This post is sponsored by Elanco. For more information, visit their hugely useful UK map for lungworm cases.

Lungworm, as a parasite, was hardly even being discussed a couple of decades ago. Yes, we knew it existed, and people on veterinary or animal science courses studied it – but it didn’t really seem to be a major worry. 

How things have changed. For whatever reason, Between 2008 and 2015, the parasite more than doubled its presence across the UK. There are many possible reasons – an increase in the urban fox population (1), bringing these potential carriers into closer contact with pet dogs; changes in the slug population resulting in more rapid population growth (2); and of course climate change making the winter environment less hostile to incubating larvae.

Regions that in 2008 were practically lungworm free were (3), by 2015, showing significant burdens (4). And in London alone, nearly 75% of the foxes examined were infested with the parasites. So, this isn’t just a marketing ploy, or scaremongering. Lungworm is not uncommon in the UK; it makes dogs seriously ill – and all too often, it kills them.

What is lungworm?

There are several species of lungworm in the UK; however, by far the most important – and serious – is Angiostrongylus vasorum. Sometimes also known as “French Heartworm” (just to be confusing), if someone talks about lungworm in dogs, it’s almost certainly this species they’re thinking of.

These are a type of nematode, or roundworm, whose eggs hatch in the lungs – hence the name. 

How is it transmitted, and what is its life cycle?

If we start with the eggs… they hatch in the lungs, releasing larvae. These are then coughed up by the infected dog, and swallowed, before passing through the gut and being excreted in the faeces. Once in the environment, they typically infect slugs, snails, and cold-blooded organisms such as frogs. Here they develop and mature (5). 

If a dog eats an infected frog, slug or snail, they are infected. However, we now know that they don’t have to eat this “intermediate host” – the slime secreted by an infected slug or snail is also teeming with infective lungworm larvae, and is a possible route of transmission. So, just drinking out of a bowl that a snail has crawled over, or playing with a toy that the slugs had been at overnight can be enough to infect a dog (6). 

Once in the dog’s gut, the larvae burrow through the gut wall and invade the blood vessels. From here, they travel to the heart and then mature into adult forms, laying more eggs, and completing the life cycle (5).

What does it do to a dog?

Unfortunately, this method of reproduction is really hard on the dog. The presence of the foreign material – eggs and larvae – in the lungs can cause significant inflammatory changes, often resulting in respiratory signs. Most commonly, this manifests as a “verminous pneumonia”, causing a chronic cough, sometimes with shortness of breath.

The worms within the heart can contribute to this – the presence of a cluster of these worms within the great blood vessels and the chambers of the heart causes changes in heart function, heart swelling, and sometimes heart failure (due to pulmonary hypertension) (5). 

However, often the most dangerous effect is the impact on the blood itself – in lungworm infestations, there is a significant change in the blood’s clotting ability. The reason isn’t entirely clear, but one theory is that, to protect themselves from the dog’s natural defences, the worms produce a blood-thinning factor that prevents the blood from clotting. An alternative argument is that the presence of the worms causes abnormal clotting in the vessels, leading to depletion of clotting factors. Either way, this “parasitic coagulopathy” results in abnormal bleeding – for example, bruising under the skin, coughing up blood, bloody vomit or diarrhoea, and internal bleeding (7)(8). 

In rare cases, too, the larvae seem to get “lost”, and invade other organs, such as the kidneys (causing renal failure) or brain (causing neurological problems such as inability to balance or seizures). The parasites can also, sadly, result in the sudden death of an apparently healthy dog.

Can vets test for infection?

Yes – the larvae in the faeces can be detected with a special test called a Baermann Test or, most commonly nowadays, with a simple blood test. If diagnosed early enough, rapid treatment to kill the worms will usually resolve the clinical signs rapidly, although severely infected dogs may need intensive care and additional treatment. Unfortunately, however, the damage done may be slow to heal, and complications are not uncommon. 

How can it be prevented?

Fortunately, there are a wide range of options available that are effective against lungworm. Some of these products can be used for prevention and treatment and some just for prevention. Preventative products will help to stop lungworms establishing an infestation in the first place

However, it is important to remember that no over-the-counter wormers will prevent lungworm. Only specific prescription-strength spot-ons or tablets from your vet will be effective.

Do I need to treat my dog for lungworm?

While lungworm is now present throughout the UK, some areas are at higher risk than others. To see whether you live in a high-risk area, check out the live online lungworm map here, and talk to your vet about your dog’s parasite control strategy.


(1) Scott DM, Berg MJ, Tolhurst BA, Chauvenet ALM, Smith GC, et al. (2014) Changes in the Distribution of Red Foxes (Vulpes vulpes) in Urban Areas in Great Britain: Findings and Limitations of a Media-Driven Nationwide Survey. PLOS ONE 9(6): e99059.

(2) BBC News (2012) Who, What, Why: Are British slugs under threat?

(3) Morgan ER, Tomlinson A, Hunter S, Nichols T, Roberts E, Fox MT, Taylor MA. (2008) Angiostrongylus vasorum and Eucoleus aerophilus in foxes (Vulpes vulpes) in Great Britain. Vet Parasitol. 2008 Jun 14;154(1-2):48-57. doi: 10.1016/j.vetpar.2008.02.030.

(4) Taylor CS, Garcia Gato R, Learmount J, Aziz NA, Montgomery C, Rose H, Coulthwaite CL, McGarry JW, Forman DW, Allen S, Wall R, Morgan ER. Increased prevalence and geographic spread of the cardiopulmonary nematode Angiostrongylus vasorum in fox populations in Great Britain. Parasitology. 2015 Aug;142(9):1190-5. doi: 10.1017/S0031182015000463. 

(5) ESCCAP Guideline 01 (2021) Worm Control 1 in Dogs and Cats, ESCCAP Sixth Edition – May 2021

(6) Conboy G, Guselle N, Schaper R (2017) Spontaneous Shedding of Metastrongyloid Third-Stage Larvae by Experimentally Infected Limax maximus, Parasitol Res (2017) 116:S41–S54 DOI 10.1007/s00436-017-5490-2 

(7) Schelling CG, Greene CE, Prestwood AK, Tsang VC. (1986) Coagulation abnormalities associated with acute Angiostrongylus vasorum infection in dogs. Am J Vet Res. 1986 Dec;47(12):2669-73. PMID: 3800129.

(8) Adamantos S, Waters S, Boag A. (2015) Coagulation status in dogs with naturally occurring Angiostrongylus vasorum infection. J Small Anim Pract. 2015 Aug;56(8):485-90. doi: 10.1111/jsap.12370. Epub 2015 May 29. Erratum in: J Small Anim Pract. 2015 Oct;56(10):633. PMID: 26032443.