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April is Heartworm Awareness Month!

Investigating drug resistance in Australian canine heartworm infection

Heartworm disease is a worrying condition for dog owners and whilst less prevalent than a decade ago, it is still a disease that kills pets. April Heartworm Awareness Month was created to help raise visibility around the importance of testing dogs for this disease.

Heartworm disease in dogs is caused by the parasitic nematode Dirofilaria immitis and is transmitted by mosquitos [1]. 

More information about the heartworm lifecycle, the signs and symptoms of heartworm infection and preventative medications used to treat heartworm can be found here:

Currently, canine heartworm disease affects hundreds of dogs in Australia every year, predominantly in Far North Queensland. Thousands more dogs live with the threat of infection, with many owners using year-round preventatives to protect their dogs from acquiring this deadly disease. 

Spread by mosquitos, canine heartworm has a wide but poorly understood geographical distribution that appears to change over time. 

Currently, central and northern Queensland are experiencing high rates of infection, however, less than 20 years ago dirofilariasis also caused heartache for owners in coastal NSW including Sydney. 

Dogs surveyed for the presence of Dirofilaria immitis antigen in Australia. a Geographical distribution of dogs tested for heartworm as part of 2016–2019 prevalence surveys. The size of the circle at each location in New South Wales and Queensland (Central and Northern) is proportional to the number of tested dogs, number in brackets indicates number of D. immitis antigen positive dogs / total number of tested dogs. b Violin plots of distribution of age of 566 dogs sampled as part of four distinct cohorts, mean and quartiles are show within the violins. On the far left of the figure is a scatter dot plot of the individual ages of 13 antigen-positive dogs

The use of preventatives to stop infection is widespread among dog owners, with the Macrocyclic Lactones (MLs) drug family the mainstay of prevention.

Heartworms are parasites able to live inside the blood vessels of the lung and chambers of a dog's heart feeding on the continuous supply of blood available to them. In very severe cases, heartworms can be found in populations numbering over 200!

The offspring of a heartworm are referred to as microfilariae, which can be found in the blood of an infected dog [2] – see Figure 1.

Figure 1: Microfilaria found in the blood viewed under a microscope (x400). Microfilariae are 250 to 300 μm (.25 to .3 mm) in length, compared to the dog’s red blood cells which have a diameter of 6 to 8 μm (.006 to .008 mm) [3, 4]

How do the heartworms affect a dog's heart?

Video courtesy of Okstate Parasit D-lab [5]

So microfilaria are present in the blood of dogs with heartworm but how does this affect the heart and lungs?

A dog can be infested with heartworm for years before clinical signs begin to show. When heartworms mature and increase in number, they begin to congregate in the heart and major blood vessels carrying blood from the heart. 

This results in impaired blood flow through the heart and decreased blood supply to the major organs. As blood transports oxygen without a sufficient blood supply the oxygen supply is also reduced. This can result in the vital organs such as the heart and lungs having impaired function [6].

Adult heartworms are unable to attach to the walls of blood vessels or the heart and cannot swim, so their movement is essentially dictated by blood flow. As such they tend to accumulate in the right side of the heart and the pulmonary arteries (carrying blood from the heart to the lungs) (See Figures 2 and 3).

Figure 2 The typical location of heartworms in the heart of a dog. The pulmonary artery takes blood from the heart to the lungs and when clogged with heartworms reduces the blood flow to the lungs where blood replenishes its oxygen supply. This results in reduced blood flow to other organs in the body.

Figure 3: Mature heartworms inside 
the heart of a dog. 
The right atrium (RA) and right ventricle (RV)
 have been opened to expose the heartworm. 
The heartworm tend to accumulate
 on the right side of the heart. 
LA = left atrium, LV = Left ventricle. 

Image from Barnette and Ward [6].
Blood flow through the heart is one way because of valves located between the atria and ventricles and in the major arteries taking blood away from the heart.

In a healthy dog, the pulmonary valve opens during contraction of the ventricle to allow blood to be ejected from the right ventricle to the pulmonary arteries carrying deoxygenated blood to the lungs. The valve closes to prevent the flow of blood back into the ventricle. 

With the accumulation of heartworms, the pulmonary valve cannot open properly, so less blood is pumped to the lungs for oxygenation. It also cannot close properly during relaxation, so blood from the arteries can flow back into the ventricle. Both these malfunctions can lead to major disruption of the heart [7].

