Heartworm Parasite: Prevention, Symptoms and Treatment

August 24th, 2018
dog health

Learn more about the parasite that can harm and even potentially kill your pet. By Carrol Baker.

Have you ever seen a cringe-worthy image of a heartworm infestation in a dog? It is like something out of a B-grade horror movie, but unfortunately, it can be all too real.

As the name suggests, heartworm is a writhing wriggling spaghetti-like parasite that lives in the heart and blood vessels of dogs, cats, dingoes and foxes. Once an infestation takes hold, it can be fatal to animals. The good news, however, is that heartworm is a disease that is largely preventable.

One little bite

Dr Hope Richards from Willoughby Veterinary Hospital explains that heartworm is distributed by mosquitoes. “The mosquito bites a dog that is heartworm-positive; the mosquito then carries the microscopic heartworm (microfilariae) and then bites another dog, who then carries the disease,” she says.

In the life cycle of the heartworm, the dog acts as an involuntary host. The microfilariae mature into adults (which takes around six months), then they find a mate and reproduce, which is why there can be so many worms inside an infected dog’s heart. Heartworms can live for around five years in dogs and, over time, these parasitic villains can grow up to 20cm long.

Dr Hope says this is why taking steps to use heartworm preventive measures is crucial. “Treating your dog is one big step in the right direction towards eradicating this disease,” she says. “We know that it almost never occurs in areas where people treat their pets with preventive measures.”

A broken heart

Over time, the parasites compromise the way the heart functions; the heart is designed to pump blood to tissues and vital organs in the body. “Once infected with heartworm, the worms actually clog the vessels that carry the blood,” says Dr Hope.

Early symptoms

In the very early stages of infestation, a dog may show no clinical signs. In fact, outwardly a dog may appear to be healthy and happy, but inside heartworms may be thriving.

Heartworm is a sinister disease that progresses very slowly. “It can take many months before the dog shows any indications it has the disease and by the time the signs appear, the dog’s heart is already under extreme stress,” says Dr Hope.

As the infestation spreads, you may notice a dry persistent cough, lack of appetite, lethargic behaviour, reluctance to go for walks, and slow recovery after exercise, she adds.

With severe cases of heartworm, the blood flow is seriously compromised, leading to collapse. At this stage, surgery can be performed to manually remove the heartworms via the jugular, but there is only a very slim chance of survival.

Heartworm can also cause secondary issues. Some dogs will develop an allergic reaction to the presence of the heartworms in their lungs.

Dogs living in tropical climates where mosquitoes thrive in the warmer weather may be more at risk of being bitten by mosquitoes but all dogs are potentially at risk, unless preventive steps are taken.

“Heartworm can affect all dogs equally,” says Dr Hope. “Even if your dog has long fur, there are still areas on the body a mosquito can get to,” she says.

Playing it safe

There are numerous preventive treatments. These range from an annual injection from your vet, monthly chewable tablets or spot-on treatments used in conjunction with flea and other worming measures. The option you choose for your pet is up to you but opt for the one that will easily fit into your lifestyle and routine, so you won’t forget to protect your pet.

Start your puppy on preventive treatment when they are around three months of age.

Treating heartworm

If you haven’t been treating your dog for heartworm, or have forgotten to administer a dose of prevention, take your dog to the vet for a blood test to rule out heartworm. This will be followed by another test in around six month’s time. “The first test may be a false negative, so a secondary test is always recommended,” says Dr Hope.

If a dog has symptoms, your vet will recommend a course of treatment, which is by no means a quick fix. Like many diseases, the earlier the dog is treated, the greater its chance of survival. The sad reality is that once a dog has heartworm, there is no guarantee that they will survive, even if they’re given the correct treatment.

If the dog recovers from the disease, there may be permanent damage to the heart’s arteries and scarring of other tissues, which can compromise their quality of life.


A word of caution: Do not give preventive medicine if you think your dog may already be infected. It can cause severe side effects.

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