Heartworms and Your Dog | Companion Care
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Heartworms And Your Dog

Heartworms are thankfully not found in the UK, but are a serious risk for dogs that travel abroad.

Transmitted by mosquitos, these parasites can cause serious illness and even death, so precautions should be taken for any dogs who leave the UK. Heartworms, called Dirofilaria immitis, are very thin, thread-worms that can reach up to 30cm in length. You won’t see these worms yourself though, as they live within the right side of the heart, and the main artery leading from the heart to the lungs.

Heartworms are nasty, and thankfully are not found in the UK. They are transmitted by mosquitos, and are therefore found in warmer countries, such as those in southern Europe.

Heartworms reproduce by producing tiny offspring, called microfilaria. These travel around the blood of infected dogs, and are ingested by mosquitoes when they bite and feed. These microfilaria can then live within the mosquito, and be passed on to the next dog that the mosquito feeds on.

The maturation process for heartworm is slow, and it can take six months for the worms to even reach the heart after an infection starts. The slow progression of this infection means that even dogs who have not travelled for some time should still be considered as ‘at risk’ of developing heartworm unless they have had appropriate parasite treatments.

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More about heartworms

If your dog doesn’t leave the UK, there is currently no risk of them being infected by heartworms. However, should you be planning a trip abroad with your dog then it is important to discuss the risk with your vet.

With the increase in imported rescue dogs over the last few years, there has been an increase in heartworm cases seen in these dogs. If your dog was imported to the UK from abroad and you have any concerns it is important to let your vet know that your dog has spent time outside of the UK.

Heartworms are spread through the bites of mosquitos that carry the infective heartworm larvae. Dogs are at risk of developing heartworm if they have:

  • Not been given preventative medication
  • Spent time in a region with mosquitos and heartworms

How serious the symptoms of a heartworm infection are depend on the number of heartworms, how long the infection has been present and the immune response of your dog.

Infected dogs fall into one of four classes:

  • No signs or very mild. Dogs may only have an occasional cough.
  • Dogs are coughing more regularly and are starting to show evidence of struggling with exercise.
  • Infected dogs lose weight, find exercise very difficult, have laboured breathing and are starting to show evidence of right-sided heart failure, such as a build-up of fluid in the abdomen.
  • The most severe cases develop caval syndrome, where the sheer number of worms blocks the flow of blood into the heart. Sadly dogs with caval syndrome usually die.

If your dog is experiencing any of these signs you should make an appointment with your local Companion Care vet.

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Heartworm preventative medication is the best way to protect your dog from contracting heartworm on holiday. Your vet can advise what the best preventative medication would be for your pet, and give you some to take with you if you will be away for a longer period.

Even missing a single dose will put your dog at risk of contracting heartworm, so it is important up-to-date with treatments.


As this signs of a heartworm infection are quite non-specific and heartworm is not endemic to the UK, if you see your dog has developed any of these signs and you have been abroad it is important to let your vet know you have spent time outside of the UK. A heartworm infection can be diagnosed via a specialised blood test. Your vet may also want to take other, screening blood tests as well as doing imaging to get as much information as possible. This will help your vet plan the best way to manage the infestation. Part of the treatment of a heartworm infestation is an anti-parasitic to kill the worms themselves. This can be a dangerous time for patients as the dead worms can cause a damaging immune response. As the adult worms live within the major vessels dead worms are also still able to block blood vessels, exercise restriction before, during and after treatment until this danger has passed is the best way to reduce this risk. Dogs with a very large parasite burden may have to have them surgically removed, but sadly the prognosis for these cases is very poor.

While humans can be affected by heartworm by being bitten by mosquitos, this risk is incredibly small and the disease is often relatively mild in people. Importantly, you cannot catch heartworm from your dog.