What is Heartworm Disease (Dirofilariasis, Dirofilarosis)?

Updated: Nov 2, 2021


Dog heartworm with exploded heart view in a jar

Heartworm disease (dirofilariasis) is a disease of the pulmonary blood vessels caused by the parasitic worm Dirofilaria immitis. It affects both wild and companion canines and felines with dogs being the most predominantly affected. The disease in cats is about 5 - 20% that in dogs. It is an emerging zoonotic disease, producing pulmonary dirofilariasis in humans. Heartworm disease is potentially fatal but preventable.

Aedes mosquito

It is a vector-borne disease, transmitted by a variety of mosquitoes (over 70 species). However, four species are the most common, i.e. Aedes, Anopheles, Culex and Mansonia mosquitos, with Aedes mosquitos (Aedes vexans) being the best vector of transmitting the heartworm.

Culex mosquito

This mosquito species is also referred to as the salt marsh mosquito and is an aggressive biter and a strong flier, living in mangrove forests along coastlines with the potential to travel between 30 km and 90 km under ideal conditions.

Anopheles mosquito

This mosquito species bites during both day and night and is an important vector of canine heartworm (Dirofilaria immitis)

Geographic Distribution

Mansonia mosquito

Cardiopulmonary dirofilariasis is mainly located along the coastlines of countries with temperate and tropical climates which are warmer. The development and activity of mosquitoes are regulated by climate, primarily temperature and humidity and the right vegetation, the mangroves.

Mangroves in a salty, marshy area

The transmission of heartworm disease depends on the presence of competent mosquito species, which is directly related to favourable climate conditions for its development and survival.

Life Cycle


The role of the endosymbiotic bacteria Wolbachia pipiens

The bacteria Wolbachia pipiens (red spots) within the heartworm gut

Wolbachia pipiens is a symbiotic bacteria that lives intracellularly within the filarial parasite, Dirofilaria immitis, and are involved in the development of heartworm disease, possibly through endotoxin production. This bacterium is necessary for normal maturation, reproduction, and infectivity of the heartworm. The eradication of Wolbachia causes the heartworm to gradually die, after first becoming sterile. This is accomplished with doxycycline therapy, which has become an important part of the treatment regime against heartworms.

What are the Risk Factors?

  • Location is a significant risk factor for canine heartworm infection as it determines the distribution of the multiple vector species of the Aedes, Culex, Anopheles and Mansonia genera of mosquitos. The preferred locations are warmer conditions (27 °C with 80% humidity) which are mostly along the tropical coastal lines.

Tropical coastline
  • Lack of the administration of preventive medication to dogs and cats in endemic areas, with up to 45% of pets that do not receive preventive medication expected to have heartworm disease.

What are the clinical signs of heartworm disease?


In dogs, at the earliest, heartworm detection is possible between 5 and 6.5 months after infection if dogs do not receive preventive medication and the infection and disease progress undetected. Clinical signs of heartworm infection include:

Comparison - Cats' vs Dogs' Heartworm Infection


Cats Dogs

  • 1%–10% infective stage survive 75% of third-stage infective L3 survive

  • Low maturation rate High maturation rate

  • Microfilariae uncommon Microfilariae common

  • Worms survive 2–4 years Worms survive 5 years

  • 1–5 worms present Many worms present

  • Smaller adult worms Larger adult worms

Stages of Heartworm Infection in Dogs


Stage Signs

I Minimal, if any, signs and none evident to the owner

II Mild to moderate signs of heartworm disease, most often including cough

III Severe signs of heartworm disease, including respiratory signs, weight loss,

exercise intolerance, and even heart failure

IV Caval syndrome

What is Caval syndrome?