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

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 or sea lions, and in rare cases, humans. 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, that 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' 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 easy 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 laboured 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 are 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 for determining 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 the sufficient time for the body to absorb the dead heartworms. Once the adult worms are completely 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 the 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 and Heartgard Plus is recommended.


Multiple drugs like ivermectin, milbemycin or moxidectin are usually prescribed as chewable tablets, pills or injection to prevent heartworm infection. When administered regularly, preventative drugs are extremely effective and protect 99% dogs and cats from heartworms. An annual heartworm exam is highly recommended for pet owners who take the minimal dosage route in order to detect heartworms earlier for a better prognosis.

Dogs living in or travelling to endemic regions with warmer climate 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.

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.

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