Back to Fact Sheets

Dirofilaria Immitis (Heartworm) Fact Sheet

Download PDF

I have heard dogs can get worms inside the heart: is this true?

Yes, Dirofilaria immitis is a parasite transmitted by mosquito bites to dogs. The parasite migrates though the tissue and into the blood where it undergoes different stages of development. Once it becomes an adult it lives in the pulmonary arteries and heart.

Can dogs become infected in the UK?

This is unlikely as, although we do have the mosquito that transmits the infection, high environmental temperatures for a sustained period of time (day and night) are required for development of the parasite in the mosquito. For this reason, mosquitoes in the UK are not able to transmit the infection. However, these conditions are met in Mediterranean counties where the risk of infection is high.

What are the symptoms?

Some dogs can have mild symptoms such as a mild cough or exercise intolerance. Sometimes the symptoms can be severe such as collapse, severe anaemia and even death. Some dogs with low worm numbers may not have any symptoms.

How is the diagnosis made?

The parasite can be found in the blood in most cases (antigen or microfilaria tests). In cases where there are only a small number of worms, the tests can be negative even though the dog is infected. Sometimes the parasites can be seen in the heart or pulmonary artery using ultrasound.

Is it a treatable disease?

Yes, but treatment carries risks and the damage done to the heart and lungs may be irreversible. Some dogs can have serious anaphylactic reactions when they receive treatment to kill the worms, so this must always be done under strict veterinary supervision. The treatment is usually carried out in a phased manner over a few months to minimise the risk of severe reactions, which can lead to death. It is also important that the dog is rested for the entire period of the treatment. In some cases, your vet will advise that the worms are physically removed from the arteries and heart using forceps – especially if there are large numbers of worms and/ or they are obstructing blood flow.

How can I prevent my dog from becoming infected? I am due to travel to an area where the disease is present.

It is a preventable disease, as long as the dog receives monthly treatments to stop the parasite from developing inside the dog. There are various brands but the three active ingredients that are effective at preventing infection are: moxidectin, milbemycin and selamectin. Your vet can advise you on the best product for your pet. The treatment must given monthly while you are abroad and it is important that your dog does not miss any doses – this is because these drugs kill the early stages of infection and if the treatment lapses by a few weeks and the parasite matures, these treatments will no longer be effective. If you are going abroad for less than a month, give the treatment as soon as you come back into the UK.

It is also useful to prevent the bite of the mosquito in the first place; this can be done by using insect repellents (always use a product that is licensed in dogs and has been prescribed by a vet) as well as avoiding times when the mosquito is active (dawn and dusk).

If I miss a dose, can I test my dog immediately for infection?

No, we can only detect adult worms and they take up to seven months to develop, so you would have to wait at least seven months to know if your dog has become infected. Give the treatment as soon as you realise the treatment has lapsed and continue monthly treatments from there. This will kill any developing, immature worms. An antigen and microfilaria test can be carried out by your vet seven months later to determine if any adults have developed.

Can cats become infected?

Yes, but they are a lot more resistant to infection than dogs. Treating cats with heartworm is very difficult though as the drugs used to kill the worm are toxic to cats. Preventative treatment is therefore extremely important in cats that live or travel to areas where the parasite is present.

What about people?

Dirofilaria immitis does not affect people.

Linnaeus Veterinary Limited trading as Davies Veterinary Specialists 01582 883950

©2020 Davies Veterinary Specialists