Heartworm in dogs

Written by Dr Andrew Miller MRCVSDr Andrew Miller MRCVS is an expert veterinary working in the field for over 10 years after graduating from Bristol University. Andy fact checks and writes for Pure Pet Food while also working as a full time veterinarian. Pure Pet FoodPure Pet Food are the experts in healthy dog food and healthy dogs featured in media outlets such as BBC, Good Housekeeping and The Telegraph. Working with high profile veterinary professionals and nutritionists, Pure Pet Food are changing dog food for the better. - Our editorial process

Heartworm is a horrendous parasite that does exactly what it’s name implies. It’s a worm that lives in your dog’s pulmonary artery and heart, damaging their blood vessels and organs, and it is potentially fatal. As well as dogs, heartworm can infect cats and ferrets.

Although heartworm isn’t considered a risk to dogs in the UK, your pooch can catch this disease if they come from another country or travel abroad with you. Thankfully, this nasty parasite can be prevented with the use of routine preventative medication.

Because heartworm isn’t a considerable risk to owners in this country, many aren’t aware of it. So, we’ve created this detailed guide on heartworm in dogs to help you understand what this parasite is, what it does, the symptoms of infection, and how it’s treated.

What is heartworm in dogs?

Heartworm is a type of parasitic worm called ”Dirofilaria immitis” that lives inside an infected dog’s heart and the large arteries and blood vessels attached to it. In some cases, these nasty worms might also live inside a dog’s lungs and other organs and blood vessels within the body.

If the thought of worms living inside their internal organs wasn’t creepy enough, they’re much bigger than you’d imagine too. Adult female worms can reach a hideous 15-31cm long and male heartworms around 7-15cm long, and they resemble cooked spaghetti.

But as well as being a parasite, heartworm is considered a serious disease that causes damage to your dog’s heart, lungs, blood vessels, and other organs. In severe cases, heartworm can cause heart disease, heart failure, and death.

How do dogs catch heartworm?

Dogs only catch heartworm when they’re bitten by mosquitos carrying heartworm larvae. To understand the spread, let’s talk about the lifecycle of these worms.

How is heartworm spread?

Heartworm has two hosts, an intermediate host that carries the parasite and spreads it, and a definitive host that the parasite lives and reproduces inside.

The intermediate host is a mosquito. When a mosquito feeds on an animal infected with heartworm, it sucks up the microscopic larvae (microfilariae) along with the blood it’s eating.

When the mosquito then feeds on another animal, it injects its saliva into the bloodstream when it bites. The larvae are injected into the dog’s blood inside the saliva, infecting the dog.

The dog then becomes the definitive host, meaning the worm lives out the rest of its life inside the dog.

What happens when a dog is infected?

The larvae live under the skin for a few weeks, growing and burrowing through the subcutaneous tissues until they can enter the dog’s bloodstream. The larvae then travel around the body until they reach the pulmonary artery in the heart. Once there, they begin to grow and mature into adult heartworms.

Once a worm reaches maturity at six months of age they begin to reproduce, birthing thousands of microscopic microfilariae. Since these larvae are so small, they can travel throughout the blood vessels in your dog’s body.

An infestation of heartworms can cause a lot of problems as the worms reproduce and infect your dog’s body, damaging their heart, lungs, and blood vessels.

Your dog could have a large number of worms filling their artery and heart, causing obstruction or complete blockage. This is especially worrying when a single mature heartworm can reach almost a foot long, and a dog could have hundreds of these horrible parasites living inside them.

The worms don’t just cause damage when they’re alive though. Once they die, their bodies are carried along by flowing blood into smaller blood vessels, where they can obstruct or completely block the artery and cause the formation of blood clots.

How long is the heartworm’s lifecycle?

Once a dog is bitten by a mosquito and infected with heartworm, it takes 6-7 months for the worm to mature and reproduce, completing the lifecycle.

The larvae can survive for a year in the bloodstream, ready to mature into adult worms or be removed by mosquitos and spread to other animals.

