What is Canine Heartworm?

heartworm prevention 2010 incidence map

And is it a death sentence for your dog?


WARNING: Graphic Pictures

Heartworm infection in your canine will more than likely cause serious disease if left untreated. Heartworm (Dirofilaria immitis) is a parasitic roundworm that is spread by mosquitoes. It is a nasty and ugly infection and will eventually cause your dog great pain and suffering if not treated.

It is referred to as “heartworm” because the adult worm stays mostly in the right ventricle of the heart where it will live a long time if not exterminated.  Heartworm in canines was first written about in medical journals in 1847 while felines were not recognized victims until 1920. It was once thought that heartworm affected mostly dogs in the south eastern United States but it’s now known that dogs everywhere are affected by heartworm and only Alaska and some northern parts of Canada have some immunity from infection in dogs. That means pretty much any place that is too cold for mosquitoes will have a very low incidence of heartworm infections.

The Technical Stuff:

This is the cycle of the heartworm as it goes through several stages: (taken from Wikipedia, The Free Encyclopaedia)

“Heartworms go through several life stages before they become adults infecting the heart of the host animal. The worms require the mosquito as an intermediate stage in order to complete their life cycle.Development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27 °C (80 °F). Below a threshold temperature of 14 °C (57 °F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically. The period between the initial infection when the dog is bitten by a mosquito and the maturation of the worms into adults living in the heart takes 6 to 7 months in dogs and is known as the “prepatent period”.

After infection, the third stage larval heartworms (L3) deposited by the mosquito grow for a week or two and molt to the fourth larval stage (L4) under the skin at the site of the mosquito bite. Then they migrate to the muscles of the chest and abdomen and 45 to 60 days after infection, molt to the fifth stage (L5, immature adult). Between 75 and 120 days after infection these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. Over the next 3 to 4 months they increase greatly in size. The female adult worms are about 30 cm in length, and males are about 23 cm with a coiled tail. By approximately 6.5 to 7 months after infection, the adult worms have mated and the females begin giving birth to live young, called microfilariae.

The microfilariae circulate in the bloodstream for as long as two years, waiting for the next stage in their life cycle in the gut of a bloodsucking mosquito. When ingested by a mosquito, the microfilariae 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. The incubation period required to reach the stage where the microfilariae become transmittable to another host can be as little as two weeks or as long as six weeks, depending on the warmth of the climate, and the larval life cycle ceases entirely if the ambient temperature drops below 14° Celsius (57° Fahrenheit).”

Watch this video showing the removal of 87 heartworms from this poor dog

Who gets heartworm and what are the symptoms?

heartworm prevention 2010 incidence map
The 2010 Heartworm Incidence Map as provided by the American Heartworm Society

It is difficult to detect heartworm infection under normal circumstances. No symptoms occur whatsoever during the “prepatent period’ referred to above and it’s not until the adult worms have taken up residence in the heart, that your dog might even start showing signs of being ill. Coughing and early exhaustion after exercise are the first and earliest symptoms. If a dog goes untreated for heartworm infection, more serious symptoms will start to occur such as weight loss, fainting and coughing up blood. Ultimately congestive heart failure will kill your beloved pet.

Detection and Treatment of Canine Heartworm

Antigens released by the adult female worm can be detected through blood tests. These tests are the most commonly used but are useless prior to the period where the antigens are being produced. In other words, blood tests for heartworm done within the first 6-7 months of the initial infection would not return positive results for heartworm infection. After a dog is diagnosed positive for heartworm, a test for the presence of microfilariae should be performed as well as x-rays of the lungs to assess the damage done by the worms.

Treatment for heartworm is common and most often successful if conducted properly and with care. Immiticide, an arsenic-based formula is administered by your vet to kill the worms. Treatment will take several weeks while the dog’s body absorbs and passes the dead worms and your dog will have to be kept at a restful state. Exertion could cause the dead worms to break up and lodge into the lung areas causing respiratory failure. Of course, your dog should be checked for any liver, kidney, heart or lung problems prior to HW treatment to ensure he/she is healthy enough to undergo treatment. Once the microfilariae have also been destroyed, another heartworm test is done which should come back negative.

Can Heartworm Be Prevented?

ABSOLUTELY! Heartworm can and SHOULD be prevented especially if you live in an area that has a lot of mosquitoes and maintains moderate temperatures year round. Several forms of Heartworm treatment, tablets, injections or topical applications are available from your vet and you should consult with them to get the best one for your beloved critter.

Note: Homeopathic treatments have not been proven to work and should be used only after careful consideration.

Is Heartworm a Death Sentence for Your Beloved Critter?

ABSOLUTELY NOT! Heartworm is a very serious illness for your beloved critter to come down with but it is not a death sentence. Treatment is available and is relatively inexpensive. Of course, prevention and early detection are the best ways to ensure that your dog never has to wage a battle against serious infections. Have your dog checked annually for heartworm and start your preventative treatments today.


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