Is the threat of heartworm in UK dogs increasing?

Heartworm is a parasitic worm that infests the heart and can seriously harm dogs. Though traditionally very rare in the UK, recent evidence suggests the threat is increasing due to climate and ecological changes.

Heartworm infection happens when mosquitoes bite dogs, transmitting immature worms into their bloodstream. These mature in the heart, causing inflammation and damage. Adult worms can grow up to a foot long and live up to seven years, often existing as around 10-15 worms per infected dog.

What is heartworm?

Heartworm, scientifically known mainly as Dirofilaria immitis, spreads through mosquito bites rather than direct dog-to-dog contact, so transmission relies on mosquito prevalence. The UK hosts over 30 mosquito species, but historically these have been too few and seasonal to sustain the parasite’s life cycle widely.

Why hasn’t heartworm been common in the UK?

Due to historically cooler climate and less vector activity, heartworm has traditionally been rare in UK dogs. Most cases originated from dogs imported from endemic countries or recently travelled abroad. Dog-to-dog contact does not spread heartworm, limiting risks to local dogs unless mosquitos infect them.

Why is heartworm risk increasing?

Recent studies, especially from the University of Bristol, reveal new concerns. A particular strain known as French heartworm (Angiostrongylus vasorum), distinct but related, is prevalent in wild red fox populations in Greater London, the southeast of England, and south Wales. About 75% of tested foxes in London carried it, with infections stable year-round, increasing transmission potential.

This strain can be fatal to dogs and is spreading, with infected areas showing a fivefold increased risk to domestic dogs. Battersea Dogs and Cats Home found 2-5% of tested dogs had larvae, though often without serious symptoms yet.

Geographic hotspots and spread

Currently, the main hotspots are Greater London, the south east of England, and south Wales. Climate change is lengthening mosquito seasons and expanding their habitats, raising risk areas beyond traditional limits. This enables heartworm, especially French heartworm, to become a growing concern for UK dog owners.

Veterinary advice and prevention

UK veterinary bodies, including the British Small Animal Veterinary Association (BSAVA), stress increased vigilance. Dogs showing coughing, exercise intolerance, or unexplained symptoms should be tested for heartworm using antigen, microfilaria tests, radiographs, or echocardiography.

Preventive medication, including monthly heartworm preventatives, is recommended especially when living in or travelling to risk areas. Responsible dog ownership includes consulting your vet about heartworm risks in your locality.

Summary of current UK situation

• Heartworm cases rising due to imported infected dogs, climate warming, and wildlife reservoirs
• French heartworm is a dangerous emerging strain in foxes and dogs in key UK regions
• Greater London, south east England, and south Wales are current hotspots
• Longer mosquito seasons increase transmission risk
• Veterinary vigilance and preventive treatments are vital to protect dogs

By staying informed and proactive in prevention, dog owners can help mitigate heartworm risks and keep their pets healthy as the situation evolves in the UK.