A Less Romantic Type of Valentine: Heartworm By Dr. Emma Bush

In February we often think about and send one another love hearts and valentines. However, today I wanted to talk about a less romantic type of heart-lover. This is the parasite heartworm.

What is Heartworm?

Heartworm (or Dirofilaria immitis) is a large worm that, in its adult stage, lives in the heart and pulmonary (blood vessels that flow from the heart to the lungs) vessels of infected dogs. They can grow to be up to 14 inches in length!


Adult worms do not lay eggs. Instead, they give birth to baby worms (microfilaria) that are released into the circulatory system. These can last up to 2 years in host dogs! (They can also pass across the placenta of a pregnant dog into her unborn fetuses – though these do not grow into adult worms.)

The parasite is spread through mosquito bites that pick up larval or immature forms from the blood of infected dogs – the larvae then mature in the mosquitoes over the course of a few weeks, and then are passed on to non-infected dogs when the mosquito bites them. The larvae further develop in the host dog’s skin for several months prior to entering circulation where they then migrate to the heart and pulmonary arteries where, if other worms are present, they will mate.

Consistently warm temperatures are required in order for this life cycle to be completed and, thankfully, it is still too cold in the Maritime provinces for this. However, with global warming and our ever-warming winters, along with the increasing popularity of adopting and rescuing dogs from warmer parts of the world, the risk of heartworm and the importance of its awareness is significantly increasing.

What does it do?

As noted above, worms will live in the heart and the vessels that go from the heart to the lungs for blood to become oxygenated. Heavy infections (up to dozens of worms) can take up a significant amount of space. This, unsurprisingly, makes the heart work harder to pump around them.
In addition, the immune system of dogs recognizes proteins in the heartworms and creates a significant inflammatory response where it tries to kill the worms. Unfortunately, the worms are too big for the body to kill off in this way, and it leads to a vicious cycle of inflammation affecting the vessels and lungs around the worms. This makes the vessels abnormally thickened and scarred.

Some worms will die over time, and this is where the highest risk is present, as the body of the worms breaks down following death and can become lodged in vessels (this is called an embolism). This can cause significant and life-threatening problems.

All this together can lead to eventual right-sided heart failure with signs of coughing, shortness of breath, exercise intolerance, nose bleeds, build-up of fluid in the chest and/or abdomen, and even sudden death. In severe cases, sometimes acute collapse and even death can occur with a process called Caval Syndrome. This is when there is a massive number of worms filling the right side of the heart to the point that blood cannot be pumped through effectively. The only treatment for this is opening up the jugular vein and physically removing the worms. This is a medical emergency, and even if this emergent specialty procedure is performed, prognosis is very poor for these patients.

What about cats?

Cats are not a natural host of heartworm and as such the parasite is unable to complete its life cycle in the feline body. However, cats can still become infected with heartworm in areas of risk and, as they are so small, even just one adult worm can cause heart failure.

As they are not natural hosts, the infection affects them differently and presents as lung disease more than heart disease. The heartworms are used to responding to physiologic signals in dogs and they “misread” these in cats and as such can end up in abnormal locations in cats. This can lead to a significant immune response in infected cats. It is this intensive immune response that causes heartworm disease in cats, as opposed to the obstruction of blood flow that causes disease in dogs. On x-rays, this process looks identical to feline asthma and the only way to tell them apart is through heartworm testing.


Diagnosis of heartworm infection in dogs can be complicated as different tests detect different life stages. There are a variety of different blood tests, some more common than others, that are used to diagnose infection with heartworm. In some advanced cases, even x-rays or ultrasound can be used for diagnosis to demonstrate heart abnormalities (on ultrasound you can sometimes even see the worms wriggling in the heart and vessels!).  Testing is even more complicated in cats due to the magnitude of immune related signs to even a single worm.

If you are concerned about heartworm in your dog or cat, please consult your veterinarian to discuss next steps for testing.

An important note for testing is that the heartworm life cycle lasts 5-7 months. As such, a dog being rescued from an infected area may not test positive for up to 7 months after arriving at their new home in a low-risk area. Because of this; it is generally recommended to test for Heartworm either prior to or upon arriving in Canada (or another low risk area for heartworm) following import from a high risk area; and then again after being in the country for 7 months. This also applies to puppies; it is not helpful to test prior to 7 months of age in a puppy as the life cycle would not yet be completed.


There are different types of treatments for heartworm that are indicated for different types of patients: this is proceeded with following diagnosis and staging of the degree of infection.

The most common type of treatment in dogs consists of parasite prevention to kill microfilaria and migrating worms; an antibiotic to kill a type of bacteria that is often present with heartworms; and repeated injections of a medication in a veterinary setting that kills adult worms.
One of the most important parts of this treatment is keeping the dog quiet with strict confinement during treatment to minimize the risk of worm-related embolisms.

There are other treatment options for dogs, and treatment in cats is different as well due to the different nature of the disease. However, keeping the infected animal quiet during treatment is important for all patients.

If you have any questions about the treatment of heartworm in more detail, please speak to your veterinarian.


Heartworm prevention medications are used to kill the larval form of heartworms that are introduced into the dog’s blood through mosquito bites as discussed above. There are a variety of different products that are effective for this with risks to be aware of present with different products.

In high risk areas, or when coming from a high risk area, prevention is recommended year round.

If you have questions about parasite prevention in your pet, please speak to your veterinarian about your options.

This serves as a brief introduction to heartworm infection in dogs and is by no means an exhaustive summary of the parasite or the infection. If you are concerned about heartworm in your pet or have any questions, please do not hesitate to contact your veterinarian and veterinary professional team.

References Used:

Heartworm: the Parasite:

Heartworm Disease in Dogs:

Heartworm Disease in Cats:

Heartworm Diagnosis in Dogs:

Heartworm Treatment for Dogs: https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4951486 Heartworm Prevention: https://veterinarypartner.vin.com/default.aspx?pid=19239&catId=102894&id=4951473