What is heartworm?

Heartworm disease in pets is caused by the parasitic roundworm Dirofilaria immitus, which is endemic in many regions of the world, including large areas of Australia. The disease is prevalent in areas with humid, tropical conditions, where mosquitoes are found. However, it can also be found in cooler climates if mosquitos carrying heartworm larvae are blown by wind to new areas, or if an infected dog travels to another area and is bitten by a mosquito. This means that indoor-only pets are also at risk because it is very difficult to keep a home 100% mosquito-free.

Dogs are the natural hosts of heartworm, and are more susceptible to the disease than cats are. However, cats can still become infected and suffer serious illness and death from heartworm disease.

Heartworm life cycle:

The heartworm has an indirect life cycle, meaning it requires an intermediate host or a vector to complete its development into an adult. There are actually two types of host in its life cycle: 

1. A population of infected dogs in the environment. In Australia, the wild dingo is a major source of heartworm, with one study in 2016 showing that >70% of wild dingoes were heartworm positive. Domestic dogs are also natural hosts.
2. The mosquito, which acts as a vector, or intermediate host, transmitting immature heartworm stages from an infected host to a new host.

Therefore, heartworm cannot be transmitted directly from one dog (or cat) to another. The mosquito is needed to transmit the parasite from an infected animal to another animal.

Only female mosquitos feed on blood. When they bite a host infected with heartworm, the immature mirofilaria develop inside the mosquito for 10-30 days, reaching larval stage 3 (L3). The mosquito then transmits the L3 larvae when it feeds on a new host (dog/cat), delivering the larvae under the host’s skin. In both dogs and cats, the larvae will develop to stages L4 and then L5 in the 60-day period after transmission.

These immature heartworms become young adults in 6-7 months and travel via the host’s bloodstream to the blood vessels in the lungs and the heart. Adult females can grow to a length of 15-35cm and a width of 3mm. Adult males are around half this size. At 8 months they produce a new generation of microfilaria, which are carried through the host’s bloodstream and picked up by a mosquito during feeding. If not treated, heartworms can live for up to 5-7 years in dogs and 2-4 years in cats.

Signs of heartworm infection in dogs:

It can take several years for some dogs to show signs of illness, depending on the worm burden of each individual dog. Signs result from the heart and lungs having to work harder, with heartworms damaging the tissue of these organs and the lining of their blood vessels. Signs you might notice are generalised, and include:

• Exercise intolerance
• Lethargy
• Shortness of breath
• Coughing
• Weight loss
• Damage to other organs in severe cases

Signs of heartworm infection in cats:

Although cats are believed to be more resistant to heartworm infection than dogs, the condition is generally much more serious in this species. Additionally, the disease is more difficult to identify in cats, possibly because it presents similarly to feline asthma and therefore goes undiagnosed. For these reasons, it is thought that the incidence of heartworm infection in cats is much higher than previously believed.

Because a cat’s heart is smaller than a dog’s, it potentially takes only 1-2 resident adult heartworms to cause a fatal blockage. Cats can mount an immune reaction against the immature microfilariae in their bloodstream, but this immune reaction itself can cause damage to the cat’s lungs and other body tissues. Signs you might notice in a cat are also generalised and include:

• Sudden death (caused by adult worms blocking the heart)
• Increased respiratory rate and/or difficulty breathing (any breathing difficulty in a cat is an emergency and requires immediate veterinary attention)
• Vomiting and/or diarrhoea
• Reduced appetite
• Possibly coughing
• Lethargy
• Collapse

Treatment for heartworm:

***The best treatment for heartworm disease is PREVENTION!***

Because treatment is often delayed due to the disease not being diagnosed until it has progressed to a severe level, this makes treatment itself quite dangerous, particularly in cats but also in dogs. This is because once adult heartworms have been killed, they are carried in the bloodstream to the lungs where they can block blood vessels. If there is a relatively small worm burden only the smaller vessels will be blocked but in large worm numbers the larger blood vessels can also become blocked, leading to death. As already mentioned, in a cat it only takes 1 or 2 adult worms to cause a blockage and death. It is also possible for anaphylaxis (an allergic reaction to dead heartworms) to occur in dogs.

Treatment in dogs commonly involves administering drugs to kill both the adult and immature heartworms. This is done under close veterinary supervision. Some dogs might first receive treatment to minimise blood clot formation and the effects of damage to blood vessels. In cases with a large worm burden, surgical removal of worms from the right side of the heart (where they live) might be warranted.

In cats, treatment options are restricted. Adult heartworms can be removed surgically from the cat’s heart at a specialist veterinary centre, however the mortality (death) rate is still quite high in cats undergoing this procedure so it is generally considered a last resort. The drugs commonly given to dogs can be less effective, and quite dangerous to cats, so the primary option for treatment, if the disease is not too severe, is to try to control the signs of illness in the hope that the cat will live longer than the heartworms.

How can pet parents reduce the risk of heartworm disease in their dog or cat?

• Minimise contact between mosquitoes and your pets by removing areas of stagnant water from your home environment. This includes ponds or fountains, swimming pools and pot plants with catch trays underneath, where water can accumulate.
• Install fly screens on all doors and windows, to reduce the mosquito population in your house.
• Treat all dogs and cats with regular heartworm prevention, at the dose and interval recommended by the product’s manufacturer. Because heartworm takes around 6 months to mature to adulthood, any adult pet which has lapsed in preventative treatment for longer than this, or which is over 6 months of age and beginning prevention for the first time, should first be heartworm tested by your veterinarian, to reduce the risk of adverse reaction or death from treatment.
• Heartworm prevention is often available in a combination parasite prevention product, such as monthly spot-on’s, tablets and chews. For dogs, there is also the option of a yearly heartworm injection. For cats, ensure the product you use is specifically designed for cats (dog and cat products are not interchangeable!) Also take care to follow the age, bodyweight and life-stage advice, and other recommendations accompanying the product.
• Talk to your vet about heartworm prevention at your first visit with a new pet, and before starting any new preventative treatment, to ensure the product is safe for your pet.
• Contact your vet if your dog or cat shows any of the abovementioned signs of illness, so that an examination and diagnostic testing can be carried out.

Summary:

Heartworm disease is a serious and often fatal condition in our dogs and cats, with the incidence probably much higher than we realise. Australia’s climate and environment, along with the high number of infected canines in both domestic and wild dog populations, mean that our pets are at high risk of infection via mosquitoes.

Treatment for heartworm infection is also dangerous and often expensive and unsuccessful. However, regular heartworm prevention is very effective and relatively inexpensive. All domestic dogs and cats should receive regular heartworm prevention, based on veterinary advice for that individual, and tested for heartworm prior to commencing treatment if there is a possibility of infection.

Image source: American Heartworm Society

https://www.heartwormsociety.org/

https://www.heartwormsociety.org/veterinary-resources/practice-tools/incidence-maps

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