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Understanding the dog lungworm life cycle

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Lungworm is a parasitic worm that is potentially fatal in dogs. But the lungworm life cycle is more complex than a standard intestinal worm; it needs two hosts to fully develop and it can’t be treated by your dog’s regular pet shop-bought wormer. Break the life cycle of lungworm by preventing your dog becoming a host and getting a lungworm infection. When you know the different stages of lungworm development, it’s easier to understand how to keep your pet safe from this increasing threat.
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What is lungworm?

Lungworm, also known as Angiostrongylus vasorum, is a parasite, which can be seriously damaging to dogs if left untreated.

Different from intestinal worms, which are treated by a standard dog wormer, lungworms migrate through a dog’s body into the lungs and heart. It can lead to haemorrhages in the lungs, liver, intestine, eyes and spinal cord and at its worst, it can even be fatal. The problem is that lungworm is posing an increasing threat to dogs in the UK. Once present only in the south of England and Wales, cases of lungworm have now been reported in northern England and as far as Scotland, so all dog owners must be vigilant.

The life cycle of a lungworm

The lungworm’s life cycle is complex and they can’t develop without an intermediate host, which is often a slug or snail but the parasite can also be carried by frogs. They then need a canid – a member of the dog family – to lay their eggs in, so the next phase of the life cycle can begin.

Infective larvae

Dogs are most at risk of catching lungworm when they’re exposed to slugs and snails carrying the infective lungworm larvae. In the slug or snail the parasite develops into juvenile larvae, known as ‘third-stage’ infective larvae.

The infection can be passed to dogs when they eat slugs and snails while rooting around on their daily adventures, but also from slug and snail ‘slime-trails’. Slugs and snails can often be found in water bowls and on dog toys left outdoors, so it can be easier than you might think for dogs to accidentally become infected.

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Snail on leaf

Adult lungworm

Once a dog has contracted the parasite, the third-stage larvae will be present in the dog’s intestines. The larvae then move through the intestinal wall into the animal’s abdominal lymph nodes (small glands that are part of the lymphatic system, which runs throughout the body) where they develop into immature adult worms and then migrate to the right side of the heart.

The parasite will also move to the pulmonary artery, which carries the blood from the right side of the heart to the lungs. Lungworms can live and develop undetected because dogs may not show signs until the infection is in its later stages.

Find out more about lungworm in dogs – the symptoms and treatment.

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If you keep your dog’s water bowl outside, change the water and wash the bowl regularly. Slugs and snails are attracted to water, and could easily contaminate the area.

Laying eggs

In as little as 28 days after infection, the female adult worms can begin to lay eggs.

Hatchling larvae

Lungworm eggs travel in the blood to the dog’s lung tissue where they then hatch into ‘first-stage’ larvae. It’s often here that lungworm will make the dog sick, displaying clinical signs or symptoms of illness.

Once here, the first-stage larvae migrate into the lung alveoli (which are tiny air sacs in the lungs). This causes the dog to cough up the larvae and swallow them back into the intestines. From here, they are passed out of the dog in their faeces.

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Dog coughing can be a sign of lungworm

The cycle continues

Once the parasite has been passed out of the dog in their poop, it waits for slugs and snails to pass over it, which are then infected by the parasite.

It’s here, in the intermediate host, where the larvae develop into the infective third-stage larvae, lying in wait to be ingested by an unsuspecting dog to start the cycle once more. 

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slugs and snails can carry lungworm
How do foxes contribute to the lungworm cycle?

Lungworm affects the dog family, members of which are called canids. Foxes are a canid and sadly, they too can be infected with lungworm.

Just as dogs can’t pass on their infection from dog to dog, foxes cannot spread their infection directly from fox to dog. But foxes (and dogs) with lungworm are indirectly contributing to the spread every time they poop. The more contaminated fox and dog poop left on the ground, the more likely it is that slugs and snails will find it and become infected, increasing the infection risk to your dog.

A survey of dog owners in Britain found one in six have spotted fox poop in their garden and one in seven are visited regularly by a fox, indicating the potential risk of lungworm to pet dogs.

How can I break the lungworm cycle to protect my dog?

Lungworms need the right conditions to develop, that means when their life-cycle is disrupted, it prevents growth and helps protect your dog.

  • Treat your dog

Lungworms can’t develop without a canid. Take your dog out of the lungworm life-cycle by using an effective treatment. The good news is lungworm can be prevented with a monthly treatment, which is only available on prescription from a vet practice. Speak to your vet to find out more.

  • Pick up your dog’s poop

Infected dogs spread lungworm to slugs or snails through their faeces. Responsible dog owners always pick up their pooch’s poop immediately, even in their own garden. This helps break the life-cycle of the lungworm larvae, preventing them from coming into contact with slugs and snails.

  • Keep your dog’s toys clean

When you play with your dog regularly, leaving their toys outside can become a habit. The problem is slugs and snails love to explore, especially in damp or wet conditions, often choosing dog toys to slide across.

To cut the risk of your dog eating a slug or snail, bring dog toys inside after playtime and in the event they are left outside, give them a good wash before letting your pup go near them.

  • Change your dog’s water regularly

If you keep your dog’s water bowl outside, change the water and wash the bowl regularly. Slugs and snails are attracted to water, and could easily contaminate the area. For the same reason, it’s important to stop your dog drinking from puddles.

Are cats also at risk of lungworm?

Cats are also at risk of lungworm, but it is a different species of lungworm called Aelurostrongylus abstrusus. It can cause cats to cough, wheeze and have difficulty breathing.

Read more about lungworm in cats.

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