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STAGE 1  No symptoms or slight cough.

STAGE 2  Mild to moderate, symptoms such as the occasional cough and tiredness after just moderate activity. X-Ray should usually show heart and lung changes as a result of the heartworm.

STAGE 3   Overall loss of body condition, a persistent cough, with tiredness after mild activity.  Trouble breathing and signs of heart failure are should usually show heart and lung changes as a result of the heartworm.

STAGE 4  This stage is also termed caval syndrome.  The heavy worm burden is so high that it is physically blocking blood flow and preventing the blood from flowing back to the heart. Caval syndrome is life-threatening and emergency surgery to remove the heartworms is the only treatment. At this stage even with surgery, most dogs with caval syndrome die.

Not all dogs with heartworm disease develop caval syndrome. If left untreated however, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death.

Heartworms (Dirofilaria immitis) were at one time seldom heard of outside the Southern regions of the United States, now they are commonly known about and have spread to all areas of the USA where the mosquito is found. Cases have been reported in Alaska, whilst outside of the USA, heartworm can now be found in Canada, South America , southern Europe, Southeast Asia, Australia, Japan and the Middle East.

Although dogs have usually been considered the preferred host for heartworm, it is believed that over 30 other animal species are also being infected. These include foxes, wolves, jackals, coyotes, raccoons, beaver and other wild canids (there are 35 species of wild canids in the world), domestic cats and wild felids (wild cats), ferrets and sea lions. Another host, or victims are Humans themselves, surprisingly yes.


Dogs will show no indication of heartworm infection in the the six-month prepatent period, before the worms reach maturation, and there are no current diagnostic tests which will show the presence of microfilariae or antigens in prepatent infections. Until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness.

On occasions things go wrong, even if rarely, and migrating heartworm larvae have got "lost" and have ended up in unusual areas such as the eyes, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizures and lameness.

The severity of the heartworm disease relates directly how many worms are living inside the dog, how long the dog has been infected and exactly how the dog’s system responds to the heartworms, as each dog will respond differently.

Many dogs will show little or no sign of infection even after the worms become adults. These animals usually have only a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections will usually show the classic signs of heartworm disease.

Early signs include coughing, especially on exercise or physical exertion as well as early exhaustion upon exercise. In the most advanced cases where many adult worms have built up in the heart without treatment, signs may include severe weight loss, fainting, coughing up blood and lethargy. Without treatment there then follows congestive heart failure.


Similar to most parasites, Heartworms need to go through several life stages before becoming adults able to infect the pulmonary artery of the host animal. For the worms to complete their life cycles they require a mosquito for their intermediateheartwom stage, the mosquito is a vital part of the life-cycle of the Heartworm.

The speed of development in the mosquito depends upon the temperature and requires around two weeks of temperatures at or above 27°C (80°F). Anything lessheartwom that around 14°C (57°F), and development cannot occur, and the cycle will be stopped. Therefore transmission is limited to warm months, and the transmission season will in turn vary geographically.

Once the dog is infected, the third-stage larval heartworms deposited by the mosquito will grow for a week or two and molt into the fourth larval stage just under the skin at the site of the mosquito bite. Now they can migrate to the muscles of the chest and abdomen, and approximately 45 to 60 days after the infection, they will molt into the fifth stage known as the 'immature adult'. Approximately 75 to 120 days after the original infection, the immature heartworms can now enter the bloodstream and be carried through the heart to begin life inside in the pulmonary artery from where they will gradually occupy the heart. Over the following three to four months, the heartworm will grow greatly in size. There are both male and female worms growing, the female adult worm will average about 30 cm in length, and the male average around 23 cm in length. 7 months after infection, the adult worms will have mated and the females begin giving birth to live young, known as microfilaria (microfilariae = plural), which will remain in the bloodstream for as long as two years. They will remain in the bloodstream until a mosquito lands on the dog to suck some blood! At this stage the mosquito is again vital in the lifecycle of the heartworm, as once ingested by the mosquito, the microfilaria undergo a series of molts to the infective third larval stage, and then migrate to the salivary glands of the mosquito, where they wait to infect another host and begin the cycle again.

So the mosquito is vital at both the infecting stage and again when they land on the dog to suck blood and will suck microfilaria back with the blood to begin the process again. The mosquito has a similar role in many illnesses, including Malaria.

The incubation period required to reach the stage where the microfilariae are transmittable for infecting another host, can be range from two to six weeks, depending upon the climate. warmth of the climate.

From the point a dog is bitten by the mosquito, to the point where worms have grown into adult worms living in the heart, will take six to seven months in and is called the "prepatent period".

heartwom heartwom heartwom

Heartworm sectional diagram

Heartworm from a dog ‘Necropsy’ (autopsy)

A heart infested with heartworm

Inside the dog, the heartworm’s life span is five to seven years.  Adult heartworms resemble strands of spaghetti, with females reaching about 30cm in length and males reaching about 23cm. Worms living inside the infected dog is called the ‘worm burden’, the average worm burden in dogs ranges from 1 to 250 worms, but considering the size of the worm, the effects are devastating.

Course of Infection

Heartworm disease is not contagious, that means that an infected dog cannot pass it on to another dog.

 Heartworm disease can only be spread through the bite of a mosquito.

If  caught in time, Heartworm is easy and low cost to treat

Heartworm is categorized into 4 stages, with stage 1 being the lower and stage 4 being the most severe.