Pet Info

Heartworm History and Update


Heartworm has been known for over 140 years and until recently (about 1960 -1965), it was regarded as a disease limited to the warm coastal regions of the world e.g. France (but not the UK), USA, Japan, Northern Territory and the eastern coast of Australia (especially Queensland). In areas with high mosquito numbers, 100% of unprotected dogs catch heartworm. 

In the USA, surveys over the last 30 years have shown a spread from the sub tropic southern and south-eastern states into the colder northern states, both along the coast and inland. Now only the upper north-western states of the USA (e.g. Washington) remain relatively free from the disease. The spread has been particularly noticeable along the river valleys of the Mississippi, Missouri and Ohio rivers, which extend from the warm south to the colder north.

History of Heartworm in Australia

Heartworm (Dirofilaria Immitis) arrived in NSW in the 70’s from Qld. I was working in my first job at Five Dock in 1982, and we were seeing numerous cases each month. The only preventative was Dimmitrol, a daily tablet. Trouble was, if you missed just one day, your dog could catch heartworm, even if you doubled the dose the next day.

The only treatment at that time was an intravenous injection of an arsenic containing drug (Carparsolate) which sometimes wiped out a dog’s liver resulting in death. If the dog moved whilst injecting and a small drop of the injection accidentally went under the skin, you would get a huge area of dead skin that fell off (sloughed) 2 days later. In other words, a really nasty chemical burn developed.

What is Heartworm?

Heartworm is a long thin parasitic worm that grows to approx.10 - 20cm in length. They are a bit thinner than your regular garden worm. Adults live in the right side of the heart. They block up the right heart chambers and major blood vessels trying to send oxygen poor blood to the lungs (pulmonary arteries).

Heavily infected dogs can have 200 or more adult worms inside the heart, and in severe cases, the major blood vessels returning oxygen poor blood to the heart (vena cava) also get blocked up. Dead adults leave the heart and travel up to the lungs where they cause a foreign body reaction (embolus) resulting in severe lung damage. Live adults cause damage to
the pulmonary veins which scar up become narrower, meaning it is harder for the heart to pump oxygen poor blood to the lungs.

How common is Heartworm?

Just last month, Simone (my assistant vet) received a call from a friend on the central Queensland coast who had purchased a vet practice. They diagnosed 60 dogs with heartworm in the first month! The previous vet had only promoted monthly heartworm preventatives and many clients had forgotten to give them on a regular basis.

In the 80’s, Sydney infection rates were spiraling upwards. Not many dogs were on prevention, and the heartworms and mosquitoes had a huge population of unprotected dogs to “work on” and spread the disease, so to speak. We saw 50-60% infection rates in unprotected dogs in those days.

In the Shoalhaven area, approx. 20-25% of unprotected dogs are at risk, depending on how many mosquitoes are in the area. There are regional hot spots along certain waterways and ponds. Going for walks in these areas exposes unprotected dogs to a much higher risk of contracting heartworm. Ipersonally treated 3 dogs in Wollongong that all lived up against Cabbage Tree Creek in Fairy Meadow. In fact, one dog had to be treated twice over 5 years after the owners forgot to stay on monthly prevention after the first treatment. A very expensive exercise!

How does Heartworm spread?

Adult worms mate and produce microscopic babies that leave the heart and travel around in the blood stream all day long. A mosquito biting this dog sucks up these babies (microfilaria) where they spend a few months in the mosquito’s body developing. Eventually, they find their way to the salivary glands. When the mosquito bites another dog, they “abandon ship” and enter the dog’s skin. After a few months of traveling through the dog’s tissues, they find their way to the heart where they “setup base” and start growing to their adult size. Incidentally, the occasional human picks up adult heartworm infection but usually the immune system manages to destroy them before they get too big.

What are the symptoms?

Affected dogs have severe damage to the lungs and major blood vessels trying to pump oxygen poor blood to the lungs.

Classic symptoms are:


Going back to the early days, we struggled for a really accurate test. A positive result had to be posted to Queensland for a confirmation to rule out an infection by a “heartworm cousin” that did not cause major illness in dogs (Dipetalonema). In other words, we had to double check the result to make sure it was the real thing. All this took a week or so. Some of the tests were so inaccurate that dogs escaped detection and went on to develop severe life threatening infection several months later.

Some dogs were diagnosed by classic changes to the lungs and heart on X-rays. When ultrasound became available to the veterinary industry, specialists could actually see the worms inside the pulmonary arteries in those difficult to diagnose cases (see video below).

Nowadays, we have really simple blood test kits that are very accurate and only take 5 minutes to run. How easy is that!


The good news is we stopped using the hasty arsenic injections in the 90’s with the advent of a French product that is much safer to use (Immiticide). Depending on the severity of the infection, treatment varies.

The main complication to avoid when treating is not to kill all the adult worms in one go. If you do, a huge number leave the heart all in one go and causes a major life threatening blockage of dead worms in the pulmonary arteries (a pulmonary embolism). This is especially the case in cats where treatment is actually more likely to kill the cat than doing nothing.


There is no argument on this one. The yearly Proheart injection is the best invention since sliced cheese. Twelve months of peace of mind and not having to remember to give your dog a monthly preventative. The cost of the injection is often cheaper then 12 months of monthly preventatives, depending on a dog’s weight.

Can I just start my dog on the Proheart injection if it’s not been on prevention at all?

Unfortunately, Proheart is a little too effective in preventing heartworm, as it also kills immature worms up to 2-3cm in length living inside the right heart chambers. If you kill all these in one go, they leave the heart in a “big blob” and cause a pulmonary embolism characterised by a very sick dog coughing up blood and mucous and gasping for air.

So, before starting Proheart injection in an adult dog that has not been on prevention, a blood test must be performed. The same holds for dogs that are more then 3 months overdue on their Proheart  injection.

Puppies can have their first Proheart injection at 5 months without testing as they are not old enough for the immature heartworms to have grown to the 2-3cm length. That’s why we give tshe Proheart injection at desexing time (5 months of age)

N.B. Daily heartworm prevention tablets have been shown to have a significant failure rate in high risk areas i.e. where there are lots of mosquitoes. For this reason, we do not encourage owners to use them. Similarly, owners quit eoften forget to give their dog its monthly oreventative (as seen in the story form Queensland above).