Heartworm in Dogs and Cats

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Bakers Hill Veterinary Hospital
4609 Great Eastern Highway
Bakers Hill
WA 6562

08 9574 1061

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Heartworm (scientific name Dirofilaria immitis) is a thin, long spaghetti-like worm that lives in the right chamber of the heart and in the major arteries of the lungs. In severe infections it can back up into the veins of the liver. It is transmitted by mosquitoes. It was brought over from America approximately 40 years ago. It is more common in wet warm areas where mosquitos can breed easily, as this increases the number of transmissions, but occurs wherever there are mosquitoes. Indoor dogs have less risk than outdoor dogs. It is rarely contracted in cats but does occur.

The heartworm lifecycle

The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms living in an infected dog, fox, coyote, or wolf produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal's skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. Because of the longevity of these worms, each mosquito season can lead to an increasing number of adult heartworms in an infected pet.

Dogs. The dog is a natural host for heartworms, which means that heartworms that live inside the dog mature into adults, mate and produce offspring. If untreated, their numbers can increase, and dogs have been known to harbor several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. For this reason, prevention is by far the best option, and treatment—when needed—should be administered as early in the course of the disease as possible.

Cats. Heartworm disease in cats is very different from heartworm disease in dogs. The cat is an atypical host for heartworms, and most worms in cats do not survive to the adult stage. Cats with adult heartworms typically have just one to three worms, and many cats affected by heartworms have no adult worms. While this means heartworm disease often goes undiagnosed in cats, it’s important to understand that even immature worms cause real damage in the form of a condition known as Heartworm Associated Respiratory Disease (HARD).

Moreover, the medication used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the effects of heartworm disease.

Clinical signs of heartworm


Unfortunately there are no early signs, and many animals show no symptoms at all. The longer the infection is present and undiagnosed, the more likely symptoms will develop and irreversible damage will be done to your pet. Animals that are active, obese or have other disease are more likely to show signs earlier. As the disease slowly progresses, the dog may show symptoms such as coughing, decreased appetite, tiredness, loss of endurance during exercise, weight-loss, a pot belly and, in advanced stages, liver and heart failure. The worms interfere with the ability of the heart to pump blood. Clots can form and shower the lungs, causing parts to lose blood supply and die. The body reacts to the worms causing further damage especially to the lungs as well as to the vessels. Dogs with large numbers of heartworms can develop a sudden blockage of blood flow within the heart, leading to a life-threatening form of cardiovascular collapse. This is called “Caval Syndrome”, and is marked by a sudden onset of laboured breathing, pale gums, and dark bloody or coffee-coloured urine. Without prompt surgical removal of the heartworm blockage, few dogs survive.

Prevention of heartworm

Heartworm disease is risky and difficult to treat, but prevention is easy!

Heartworm prevention should begin at 12 weeks of age and, to be effective, must continue for the whole of the pet’s life. Older pets that are not on heartworm prevention should be tested and be negative before going onto heartworm prevention medication. This is because, if they receive preventative medication when they have adult worms in their heart, they can have a severe reaction where parts of the worm may die off and be sent through the arteries of the body, where they act as clots and points of infection. This can lead to serious life-threatening complications.

There are three groups of drugs available to prevent heartworm:

Daily: Diethylcarbamazine citrate or DEC is found in all the daily tablets. It's cheaper than the monthly tablets but must be given every single day. Once the dog is over six months old, if two tablets are missed in a row, the dog must be re-tested before another tablet can be given. DEC given to an infected dog can be fatal.

Monthly: There are various types of monthly preventative products – these come in the form of tablets or spot-ons. Heartguard, Interceptor, Sentinal, Panoramis, Revolution and Advocate are all examples of monthly products that can be used to effectively prevent Heartworm. However, monthly products need to be given on the same day each month to be effective. If a dose is missed you need to continue prevention as soon as you realise this, but your pet will have been at risk of getting heartworm in the time that they were unprotected. Therefore we recommend in this case to have your pet be re-tested for heartworm after six months (ie the life-cycle of the heartworm).

Yearly: Proheart is a once yearly injection given by us your vet to prevent heartworm. This is suitable for pups from 12 weeks and is the preferred method of preventing heartworm at Bakers Hill Veterinary Hospital. This injection is ideal for owners who want the convenience of not having to remember daily or monthly products. The injection is based on the weight of the dog, is given twice in the first year of life and then once a year. It can be given at the same time as your dog's yearly vaccinations, or you can offset it so that you have a visit with us every 6 months.