Is the threat of heartworm in dogs increasing in the UK?

Heartworm is, as the name implies, a type of parasitic worm that infests the heart; and in some countries it poses a serious threat to the health of the canine population.

However, heartworm in dogs is so uncommon in the UK that many veterinary clinics have never even treated a single case of it, and when cases do present, they tend to be in dogs imported from abroad or that have recently been brought back into the UK from abroad.

This means that many dog owners in the UK too aren’t even aware that heartworm exists, which is quite understandable as it has never really been a problem – but studies undertaken in 2018 at the University of Bristol and published in the Vet Times have revealed that heartworm might actually be a greater threat to dogs in the UK than previously thought, and that evidence is mounting for the existence of new and previously undiscovered strains of heartworm in the UK.

In this article we will examine the facts surrounding the recent research into heartworm in the UK, and share the information that is currently available in terms of the threat it poses, what areas of the country may be affected, and what we might be dealing with.

Read on to find out if the threat of heartworm for dogs in the UK is increasing.

What is heartworm?

Heartworm is a type of parasite that comes in various different forms with different scientific names, but they all have a similar lifecycle and effect. Heartworms are spread by mosquito bites, and when a mosquito bites a host like a dog, it transmits immature worms into the bloodstream of the host, which will circulate through the body for weeks or even months in some cases, before settling in the heart itself.

Once in the heart, the worms grow to maturity and can have a lifecycle of up to seven years – with each worm reaching up to a foot in length, and reproducing offspring all the time. A dog with heartworm may play host to hundreds of them, but due to their potentially large adult size, 10-15 individual worms is more common.

As you might expect, heartworms damage the heart and cause inflammation, and may also affect the arteries and lungs too.

Why hasn’t heartworm ever been a real issue in the UK?

Unlike most type of parasitic worms that can infect dogs, heartworm isn’t passed from dog to dog by direct contact – and so even if one dog had a serious heartworm infestation, close contact with another dog would not pose a risk to the second dog.

Heartworm is spread by mosquitos, which of course are not a huge issue here in the UK, although we do host over 30 different mosquito species, some of which can be a biting nuisance! In other countries where mosquitos are more prevalent and problematic, heartworm is more common in dogs, and more widely distributed.

However, mosquitos from abroad that make their way over here on flights, in luggage and in freight can carry heartworm, and it can be brought over in infected dogs too, although as mentioned, dog to dog contact won’t spread it further.

Why do experts think the threat of heartworm in the UK might be increasing?

So, what’s the problem now? Well, University of Bristol studies indicate that red foxes in the UK might be playing host to some of the same heartworm strains as those that can affect dogs, meaning that they could serve as a potential reservoir of infection just waiting to be spread around.

Studies conducted on red fox populations in the London area in 2016 found a very high prevalence of infection with heartworm within the population – almost 75% of tested foxes – and it also found that unlike most heartworm infestations, the level didn’t drop off in winter, remaining stable year-round and so, having a larger scope for infection.

Evidence also suggests that although recorded cases of heartworm in UK dogs are still uncommon, such infections are becoming more widely distributed, particularly those caused by a potentially very dangerous strain of heartworm, known as French heartworm, which can prove fatal (in contrast to most other variants).

Greater London and the south east of England, as well as south Wales, have been identified as hotspots for French heartworm infestations in red fox populations, and in such areas, the risk of the same infections arising in local domestic dogs increases fivefold.

Additionally, heartworm tests performed on dogs in Battersea Dogs and Cats Home have identified an infection rate of 2-5% of all dogs tested showing evidence of French heartworm larvae in their faeces – although it is important to note that said dogs were not symptomatic nor found to have heartworms in their hearts themselves.

Whilst all of this adds up to a fairly vague picture in terms of risk and whether or not French heartworm is likely to cause serious problems in the UK and become symptomatic within the wider dog population, the evidence of its widespread nature plus the possibility of genetic mutations causing further strains to develop are certainly cause for concern.

Talk to your vet if you are concerned about heartworm to find out the risk factors for your own area and dog, as your vet will have a broader overview of the state of play in the local area and be able to provide specific advice pertaining to it.