Heartworm breaks the heart

Part 4 of our parasite series 

pic by Caitlin of Tom and Captain



Adult heartworm are located in the pulmoary artery, and in bad cases, the right ventricle of the heart. The female adults release stage 1 larvae into the blood stream. These larvae are ingested by mosquitoes when they bite the infested dog. The larvae undergo further development within the mosquito, and migrates into the mouth parts of the mosquito. From here, the mosquito bites another dog. This stage crawls through the skin and into the blood stream. IT then follows the blood flow to its final destination, the pulmonary artery and right ventricle.



For heartworm disease to spread, the right species of mosquitos must be present in high enough numbers. The prevalence of heartworm increases the further north you travel in Australia. Victoria is a low risk area, NSW is higher risk, and Queensland is a very high risk area for this diease. The parasite is also carried by feral animals such as dogs and foxes.

Health Effects on Dogs and Cats

The health effects on dogs depend on how many adult heartworm and lodged in the blood stream. The end stage of heartworm disease is congestive heart failure. Worms that dislodge may wedge in the smaller blood vessels of the lungs, leading to pulmonary thromboembolism (similar to deep vein thrombosis). Dogs may not show signs of heartworm until the disease is very far progressed. Treatment is difficult, especially in heavy infestations.

Cats are much less likely to be affected by the species of heartworm present in Australia.

Health Effects on Humans

Canine and Feline heartworm is not known to affect humans


Canine heartworm disease can be prevented quite easily. An annual long acting injection of a drug that can prevent this infections. A range of monthly products are available. Most combined products that cover fleas and roundworms will also protect against heartworm if used to product directions.

If you travel with your dog, and are likely to be entering higher risk areas at times, the annual injection may be the best option. In this case you do not have to worry about forgetting to give prevention in time, and the injection can be given at the same time as you dog’s annual health check and vaccination.


Which of the following is true?

  1. Heartworm disease is very common in Victoria

  2. Disease caused by heartworm is mild, and easily treated

  3. Heartworm can be easily and effectively prevented

  4. Heartworm is spread by close contact with other dogs










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