Heartworm Disease in Dogs

What are heartworms?

Caused by Dirofilara immitis, heartworms are a potentially fatal parasitic worm living in the pulmonary arteries, lungs & hearts of dogs. Heartworms are nematodes, a type of roundworm, they are several inches long, thin & white.

The disease is spread from mosquito to animals when the mosquito feeds from it's host. This is known as vector borne.

Life cycle of heartworms:

In an infected animal, the adult heartworms produce their young, known as microfilaria, which swim around the bloodstream. Microfilaria require an intermediate host in the form of the mosquito. The mosquito bites an infected dog & takes up some of these microfilaria circulating in the dogs blood. Once inside the mosquito, they undergo further maturation, which takes 10 - 14 days, in which time they become infective larvae. When the mosquito feeds from a dog or dog, these infective larvae are injected into the animal.

Once in the dog, they take around 8 months to mature to adult worms. When they have matured they make their way to the heart (although other organs can also be infected with heartworms). Due to their resistance, cats are usually only infected with a small number of heartworms (usually between 1-3 worms), whereas in dogs numbers are generally higher. However, cats do not tolerate heartworm infection as well as dogs & even one or two heartworms can cause death.

Heartworms live in dogs for around 5-7 years & in cats for around 2-3 years. Cats are commonly found to have only one sex of heartworm, and it is unusual for cats to have microfilaria in their bloodstream.

Symptoms of dog heartworm:

Symptoms of heartworms are often non-specific in dogs. Some symptoms include:

• Coughing

• Wheezing

• Difficulty breathing

• Anorexia

• Vomiting

• Lethargy

• Weight loss

• Some dogs show no signs at all, but die suddenly

Diagnosis of dog heartworm:

Diagnosis of heartworms in dogs is often difficult and not always 100% reliable. There are several tests which your veterinarian may perform, including;

• Antibody test: Detects antibodies made by the dog, to adult heartworm antigen. This may give false positive results if the dog has had a prior heartworm infection which has cleared up. Also, it is possible for the dog to have had microfilaria in the blood & removed this without them developing into adult worms.

• Antigen test: Detects the presence of heartworm antigen in the blood. This relies on the dog being infected with an adult female heartworm, so may give false negatives if the dog is infected with male only or immature heartworms.

• Echocardiogram (ultrasound reading of the heart): To detect the presence of heartworms.

• Radiography: May detect enlarged pulmonary arteries possibly with ill defined margins & an enlarged right hand side of the heart & lung changes.

• Microfilarial Tests: This tests for the presence of microfilaria in the blood. Less than 20% of dogs will have microfilaria in the blood. This may be due to several reasons. As dogs often only have one or two heartworms, they may male only or female only, which would rule out mating of worms producing microfilaria. Also, the dog's immune system may be attacking & destroying any microfilaria present. Therefore a negative blood test will not rule out the presence of heartworms.

Treatment of dog heartworm:  

There are no approved methods to treat heartworm in dogs. The treatments which are available are themselves dangerous. A single dead worm can be fatal in dogs as it can break away & cause a blockage of the pulmonary artery (pulmonary embolism.

If there are no clinical symptoms your vet may decide not to treat the dog & wait for it to clear the parasite in it's own time. As stated earlier, heartworms live for around 2-3 years in dogs. If this is the chosen method, your veterinarian will want to monitor your dog every 6-12 months for signs of complications.

If the dog is displaying symptoms of heartworm disease supportive therapy may be recommended. Prednisone may be given to the dog to reduce the inflammation & reaction to the worm.

Dogs with severe symptoms may require additional supportive therapy such as a bronchodilator to open the airways, oxygen therapy & intravenous fluids.

Adulticide treatment may be recommended for dogs with clinical signs who are not responding to supportive care. Caparsolate is the drug used & kills the adult worms. This carries risks, as a dead worm can result in a pulmonary embolism. Around 1/3rd of dogs receiving treatment will face life threatening complications as a result of the dying worms. Confinement will be necessary for a few weeks after treatment. Either way, if you choose to let nature take it's course & hope that the worm lives out it's lifespan within the dog, or if you use an adulticide there are risks. These must be weighed up by your veterinarian before a decision is made. Surgical removal of the worms has been used in some cases.

How are heartworms prevented?

There are several products on the market which can be used to prevent heartworm. These are administered monthly. It is recommended that your veterinarian perform the necessary testing for heartworm infection prior to using these products. Heartgard®, RevolutionTM & Interceptor®.

Can I catch heartworm?

Yes, but not from your dog, but it is possible to become infected via a mosquito bite. Heartworm infections in humans are extremely rare.

Please note:

The medical articles on this site have not been written by a veterinarian & should not be considered a replacement for a veterinarian visit. The articles are provided for informative purposes only. Always seek immediate veterinary advice for any problems (health or behavioral) in your pets. While great care has been made in the creation of these articles, we cannot guarantee the accuracy or omissions on these pages. If in any doubt whatsoever, seek professional medical advice.  

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