every pet parent needs to be aware of the risks heartworm poses to their pooch

Heartworm in Dogs: Symptoms, Diagnosis & Prevention

Thumbnail of Interceptor


Heartworm & Deworming
{{petcare_price|currency}} Price in Cart w/PetPlus {{petplus_price|currency}} See PetPlus Price in Cart

Heartworm can be prevented with medication prescribed by your veterinarian, such as Heartgard Plus.

Heartworms, commonly known as Dirofilaria Immitis, is a nematode (parasitic roundworm) that spreads from one host to the other through mosquito bites. They are small, thread-like filarial worms that cause dirofilariasis. While the definitive hosts for heartworms are dogs, they can easily infect other animals like cats, coyotes, foxes, ferrets, wolves, bears, sea lions, and in rare cases, humans. This is why you need to be aware of a heartworm medicine like Drontal for cats. The 'adult' heartworms go through several life stages before infecting the pulmonary arterial system of the dog and occasionally migrate to the heart.

The course of infection:

  • As an intermediate host, the mosquito bites a healthy dog and transfers the third-stage heartworm larvae into the dog's subcutaneous tissue. Within the next week, the third-stage larvae become fourth-stage larvae and penetrate the skin to reside in the muscles of the abdomen and the chest. After 45-60 days, the fourth-stage larvae morph into the fifth-stage larvae and take anywhere between 75 to 120 days to enter the bloodstream.

  • Through the bloodstream, the immature heartworms are carried to the heart and end up residing in the pulmonary artery for the next 3-4 months, where they also grow in size and eventually mate.

  • In another seven months, the female (30 cm) and the male (23 cm) heartworms reproduce and give birth to microfilariae, which continue to circulate in the dog's bloodstream for up to 2 years. The microfilariae require a temperature of below 14°C to survive; otherwise, their life cycle ceases entirely.

  • From the dog's bloodstream, the microfilariae move on to the next stage of their life cycle once another mosquito bites the dog and becomes the intermediate host. The microfilariae transform into second, and then third-stage larvae in the mosquito's gut while waiting to infect another dog.


Unfortunately, most dogs show no signs of being infected by heartworms during the first 6 months until they mature. The microfilariae or the immature larvae go completely undetected by the diagnostic tests.

In some cases, the fifth stage larvae get lost in the bloodstream and end up in unusual places such as the legs, an artery, eyes, or the brain, resulting in unexpected seizures, blindness, or lameness. Since dogs live a sedentary lifestyle, with little to no infections, heartworms can easily go unnoticed even after they are 'adults' and the infection is classified as 'asymptomatic'.

Early symptoms in dogs infected with Class II heartworms usually include coughing during exercise, premature exhaustion after exercising, and labored breathing.  In slightly advanced cases (Class III), symptoms include sudden weight loss, coughing up blood, fainting, pot-bellied appearance in the abdomen due to fluid accumulation, eventually leading to death caused by congestive heart failure.

In Class IV, the dog develops 'caval syndrome' characterized by the presence of way too many 'adult' heartworms in the heart that is beginning to restrict blood flow, eventually leading to congestive heart failure.


In the past, both microfilarial detection and Antigen testing have been used for microscopic identification of microfilariae in the bloodstream, especially for dogs receiving no preventative treatment. Immunodiagnostics is more commonly used now for the detection of heartworm antigens.

Note: Only the antigens released from the reproductive tract of a female heartworm test positive during immunodiagnostics, which means that an infected dog will test negative for the first 5 to 8 months.

X-rays are usually conducted to determine the severity of the infection by observing enlargement of the heart's right ventricle or pulmonary arteries.  Apart from that, a blood chemistry panel, along with a urinalysis and complete blood cell count, is also carried out to determine an accurate prognosis for the dog. 


Dogs with heartworms are always evaluated for their liver, heart, and kidney function before starting treatment. In most cases, adult heartworms are destroyed using melarsomine, an FDA-approved Arsenic-based compound that has fewer side effects and more efficacy than thiacetarsamide, a previously used drug.