Meanwhile, adult heartworms can survive in the body for 3 to 7 years.

Is heartworm contagious?

No, heartworm is not considered contagious between dogs, or from dogs to people and other pets.

For heartworm to spread from one dog to another, there would need to be an exchange of blood. Heartworm can only be spread by mosquitoes feeding on blood and spreading the microscopic worm larvae.

If your dog has heartworm, they can’t pass these parasites on to another dog through their saliva, urine, or poo. You and your other pets also won’t get heartworm from physical contact with an infected dog.

What are the risks of heartworm in dogs?

Within days of heartworms beginning to live in your dog, they begin to damage the linings of their blood vessels, causing irritation and inflammation in the blood vessels and the heart.

Inflammation of the blood vessels leads to thickening of the blood vessel walls and scarring, which stops them from working as normal.

As the infestation worsens, the inflammation can spread to your dog’s lungs and throughout their body.

Additionally, the sheer mass of worms in your dog’s heart and arteries can cause obstructions and blockages, preventing normal blood flow. This raises your dog’s heart rate and blood pressure. It also forces the blood to move through blood vessels that aren’t full of worms, which causes fluid to build up around the blood vessels and in your dog’s lungs. As a result, many dogs with heartworm can suffer from blood clots and aneurysms.

In severe cases, the worms can infest other organs. They may begin to live inside your dog’s lungs or liver. Once worms begin to migrate around a dog’s body, it’s almost certainly fatal.

If heartworm in dogs is left untreated, it will cause irreversible damage to your dog’s internal organs and blood vessels and will be fatal.

Even if your dog’s heartworm is treated, the damage is often irreversible and can leave them with chronic side effects such as a persistent cough, lack of stamina, and vulnerability to infections.

Can heartworm kill dogs?

Yes, if left untreated, heartworm will cause serious illness and could eventually kill your dog.

Is heartworm a risk in the UK?

Although the mosquitoes that spread heartworm are found in the UK, the climate isn’t right for the survival and spread of heartworm larvae.

The mosquitoes that spread heartworm are usually found in warmer countries, such as those in North America or in southern Europe. For that reason, heartworm is currently only a problem in areas with a specific climate, temperature, and mosquito population. The risk of heartworm is also higher during “mosquito season”.

However, with the effects of climate change and rising temperatures, the UK could soon become a suitable climate for heartworm. As global warming takes effect, it’s possible that heartworm will spread into new areas and become a threat to the population of dogs here in the UK.

And there is another worrying “but”... There is a species of heartworm called “Angiostrongylus vasorum”, more commonly known as “French heartworm”, which has spread to Britain.

The definitive host for this parasitic worm is red foxes, but it has the potential to infect other canines, including dogs.

Worryingly, there have been a few cases of this heartworm in dogs in the UK, and there is now widespread infection in wild foxes. The spread of French heartworm has also increased in recent years, and it is now found throughout the UK. (Previously it was only found in London and the South).

Additionally, many people travel to France or Europe with their pets, and owners must be aware of the risks to their dogs in these areas and take necessary precautions to prevent the spread of disease.

How to prevent heartworm in dogs

Unless you live in or travel through an area that is at high risk of heartworm, you shouldn’t have to worry about needing to prevent heartworm in your dogs. But if you are, there are a few things you can do to protect your pooch from parasites.

Mosquito repellents and bite prevention

Because your dog must be bitten by a mosquito carrying heartworm larvae to contract the disease, preventing mosquito bites will help to protect your pooch from parasites.

Mosquitoes are most active at dawn and dusk. Annoyingly, these are also the cooler times of day when walking your dog is advised during hot weather. If possible, walk your dog when mosquitoes aren’t so active.

You could also purchase dog-safe insect repellent to help keep the biting bugs at bay while out walking.

Otherwise, try to keep your pet indoors as much as possible, with all windows and doors shut to prevent mosquitoes from flying inside. You can also fit bug screens to your windows to keep bugs out, whilst allowing you to open your windows to let some air in.

