Heartworm is a serious infection, causing heart disease and even death in affected dogs. Mosquitoes carry the heartworm parasite and transmit infection when they bite. Heartworm isn’t present in the UK, but it’s a hazard to dogs living or travelling abroad with their owners.

Heartworm in dogs is caused by a parasite called Dirofilaria immitis. We don't have heartworm in the UK, but it’s endemic in parts of Europe and warm countries around the world. Symptoms of heartworm range from mild to life-threatening. They include coughing, weakness and difficulty breathing. Heartworm is preventable, with regular monthly prescription medicine. Talk to a vet for early advice if you are planning to travel to countries which have heartworm.

What is heartworm in dogs?

Heartworm is a life-threatening infection of the heart and the major blood vessels in your dog's lungs.

• Heartworm in dogs is caused by the parasite Dirofilaria immitis.

• Mosquitoes transfer heartworm larvae (baby worms) to dogs after biting infected animals.

• Immature parasites migrate through your dog’s tissues and into their bloodstream.

• Larvae take around 6 months to mature. Adult worms can live in dogs for 5-7 years and accumulate with every new mosquito season.

• Mature parasites live in the lung arteries and heart. Untreated dogs may end up with hundreds of worms living inside them.

• A harmful bacteria called Wolbachia pipiens lives inside heartworms. Heartworm larvae need this bacteria to become adults and reproduce. It increases the damage caused by heartworm disease. Antibiotics to kill this bacteria are an essential part of heartworm treatment.

• Heartworm is most common in Southern and Eastern Europe. Colder weather keeps heartworm out of the Northern countries at the moment. But Global Warming is changing this.

What are the symptoms of heartworm in dogs?

Symptoms of heartworm result from damage by migrating larvae, obstruction of major blood vessels with adult worms and the body’s immune responses. Common symptoms include:

• Coughing and difficulty breathing

• Nose bleeds, spitting up blood

• Weakness, loss of energy, fainting

• Vomiting and going off food.

• Anaemia: pale, blue or purplish gums

• Bloody urine

• Swollen tummy due to fluid build-up

• Sudden death

Are some dogs at more risk of heartworm infection?

Dogs of any breed, age or sex can get heartworm. They only have to be bitten by a mosquito carrying infective heartworm larvae.

Higher-risk dogs include:

• Dogs who’ve been abroad

• Imported dogs, including rescue pets

• Dogs who live outside or go camping with their owners in warm countries

• Dogs not receiving regular heartworm preventives who travel to Europe

• Medium-to large-sized dogs, between 3 and 8 years old

Dogs most likely to develop severe symptoms include:

• Small dogs with smaller blood vessels and heart chambers: It takes fewer worms to block their hearts and blood vessels.

• Dogs with active lifestyles: severely impacted by shortness of breath.

Are humans and other pets at risk?

• Humans and other pets are not at risk of infection from dogs with heartworm.

• However, animals travelling together will be equally at risk of infection.

• Heartworm infection in humans can cause benign, non-cancerous nodules in the lung. These are occasionally mistaken for cancerous changes, but they don’t cause serious illness.

How do vets diagnose heartworm in dogs?

Vets will first consider your dog’s symptoms and any history of living or travelling abroad. Additional tests include:

• Specific blood tests: detect heartworm infections with adult worms. The test won't become positive until 6 or 7 months after infection.

• Endoscopy: passing a special camera into the windpipe

Confirming diagnosis and assessing severity of the disease:

• Blood tests: general health profiles

• Chest x-rays

• Echocardiography: chest ultrasound

A dog that misses a dose of preventive treatment will need to be tested 6 months after the date of the missed dose. And again 6 months after preventive treatments started up again.

How do vets treat heartworm in dogs?

The correct treatment can clear over 95% of heartworm infections in dogs.

• Heartworm treatment is complex and can have risks.

• Killing lots of adult worms and larvae at once can trigger a deadly allergic-type reaction.

• To reduce the risk, treatment is carried out over several months and under careful veterinary supervision.

• Even dogs who are cleared of heartworm, may have limited recovery because of irreversible damage to their heart and lungs.

Example of Current Treatment Recommendations - American Heartworm Society (AHS)

Day 1 - Treatment as soon as heartworm is diagnosed

• Monthly worming medicine to kill immature larvae: moxidectin

• 28 day course of an antibiotic, Doxycycline, to kill the bacteria living in the heartworm

Days 60, 90, 91

• Injections of a drug called Melarsomine to kill adult heartworms, given into a muscle on your dog's back.

Additional notes:

• Dogs must be rested for the entire period of the treatment

• Steroid medicines help to reduce inflammation caused by dying worms

• Sometimes vets will physically remove worms from the arteries and heart with forceps. Especially where there are large numbers of worms obstructing blood flow. This procedure is only carried out in specialist hospitals.

Risks of heartworm treatment in dogs

Dead heartworms can cause severe breathing problems, especially if your dog doesn’t rest enough following treatment. Problems develop anywhere between a few days to 6 weeks after treatment for heartworm. Symptoms of post-treatment complications include:

• Coughing, spitting up blood

• Fast breathing or difficulty breathing

• Lethargy

• Loss of appetite

• High Temperature

Treatment for complications:

• Strict cage rest

• Oxygen therapy

• Medicine to reduce inflammation and reduce blood clotting

• With treatment, most recover from complications within 24 hours

How to care for a dog with heartworm at home

There are no home remedies for heartworm. It's a very serious illness, needing intensive treatment by a vet. However, homecare during and after treatment is an essential part of recovery.

• Follow your vet’s instructions carefully

• Give all medicines as prescribed, at the correct times and dose

• Reduce the risk of dead worm reactions by ensuring your dog gets strict rest. Continue this throughout treatment and for 4-6 weeks afterwards.

How to prevent heartworm infections in dogs

Preventing infection

• Any dog travelling to an area with heartworm should be treated once a month with a licensed product to prevent infection. Effective ingredients include milbemycin, moxidectin and selamectin.

• It’s important not to miss any doses. These drugs kill the early stages of infection. If the treatment lapses by a few weeks and the parasite matures, they’ll no longer be effective. If you’re going abroad for less than a month, give the treatment as soon as you come back to the UK.

Preventing Mosquito bites

• Insect repellents: always use a product that is licensed in dogs and has been prescribed by a vet (Advantix®, Bayer; Scalibor®, MSD; Vectra 3d®, Ceva).

• Preventing exposure: avoid being outside when the mosquito is most actively feeding, at dawn and dusk.

Are my family and other pets at risk?

Heartworm will not spread between dogs and other animals or between dogs and humans. However, humans and other pets are equally at risk of mosquito bites.

When to worry about heartworm infection in your dog

Call a vet immediately if your dog:

• Collapses, with pale or bluish gums

• Is having difficulty breathing

• Is coughing up blood

See a vet if your dog:

• Is having nosebleeds

• Develops a new cough

• Is breathing very quickly and less able to exercise