mosquito heartworm dog

Heartworm disease in dogs

Heartworm is a parasite, which is transmitted by mosquitoes. Heartworm infection is much more common in dogs than cats, but it can cause serious illness and even death in both species. Preventative precautions should be taken when dogs are traveling abroad or when dogs are being adopted from outside the UK. Here we will discuss how to prevent heartworm disease in your dog.

This article was written by a FirstVet vet

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What causes a heartworm infection in dogs?

What are the symptoms of heartworms in dogs?

How is a heartworm infection in my dog diagnosed?

How can a heartworm infection in my dog be treated?

How can heartworm be prevented in dogs?

What causes a heartworm infection in dogs?

Heartworm disease is caused by a parasite, Dirofilaria immitis, which is blood-borne. It is transmitted to dogs through the bite of mosquitoes, which are carrying heartworm larvae. These larvae migrate through the bloodstream to the heart. The adult heartworms live in the heart and near-by large blood vessels. Their appearance is thin and thread-like. The female heartworms are up to 30cm long and 3mm wide. The male heartworms are half the size of the females.

It is important to note that humans cannot catch heartworm from their canine friends. In regions where the disease occurs, for example in the United States, it has a mild and very rare occurrence in humans.

What are the symptoms of heartworms in dogs?

Symptoms of heartworm disease can be split into four groups:

  • No symptoms or very mild symptoms - infected dogs may only have an occasional cough.

  • Regular coughing and some evidence of struggling with exercise (shortness of breath or loss of stamina).

  • Weight loss, finding exercise very difficult (in some cases even becoming faint), laboured breathing. At this point infected dogs will start to show evidence of right-sided heart failure, such as a build-up of fluid which may cause the abdomen and the legs to swell.

  • Caval syndrome - in the most severe cases, the sheer number of heartworms present, blocks the flow of blood into the heart. Sadly, caval syndrome is usually fatal.

The disease often progresses slowly, taking around six months for symptoms to appear following the bite from an infected mosquito. Therefore, even if it has been several months since your dog travelled to a high-risk region, heartworm disease should be considered as a possibility if appropriate preventative medication was not given. The symptoms will depend on the length of time that your dog has been infected, the number of heartworms present and the individual immune response of your dog.

How is a heartworm infection in my dog diagnosed?

Diagnosis of heartworm disease in dogs is typically made using a specific blood test. Screening blood tests are often advised to rule out other diseases and assess any heartworm-associated organ damage. Chest x-rays are also often recommended to assess the extent of heart and lung damage prior to beginning treatment.

How can a heartworm infection in my dog be treated?

Treatment of heartworm disease uses an anti-parasitic drug to kill the worms themselves. However, the dead worms can cause a significant and damaging immune response. Dead worms can also block the major blood vessels where they live. Dogs undergoing heartworm treatment must have their exercise restricted during the treatment period. Additional medication may be given to support the breathing and circulatory systems. Those with a very high worm burden may need the worms surgically removed, however the prognosis for these cases is poor.

How can heartworm be prevented in dogs?

For dogs residing in the UK that never travel abroad, there is currently no risk of heartworm infection. However, with increasing numbers of pets travelling abroad, it is likely that over time this risk will increase. The risk of heartworm infection increases if a dog has not been given preventative heartworm medication prior to and during a visit to a region where there are heartworms and mosquitoes. Dogs are at ten-times a greater risk of heartworm infection than cats.

Use of preventative medication for heartworm disease is the best method of protection for dogs that travel abroad or who have been adopted from outside the UK. It is important to keep up to date with regular preventative treatments. Consistent protection is required and missing one dose can increase the risk of heartworm disease.

When to see your vet

  • If you are planning a trip abroad with your dog - be sure to read our dog travel advice!

  • If your dog has breathing problems, is coughing or vomiting.

Still have questions?

You can always book a video call with a vet at FirstVet if your cat is lame to get an initial assessment and advice about how to proceed!

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