The heartworm disease

Heartworm disease is caused by a parasite transmitted by mosquitoes from one animal to another. Heartworm disease is serious and can cause the death of the animal.

What is heartworm?

The heartworm is a nematode of the genus Dirofilaria. The adult worm, also known as wire, looks like a very thin spaghetti, it’s whitish and can measure up to 30 centimetres. It lives in the pulmonary arteries, the right heart, and the vena cava. A severely infected dog can have up to 500 adults living in his heart! The adult female will give birth to tiny babies called larvae (L1 stage ) or microfilariae , which circulate freely in the blood. The genus Dirofilaria includes about 27 species. In North America, there are Dirofilaria immitis, D. subdermata , D. tenuis , D. striata and D. ursi.

How the disease is transmitted?

By mosquitoes the parasite spreads from one animal to another. The heartworm uses the mosquito to full fill it`s growth. A mosquito can bite an infected dog and drink the microfilariae in the blood stream, then after the microfilariae grows in the mosquito, it can infect the next dog it feeds from.

Take the example of two dogs, a dog infected with heartworm named Buffy, and a healthy dog named Kari

The adult female heartworm gives birth to microfilariae and sends them into the bloodstream of Buffy. The mosquito bites Buffy and ingests blood with microfilariae. For the next 2 to 4 weeks, with an outdoor temperature of 18 º C to 27 º C, the mosquito becomes an intermediate host, that is to say that it allows immature worms (L1) to transform/grow into infective larvae (L3).

After this incubation period, the infected mosquito bites Kari and deposits a drop of saliva containing some infective larvae, it penetrates the skin through the bite wound. Once in the tissues of Kari, the larvae continue their development ( L4) , migrate to the heart where they reach the adult form reproduce and eventually release microfilariae in the blood. This cycle takes 190 days after infection.

Kari is now infected and is a reservoir for contaminating the whole neighbourhood.


Definitive host

  • The dog is the usual host and all races are likely to become infected.
  • About 21 other species of domestic and wild canines such as foxes, coyotes and wolves are infected and can play the role of host. These wild animals are important because they travel great distances, compared to the mosquito that travels over short distances.


  • Moustique du genre Culex, Aedes et Anopheles.
  • La puce ne peut pas être vectrice.

abnormal host

Un hôte anormal peut être infecté, mais ne joue probablement aucun rôle significatif dans la transmission de la maladie. Ce sont certaines espèces de:

  • félidés, dont le chat domestique et une dizaine d’autres espèces,
  • phoques et otaries,
  • animaux sauvages tels le castor, le furet, le vison, le raton laveur, l’ours,
  • équidés, tels que le cheval,
  • singes (plusieurs espèces),
  • ainsi que l’humain. Pour cette raison, la maladie des vers du coeur est considérée comme une zoonose, une maladie transmissible à l’humain.

Geographical Distribution

Globally the parasite lives in regions with tropical and subtropical climate, mainly in coastal and riverine areas. Endemic areas or problem areas where the parasite is found with many cases of infections are localized.

  • North and South Americas (especially coastal and inland)
  • Asia
  • Southern Europe
  • Australia
  • North Africa

In Canada, the parasite is present in the southern belt of provinces;

  • Quebec
  • Ontario
  • Manitoba
  • and the Okanagan Valley in British Columbia

In Quebec, with the weather data over the last 30 years, we conclude that the transmission of the parasite is possible especially in July and August ( from July 2 to September 8 ). It was in 1984 that it was declared there was a heart worm epidemic in the province of Quebec. In 2007, the area becomes endemic

  • throughout the south of the province,
  • the northern limit is north of Quebec City.
  • MBut nearly half of the cases are reported to the north of the island of Laval, between Lachute and L’Assomption.

Symptoms or signs of animal disease

  • Chronic coughs
  • Difficulty breathing or shortness of breath
  • Tired after an effort
  • Collapses following a force
  • Lose weight despite appetite
  • heart failure
  • Passive or apathetic
  • Kidneys work less
  • Death

Unfortunately, when the clinical signs of disease are evident, several organs are already affected irreversibly (kidney`s and liver).


The screening is done by a blood test for the presence of microfilariae or adult, but no test is 100% reliable (Difil, modified Knott test, smear charges, serological test).

Disease Treatment

Yes, it is possible to treat a dog with worms in the heart, but there are risks. We must keep the animal quiet with minimal exercise, because the drug kills adult worms, which can cause damage to organs already proven.


Test your animal as recommended by your veterinarian, usually every two years. Through a blood test early in the season, it is ensured that the microfilariae are absent (negative test) and immediately refer the dog to a prevention program with a drug. If the mosquito bites the animal and sends microfilariae, drug treatment prevents the development of infective larvae. The life cycle of the heartworm is thus interrupted.

When to treat

In Quebec, it is recommended to treat your dog continuously during the spring, summer and autumn, from June to November. The important thing is to treat one month after the arrival of mosquitoes and end one month after the end of the mosquito season.


En résumé

Le meilleur traitement, c’est la prévention!

Liens utiles

Références 2007

Alain Villeneuve, D.M.V.

Médecin Vétérinaire, professeur de parasitologie, Faculté de Médecine Vétérinaire, Université de Montréal

Auteur du livre « Les zoonoses parasitaires : l’infection chez les animaux et chez l’homme » ,

Edition Les Presses de l’Université de Montréal, 2003