Heartworm is a parasite that is transmitted by mosquitos and develops within the right chambers of the heart and the blood vessels leading into the lungs.

.Picture shows a heartworm microfilaria and adult heartworm clogging the heart of a deceased dog.

Heartworm is found all over the world and here in Australia. Queensland has the worst rates of infection as the disease is more prevalent in warm moist climates as it favours the transmission of the disease by the mosquitos all year round.

Here in Victoria the rates of infections are quite low but it still represents a significant risk. A survey by the Department of Agriculure on foxes killed in Melbourne found that out of 156 foxes, 7% were found to be infected with heartworm. This means that there is a reservoir of infection in Melbourne for domestic dogs to be infected from.

What does Heartworm do?

Heartworm causes heart disease and heart failure. Sudden death can occur by heartworm causing emboli in the pets lungs.

What is the Heartworms life cycle?

Adult heartworm release microfilaria (immature heart worm) into the blood stream. These are then picked up by mosquitos and they develop within the head of the mosquito. Each mosquito can carry a maximum of 3 microfilaria, any more and they die during the development of the microfilaria. The microfilaria must develop within the mosquito before it can infect the next dog. Once it has done its development in the mosquito, when the mosquito bites the next dog the parasite passes into the dog. It takes around 6 months for it to migrate and develop into the adult heartworm and start the cycle again.

How do I know if my dog is infected?

Dogs may not show clinical signs for a long time so sometimes it is detected by a blood test which is usually done prior to putting dogs on heartworm prevention. Otherwise clinical signs consistent with heart disease and certain radiographic changes will lead us to suspect the presence of heartworm. It is only confirmed by blood test which either detect the microfilaria on a blood film or detect antibodies to the adult heartworm. The pictured microfilaria was detected in a blood film on a dog that was being tested for other problems.

Any heart disease in an individual which has not been on heartworm prevention should have heartworm ruled out or confirmed as a cause of the heart disease.

How is Heartworm treated?

Heartworm is treated using drugs which are similar to arsenic. The treatment itself is not without risk as dead worms can float down fromt he heart and cause emboli in the lungs. This is why prevention is most important.

How is Heartworm prevented?

The best preventions involve giving once a month medications which are available on prescription from your vet. These once a month medications are far more effective than daily medications which can be obtained from supermarkets. The best thing is to talk to your vet about it.