Heartworm in dogs

Heartworm is a potentially fatal parasite (Dirofilaria immitis) that lives in the heart and the blood vessels around the heart.

What is heartworm disease?

Heartworm disease is a serious and potentially fatal disease in pets and is caused by worms that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body

How is the disease transmitted?

The mosquito is an essential part of the heartworm lifecycle.Adult female heartworms living in an infected dog, produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worm. Then, when theinfected mosquito bites another dog or cat, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, the larvae mature into adult heartworms in about six months. It’s important to note that the parasite requires 30 days over 18 degrees to undergo its life cycle in the mosquito so the disease is spread in the summer months only.

How do we prevent heartworms?

Puppies should be started on injectable heartworm preventatives at 12-14 weeks of age. You can usually synchronise the heartworm vaccination with your dog’s annual checkup and booster vaccinations. Other treatments require monthly topical application or feeding of a tablet every month.


It is best for any dog over six months of age that hasn’t been on heartworm prevention to have a heartworm test. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins. Some preventatives can be used safely without testing and prevention over the summer is the minimum requirement for the prevention of spread of this disease.