Dogs undergoing treatment using melarsomine are advised to rest or follow a restricted exercise regime for several weeks to allow sufficient time for the body to absorb the dead heartworms. Once the adult worms are eliminated, a separate treatment for microfilariae is administered until the dog tests negative for heartworms. For dogs infected with Class IV of the disease with considerable heart involvement, surgical removal of the adult heartworms might be carried out.

In some cases, ivermectin is administered monthly throughout the year to kill adult heartworms. However, melarsomine remains the preferred drug for treatment because of two reasons: a) Melarsomine is more effective than ivermectin or any other drug on the market at the moment, and b) Adult heartworms respond to ivermectin treatment after 18 months, which is pointless for dogs with an advanced stage or high-volume infection.

For Class I patients, a long-term treatment including Doxycycline for dogs and Heartgard Plus is recommended.


Multiple drugs like ivermectin, milbemycin, or moxidectin are usually prescribed as heartworm prevention for dogs. When administered regularly, heartworm prevention is extremely effective and protects 99% of dogs and cats from heartworms. An annual heartworm exam is highly recommended for pet owners who take the minimal dosage route to detect heartworms earlier for a better prognosis.

Dogs living in or traveling to endemic regions with warmer climates should be considered at-risk for heartworms and see a veterinarian for preventative medications. Dogs that have already been treated for heartworms should also use preventative medications and measures to avoid being infected again.

Does Your Pet Need a Blood Test for Heartworm Infection?

The answer is yes. Heartworm disease is a serious, potentially fatal condition that affects dogs, cats, and other animals. The best way to avoid heartworm disease is to keep your dog on heartworm preventive medication year-round. But what if you’re not sure whether your dog needs to be tested? Here are some common reasons why you may want to have your pet tested:

1) Your dog has been exposed to mosquitoes in the last 30 days or so

2) You live in a region where mosquitoes carry heartworms

3) You’re planning on traveling with your pet (especially during mosquito season in those areas where they carry heartworms)

Heartworm Is Most Prevalent in Places That Are Warm and Wet for a Long Period

If your dog lives in an area where there are a lot of mosquitoes, they might be more at risk of contracting heartworm than if they lived somewhere else. That's because mosquitoes are the only way that heartworm can spread from one animal to another.

It's important to keep your dog safe from mosquitos by keeping them indoors during the day and using insect repellent on them before you let them out in the evening. If you live in an area where there aren't many mosquitos, you can still get a heartworm test done every year or so just to make sure everything is okay!

Do You Have to Give a Dog Medication Year-Round for Protection Against Heartworms?

No, you don't have to give your dog medication like Interceptor or Interceptor Plus year-round for heartworm protection.

Heartworms are parasitic worms that live in the heart and lungs of dogs. They can be transmitted to dogs through mosquito bites. Heartworm disease is fatal if left untreated.

To prevent heartworms, some dog owners opt to give their dogs monthly heartworm pills for dogs or injection that kills any worms that might be in their system. It's also possible to prevent heartworms with a monthly chewable tablet that kills the worms after they're ingested.

Some dogs who live in warmer climates where mosquitoes are prevalent may need to be given this medication all year round. However, in most cases, it's not necessary for a dog living on the East Coast or other regions where mosquitoes are uncommon during winter months.

A lot of times, dogs get infected with heartworms when medications are missed or not used as per the instructions on the label. Ivermectin is usually the drug of choice in these cases, as it still offers 95% protection from the infection even with a 4-month lapse in dosage.

Was this article helpful?

You May Also Like

Image for Heartworm Symptoms - The Four Stages of Heartworm in Dogs
Heartworm Symptoms - The Four Stages of Heartworm in Dogs

Does Your Dog Have Heartworm? Track the 4 Stages of Heartworm Symptoms

Read More
Image for Reasons Why Most Pet Parents Do Not Notice Any Heartworm Symptoms?
Reasons Why Most Pet Parents Do Not Notice Any Heartworm Symptoms?

Identifying the Symptoms to Prevent Heartworm in Pets

Read More
Image for Guide to Heartgard Plus for Dogs - Ivermectin
Guide to Heartgard Plus for Dogs - Ivermectin

Ivermectin and Pyrantel Pamoate, Key Ingredients in Heartgard Plus, Prevent and Treat Heartworm Infections

Read More