Heartworm In Dogs

How Dogs Get Heartworm

Mosquitoes are the culprits. A dog infected with heartworm usually has millions of tiny larvae, called microfilariae, in the bloodstream. These are picked up by a mosquito, develop further and are deposited in another dog the next time the mosquito feeds.

In the 1980s and 1990s many dogs got sick and died from heartworm in Adelaide (click to read more). The striking thing about these dogs was that we had never seen most of them before. Back then, probably only 50% of dogs attended vets regularly. These other dogs missed the prevention message and were infected without ever leaving their backyards.

What Heartworm Disease Looks Like

Once injected, larvae grow slowly over six months until they reach full adult size. These 30cm long worms are found attached to the walls of major blood vessels such as pulmonary arteries, the aorta, vena cava, and eventually also inside the heart.

dog heartworm
By Alan R Walker (Own work) [CC BY-SA 3.0], via Wikimedia Commons

The damage to vessel walls causes clots to form and break away. This leads to blockage of pulmonary arteries and pneumonia.

The large worms restrict or block major blood vessels.

Many dogs with heartworm have no obvious symptoms. Dogs with severe heartworm infestations have a cough and are clearly unfit and unwell. They can die quite suddenly due to pulmonary embolism.

How Vets Diagnose Heartworm

Many dogs with heartworm are found during routine screening. These are the lucky ones. An instant answer can be gained with a few drops of blood in an in-clinic test.

If the test is positive, we then do other tests to determine the severity of the infection. These include X-rays to look at the degree of lung damage and more blood tests to look for larvae in the bloodstream.

Back in the 1990s we also commonly diagnosed heartworm by accident. Often heartworm would be seen by chance in blood sent to the lab for other tests.
A cool (and creepy) trick is to look at a fresh drop of blood under the microscope. Heartworm larvae could sometimes be seen wriggling among the red blood cells, often in surprising numbers.

Can Heartworm Be Treated?

Yes, but the only effective drugs are quite toxic. As you can imagine, killing large worms which live in the blood vessels also puts dogs at high risk of embolisation, or obstruction.

For these reasons we sometimes deliberately tolerate low numbers of worms if a dog is healthy.

Important: dogs with heartworm can develop fatal reactions when given many heartworm preventions. Please consult your vet before using.

How Often To Give Heartworm Prevention Medicine

Most heartworm prevention is either a liquid applied to the skin or a monthly tablet. Two products deserve a special mention. ProHeart SR-12 is a yearly injection that takes away the need for dog owners to remember a monthly treatment. Sentinel Spectrum is a monthly all-in-one treatment you can read more about at the link.

The good news is that heartworm preventions work very, very well. No dog correctly receiving one of these heartworm prevention medicines recommended by vets should ever get heartworm.

Related: Do cats need heartworm prevention?  Is heartworm prevention safe?