To try and reduce the number of mosquitoes in the environment around you and the dog, you should get rid of any standing water where mosquitoes are likely to breed. This can include still ponds, birdbaths, puddles, and outdoor dog bowls.

You could also consider planting dog-safe plants that naturally repel mosquitoes, such as catnip, lavender, lemon balm, rosemary, basil, or mint.

Preventative medicine

The most effective method of preventing heartworm is to give your dog monthly preventative medication. (These are different to regular worming medicines).

These medicines can’t prevent mosquitoes from biting your dog, so they may still become infected, but they will kill any larvae before they can mature into adulthood and reproduce.

By providing a preventative every month, the larvae are continually killed and your dog shouldn’t become infested.

However, heartworm preventatives are only effective at killing heartworm larvae. Adult heartworms cannot be killed by preventatives. Plus, prevention isn’t possible by the time your dog is hosting adult worms because an infestation has already occurred.

Because preventatives only kill young heartworms, it’s important to give your pet preventatives in a timely fashion as advised by your vet because if they’re given too late, they won’t be effective.

Stay out of high-risk areas

Heartworm is only a problem in certain climates and geographical locations. By keeping your dog out of those regions, you can be fairly sure they’re safe from heartworm. Here in the UK for instance, there’s little risk of heartworm.

If you and your dog are planning on travelling or living in an area known to be a hot spot for heartworm, you will need to provide your pet with monthly heartworm preventatives.

Is prevention better than cure?

In the case of heartworm, prevention is definitely better than cure.

Firstly, heartworm preventatives are much more cost-effective and less time-consuming than treating heartworm. It’s less stressful for you and your pooch too.

Additionally, although heartworm preventatives can have side effects, they aren’t as common or as serious as the side effects of the treatments used for heartworm.

Plus, treatment for heartworm is a lengthy and laborious task, involving several months of medication and crate rest for your dog. In which case, preventing heartworm in the first place saves you and your dog a whole lot of stress, money, time, and literal heartache.

Are some dogs at greater risk?

Heartworm can affect any dog of any age, gender, or breed. There is no genetic predisposition to heartworm.

The dogs at greater risk are those who live in areas that are at high risk of heartworm.

Additionally, dogs who live in these areas and are not provided with heartworm preventatives will be at much greater risk of infection compared to dogs who are given regular preventative medicine.

Signs and symptoms of heartworm in dogs

Sadly, many dogs affected by heartworm don’t show any signs of illness until significant infestation and considerable damage has already occurred.

Symptoms of heartworm in dogs includes:

  • Coughing

  • Lethargy

  • Weakness

  • Loss of appetite

  • Exercise intolerance

  • Unexplained weight loss

  • Excessive panting, rapid breathing, or shortness of breath

  • Crackling lungs (caused by fluid build-up)

  • Bulging chest

  • Fluid retention

  • Fainting

  • Collapse

  • Coughing or spitting blood

  • Anaemia

  • Heart failure

How is heartworm in dogs diagnosed?

If you suspect your dog has heartworm, or any other disease, you must take them to the vet for an examination.

To diagnose heartworm, your vet will talk to you about your dog’s symptoms and give them a thorough physical examination.

During this examination, your vet will listen to your dog’s heart and lungs with a stethoscope to listen for abnormalities like crackling in the lungs or a heart murmur. They may also perform an X-ray or an ultrasound of your dog’s chest to look at the condition of their heart and lungs. This will also help them to rule out other illnesses that could be causing your dog’s symptoms.

The vet will conduct a blood test, examining a sample of your dog’s blood to see if any microfilariae are present inside it. If heartworm larvae are present in your dog’s blood, then heartworm can be diagnosed.

However just because your dog’s blood sample was clear of larvae doesn’t mean they’re clear of heartworm. An additional sample will need to be taken and sent to a laboratory to be tested for the presence of adult worms.

