What is that?

Adult Dirofilaria immitis in an animal's heart. The picture is a courtesy of Bayer.

Heartworm, is a long parasite that lives mainly in the pulmonary arteries of Canidae, but also in the right part of the heart and in the caval veins of small-sized animals or those that are infected with a large number of worms. It's scientific name is Dirofilaria immitis.

Sometimes the parasite migrates erratically and ends up in other locations such as the digestive tract, where it will be vomited or excreted with the animal's feces, the eyes, the bronchioli, the cerebral arteries, the brain, the spinal cord, etc.

This parasite can be found in tropical and subtropical climates throughout the world, including southern Quebec.

The dog is the worm's main host, meaning the host in which the parasite reaches maturity, but it can also infect other species of wild and domestic Canidae, such as foxes, coyotes and wolves. These species serve as a reservoir for the parasite.

Furthermore, it can infect animal species that are considered abnormal hosts that do not play an important role in the transmission of infection. This is the case for cats, beavers, ferrets, minks, raccoons, bears, horses and humans.

The symptoms

The diagnosis

The infection is often diagnosed with use of blood tests.  

The first test, known as the Difil test, consists of passing a blood sample through a filter to concentrate the microfilariae, then stain and evaluate it under the microscope. 

Their presence confirms that the dog has contracted heartworm disease. However, the infection cannot be excluded even if none are seen. 

Consequently, there are circumstances where adult worms are present in the heart without having microfilariae in circulation. This is said to be an occult infection. 

For example, it may be that the worms are not yet fertile, as is the case if they are less than 190 days old, there are only a few worms of the same sex, the dog's immune system has overreacted towards the microfilariae and has destroyed them, or the dog receives monthly preventive medication that removes microfilariae in three to eight months. 

In Quebec, the majority of occult infections are due to the infection being unisexual, which results in a low parasite level. 

Another way to diagnose the infection is to have access to blood tests that detect the presence of antigenic substances produced by adult female worms' uterus. Needless to say, if there are only males worms present, the test will be negative. Furthermore, the test will be negative if the infection is recent because five months must have elapsed after the infection for the test to be positive. 

A thoracic x-ray can sometimes be useful to show evidence of cardiac and pulmonary lesions that are characteristic of the condition. 

The treatment

Before proceeding with treatment, it is important to determine if the infected dog is fit to receive treatment in order to minimize the risk of complications associated to it. This requires a complete physical examination of the animal, additional tests such as chest X-rays and blood tests to assess renal and hepatic function, and serological tests to estimate the parasitic load. 

The first treatment consists of injecting the patient, twice, by intramuscular injection with a medication that will eliminate the adult worms. The interval between the two injections depends on the parasitic load. Although the medication is effective and kills the adult worms in one to two weeks, it is not without risk to the animal. Thus, because of the possibility of pulmonary thromboembolisms,  the pet must be always be kept in their cage at rest for 2 weeks. He should also not engage in any vigorous exercise for a period of one month. 

 A dog with a heavy parasitic infestation can experience some side effects, usually transient, related to treatment about 4 to 8 jours later. These include lethargy, loss of appetite, salivation, nausea, pale gums, defecation, and an increase in heart rate. 

In dogs that are determined, through use of tests and absence of symptoms, to have a weak parasitic infestation, it is sometimes prefered to administer the monthly preventive treatment for a period of twelve months. This will result in the infertilization of male worms and thus gradually eliminate, over a period of six to nine months, the production of microfilariae by female worms. 

Three to four weeks after the treatment used to kill adult worms (adulticide), a medication to kill the microfilariae (microfilaricide) will be used. The absence of microfilariae should be verified three weeks later. The treatment can be repeated only twice. After that, the efficiency of the adulticide should be evaluated. 

The parasite cycle

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.


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. 


There are different medications, sold under a prescription, that are safe and effective for the prevention of heartworm in dogs. 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 dog. 

The transmission

Animals get infected when they are bitten by an infected mosquito.