Microfilariae can also have an effect on the dog’s general health. As microfilariae circulate through the body, they are also commonly found in the smallest blood vessels called capillaries where they are can become lodged and block blood flow.

The tissues mainly effected by microfilaria are the lungs and liver as both of these organs are the major sites of oxygen and nutrient exchange, so contain the greatest number of capillaries. Without oxygen, these organs begin to deteriorate [6].

What is the current situation of heartworm in Australia?

Canine heartworm is currently endemic in Australia, with high infection rates occurring in Queensland [1, 8]. 

A recent case of canine heartworm infection also occurred in Sydney in 2020 when adult heartworms were obtained from the heart of a dog [9]. To prevent heartworm infections, veterinarians and pet owners mainly use the macrocyclic lactone (ML) class of drugs. These drugs are administered orally or topically each month, or via yearly injections [1]. 

However, resistance to these drugs has emerged in heartworms in the USA… but what about here in Australia? Do we also have drug-resistant heartworms infecting our beloved canines? The potential emergence of resistance in Australian heartworms is a major concern for the veterinary industry, as it would limit the effectiveness of current preventive drugs.

In the Veterinary Parasitology Laboratory at The University of Sydney in Prof Jan Slapeta’s research laboratory, PhD student Rose Power is investigating the drug resistance status of the Australian canine heartworm population. The work was initiated with funds from the Canine Research Foundation

To do this, a variety of techniques and tools are utilized. Blood samples from heartworm-infected dogs residing in Queensland, with a few dogs occasionally from New South Wales are sent to the lab for analysis. 

By obtaining heartworm microfilaria from infected Australian dogs before and after drug treatment, it is possible to determine whether the drugs are still working [10].

Microfilariae are also genetically tested to determine whether Australian heartworm populations possess resistant signatures identical to those in the USA.

Now the good news - by performing these genetic tests, no evidence of drug resistance in canine heartworms in Australia has been found. Surveillance, however, is critical. 

The question is a case of not “if” but “when” ML-resistance in Australian canine heartworm makes headline news. The team at the University of Sydney has the tools to survey and inform the veterinary profession to keep our dogs safe from heartworm.


We thank Emeritus Prof Brian Corbitt and Dr Steven Holloway (BVSc MVS PhD MACVSc Dipl. ACVIM) for their valuable comments on this article.

About the Contributors

Prof Jan Slapeta is Professor of Veterinary and Molecular Parasitology at the University of Sydney.

Rose Power is a current PhD student in the School of Veterinary Medicine at the University of Sydney. Her thesis is entitled “Investigating macrocyclic lactone resistance in canine heartworm (Dirofilaria immitis) from Australia”

Meg Hamill completed her Bachelor of Biomedical Science at Deakin University at the end of 2021 and worked with the Canine Research Foundation as part of her Professional Practice Placement researching a writing a series of articles.

Assoc Prof Jan West Jan is a lecturer in the School of Life and Environmental Sciences at Deakin University. Jan was appointed as a trustee to the Canine Research Foundation in 2014.

To learn more, visit the website: or 
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  5. Okstate Parasit D-lab. Dirofilaria immitis – microfilaria [video on internet]. Oklahoma: Oklahoma State University Centre for Veterinary Health Sciences;2012 Dec 29; [cited 2021 Sept 16]. Available from:
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  7. Jones S L. TVA [Internet]. Today’s Veterinary Practice; c2021. Canine Caval Syndrome Series, Part 1: Understanding Development of Caval Syndrome. N.d. [cited 2021 Sept 15]; [about 9 pages]. Available from:
  8. Orr B, Ma G, Koh WL, Malik R, Norris JM, Westman ME, et al. Pig-hunting dogs are an at-risk population for canine heartworm (Dirofilaria immitis) infection in eastern Australia. Parasit Vectors. 2020;13(1):69.
  9. Lau DC-W, McLeod S, Collaery S, Peou S, Tran AT, Liang M, et al. Whole-genome reference of Dirofilaria immitis from Australia to determine single nucleotide polymorphisms associated with macrocyclic lactone resistance in the USA. Current Research in Parasitology & Vector-Borne Diseases. 2021;1:100007.
  10. Pulaski CN, Malone JB, Bourguinat C, Prichard R, Geary T, Ward D, et al. Establishment of macrocyclic lactone resistant Dirofilaria immitis isolates in experimentally infected laboratory dogs. Parasites & vectors. 2014;7(1):494.
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