Treatment for heartworm in dogs

The treatment for heartworm in dogs is a long and laborious process, that can cause side effects for your dog. However, if heartworm isn’t treated, the infected dog will suffer organ damage and may die. Fatalities from heartworm treatment are rare, whereas untreated heartworm is almost guaranteed to be fatal.


The most common treatment for heartworm in dogs is a combination of oral and vaccination medications to kill the worms and treat any symptoms or secondary infections that they have caused.

Initially, your dog will be hospitalised so the veterinarians can stabilise their condition and begin heartworm treatment in a controlled environment. They will be given tablets of prednisone and doxycycline to reduce the likelihood of a reaction to the heartworm medication. Because doxycycline is an antibiotic, it can also treat any secondary infection that has been carried or caused by the parasite.

After this, your dog will be given a course of heartworm preventatives to kill any heartworm larvae living in their bloodstream. Because of the risk of reactions to heartworm preventatives, the first dose will be given while your dog is hospitalised so if there are any adverse reactions, the veterinary team can treat them immediately.

Once the veterinary team know your dog can tolerate the medication, treatment can continue at home and your dog will need to take a monthly heartworm preventative.

As well as heartworm preventatives to kill off the heartworm larvae, your dog will be given a series of three vaccinations to kill the adult worms. These vaccines contain a drug called melarsomine.

Your dog will receive their first injection when they’re hospitalised and will need to take another vaccine 30 days after the first. During this 30-day waiting period, you must keep your dog well-rested and monitor them for signs of illness or adverse reactions and take your dog to the vet if they become ill. The final vaccine will be given within 24 hours of their second dose.

Surgical removal

In some cases, surgery may be carried out to try and remove the adult worms from your dog’s heart and clear any obstructions in their blood vessels. This is a highly complex and high-risk operation.

Exercise restriction during recovery

When your dog is being treated for heartworm it’s vital that they rest and aren’t allowed to exercise.

You will need to strictly limit your dog’s exercise as soon as they begin treatment for heartworm and continue to do so until 4-8 weeks after their final vaccine. That means they should be on crate rest when unsupervised and only allowed out of the house on a lead for a brief walk to go to the toilet.

Your vet will be able to advise you on when you can start exercising your dog again, and how long for.

Strict restriction of exercise might seem boring for your pooch, but it’s vital to keep your dog safe during treatment for heartworm.

Why does my dog need crate rest?

This is because the adult worms die within days of your dog’s injection and their bodies are pushed into smaller blood vessels by your dog’s blood flow. These smaller blood vessels are then essentially blocked by the bodies of the dead worms. (The biggest risk with heartworm treatment isn’t the medication, it’s actually the accumulation of these dead worms).

If your dog exercises, the elevated heart rate and blood pressure caused by the blood being forced around these obstructions could cause serious complications.

Your dog must not exercise until the dead worms have broken down and been removed from the body, which takes a few weeks.

Additionally, because of the damage the heartworms have done to your dog’s respiratory system, exercise will probably be demanding and uncomfortable for your dog at first. Limiting their exercise will give them time to rest and recover.


Once treatment is complete, your dog will need to return to the vet for further tests to confirm that heartworms have been killed. They will need a blood test for microfilariae 3-5 months after treatment, and testing for adult worms 6 months after treatment. If these tests come back clear, treatment has been successful.

Additionally, some dogs may require a change of lifestyle or continued medication for the rest of their life because of the damage to their organs caused by heartworm. Some pets may require moderated exercise, dietary changes, or treatments such as ACE inhibitors or beta-blockers to treat any heart disease resulting from heartworm infection.


Heartworm in dogs is a debilitating disease caused by a terrifying parasite that lives in your dog’s heart and arteries. An infestation can cause damage to your dog’s heart, blood vessels, and other vital organs and if untreated it will certainly prove fatal.

Treatment for heartworm is a long and laborious task though, involving several weeks of crate rest and medication for your pet. That’s why it’s always advised to be conscious of the risks and use preventatives as needed to protect your pooch and save them the heartache of heartworms.