Heartworm disease

What is that?

Adult Dirofilaria immitis in the heart of an animal. The photo is courtesy of Bayer.

Heartworm is a long parasite that mainly lives in animals' pulmonary arteries, but also sometimes in the heart. 

It is a parasite that can be found in regions with tropical and subtropical climates, which includes the south of Quebec.

The dog is the principal host, meaning the host in which the parasite attains maturity, but it can also infect other wild and domestic canine species such as foxes, coyotes, and wolves, which serve as reservoirs (source of the parasite). 

Furthermore, the parasite can infect other animal species that are considered abnormal hosts that do not play a significant role in transmission. These include, among others, the cat, beaver, ferret, mink, raccoon, bear, horse and human. 

Even cats not having access outdoors are susceptible to a heartworm infection. Several studies have shown that 25% of infected cats lived exclusively indoors.

In addition, it has also been shown that cats living in proximity to infected dogs were probably infected as well. 



The infection is acquired from a mosquito bite. 

The parasite cycle

Parasite cycle in the dog: 

The development of the parasite takes place as follows in dogs: the female parasites, lodged in the dog's pulmonary arteries, produce larvae, called microfilariae, that spread in the blood. 

When a mosquito bites an infected dog, it draws blood containing microfilariae. Once inside the mosquito, the microfilariae continue their development for a period varying from 10 days to a month, depending on the external temperature. 

This phase of the parasite's development in the mosquito is essential for the microfilariae to become infectious, meaning it is able to infect another animal. 

Afterwards, when the infected mosquito bites another dog, it will deposit a drop of saliva containing larvae on the surface of the skin, which will penetrate the skin and invade the organism. 

During this approximate four month period, the parasites will continue their development and enter a vein, where they will reach the pulmonary arteries. Once there, they will continue to grow up to 12 cm in the male and 30 cm in the female.

About six and a half months will elapse between the time the parasites infect the animal and the female worms produce microfilariae. 

Adult worms can live up to four years. 

Parasite cycle in cats: 

The parasite's cycle in cats is a bit different from that of dogs. 

On the one hand, the worm does not usually live long enough to produce microfilariae because most of them die during their development. On the other hand, some cats' immune system often elimiate the larvae before they become adults. 

Consequently, cats generally only hold a small number of worms. Despie this, heartworm infection can still cause them to have serious health problems and sometimes be fatal.  

The symptoms

In addition to often having less adult worms than dogs, the worms do not become as big in cats either. 

However, because cats are usually smaller than dogs and their blood vessels are also smaller, only a some worms present in their pulmonary vessels can be enough to cause them harm. 

Just as in dogs, some cats will never show any clinical signs caused by the infection. 

When there are symptoms present, they can easily be related to other diseases, such as feline asthma. Cats can vomit, cough, and have difficulty breathing. This syndrome is known as «Heartworm Associated Respiratory Disease» or «HARD». 

On the other hand, the only sign to give away the infection can be sudden death. 

The diagnosis

As previously mentioned, the parasite's cycle differs in the cat from that of the dog. Consequently, it is more complicated to diagnose the infection in cats than in dogs. 

For example, it is often necessary to run the tests on different occasions to identify the presence of the worm. Furthermore, negative results do not necessarily exclude the infection and positive results, depending on the tests performed, do not confirm the infection every time. 

In cats, it is possible, as a first attempt, to use antigenic and serological tests to detect the worm. 

Each of these tests have advantages and are limited, and on their own, can not identify the disease in all infected cats. 

For the diagnostic methods used in dogs, we invite you to consult the following link: http://monvet.com/fr/fiche-informative/411/ver-du-coeur


The treatment

Currently, there is no existing treatment for heartworm disease in cats. 

The best that can be done is to monitor the cat's health closely and control signs of the disease. 

In some cases, a surgery to remove the worms can be recommended even though it could be costly and risky.  


There are different medications, sold under a prescription, that are safe and effective for the prevention of heartworm in cats. They come in different forms: chewable tablets, topical solutions applied on the pet's back, etc. 

A monthly administration is recommended between the months of june to november, every year.

Do not hesitate to seek advice from one of our staff members who will be happy to advise you on which medication is best suited for your cat. 


Every year, it is recommended that dogs be tested for the presence of heartworm not only to treat them so that they get rid of their parasites, but also to prevent them from being used as a reservoir of infection for themselves, other animals, and humans around them, and so that they do not suffer from complications associated with the treatment which can sometimes be serious. 

Nevertheless, it is always important to have the screening test performed even with pets that are on a preventive program every year, as the owners might forget or delay a dose, the dog spits out the medication, there is an error in the dosage of a growing dog, the medication might have been poorly absorbed, or the heartworm might have developped a resistance against the product used. 

Are cats susceptible to heartworm?

Cats can be affected, but will develop very little microfilaria and only for 2-3 months. The worm will die after about 2 years. The majority of cats reported to be affected only had one worm. No prevention is recommended for cats at this time. 

And humans?!?!

The first reported case of a heartworm infestation in a human was in 1991. Since then, we have seen a rise in both human and canine cases. The clinical signs observed in humans are the following: thoracic pain, cough, and fever. No diagnostic test that has been used in humans has had good results. A chest x-ray can point to an indication of dirofilariosis, but a thoracotomy (open the chest cavity and confirm on the spot) must be performed to confirm the suspicion. Thus, to prevent a future infestation in humans it is necessary to test and do prevention in dogs.

Antigenic tests

An antigen is a protein produced by a  foreign organism. 

In the case of heartworm, research has led to the development of tests identifying certain antigens produced by adult female worms. There is a very reliable test that can be performed in clinic that can quickly identify their presence.

Nonetheless, if the infection limits itself to just a few male worms or microfilariae, it goes without saying that the test will not detect them, thus, it will be negative. 

As a result, other diagnostic methods often have to be used to try to confirm or exclude the infection. For example, it is sometimes possible to see the worms on an x-ray or ultrasound of the heart and lungs. Unfortunately, even these tests can be inconclusive. 

Serological testing

A serological test consists of identifying antibodies produced by the animal's body following invasion by a foreign microorganism, in this case, the heartworm. 

A positive test can indicate the onset of an infection or an earlier infection that was already eliminated by the cat's immune system, but not necessarily an current infection. In fact, several cats with a positive serological test do not host any adult worms.

In contrast, some cats parasitized by the adult worm do not produce antibodies against them during the entire period during which they are infected. Therefore, a serological test performed in these cats will often be negative.