Heartworm in Europe: A Growing Threat
Heartworm disease, caused by the parasitic worm Dirofilaria immitis, has long been associated with tropical and subtropical regions — North America, South America, and Southeast Asia. Many European dog owners therefore assume it is not their concern. That assumption is increasingly dangerous. Heartworm is endemic across large parts of southern Europe, and its range is expanding northward as the climate warms and vector mosquito populations spread.
ESCCAP Guideline 6 (GL6) provides detailed guidance on the prevention and control of dirofilariosis in Europe. Every dog owner in Europe should understand the risks outlined in that document.
How Heartworm Is Transmitted
Dirofilaria immitis is transmitted exclusively by mosquitoes — it cannot pass directly from dog to dog. Infected mosquitoes of the genera Aedes, Culex, and Anopheles deposit infective larvae (L3) into the dog's skin during a blood meal. These larvae migrate through the tissues over several months, eventually reaching the heart and pulmonary arteries, where they mature into adult worms that can reach 30 cm in length. Adult worms can live inside the dog for five to seven years.
The pre-patent period — the time from infection to detectable adult worms — is approximately six months. This means a dog can be infected for half a year before any test can confirm it.
Where Is Heartworm Endemic in Europe?
The risk is not uniform across Europe. Transmission requires mosquito populations large enough and warm temperatures sustained long enough for the parasite to develop within the mosquito host. Currently, the highest-risk areas in Europe include:
- Southern Spain, particularly the Mediterranean coast, Andalusia, and the Canary Islands
- Portugal, especially the Algarve and Alentejo regions
- Italy — historically the most affected country in Europe, particularly the Po Valley and southern regions
- Greece and the Greek islands
- Southern France, particularly Languedoc and Provence
- The Balkans, including Croatia, Serbia, Romania, and Bulgaria
Crucially, this range is not static. Climate change is allowing mosquito vectors to persist further north and at higher altitudes, and confirmed cases of heartworm have now been reported in previously non-endemic areas of central Europe. Dogs that travel between EU countries — or whose owners bring them on holiday to endemic regions — face genuine risk.
Clinical Signs of Heartworm Disease
One of the most dangerous aspects of heartworm infection is how slowly clinical signs develop. Early infection is typically silent. As the worm burden increases and worms mature, signs appear — but by this stage, significant damage has already been done.
- Early stage: No obvious symptoms. Dog may appear completely healthy.
- Moderate disease: Exercise intolerance, soft persistent cough, reduced appetite.
- Severe disease: Marked lethargy, difficulty breathing, right-sided heart failure, distended abdomen due to fluid accumulation (ascites).
- Caval syndrome: A life-threatening emergency caused by massive worm burden blocking blood flow through the heart — requires immediate surgical intervention.
By the time owners notice something is wrong, the disease is often well advanced. This makes prevention so much more important than relying on early detection.
Diagnosis
Two main diagnostic approaches are used in veterinary practice:
- Antigen test: Detects proteins produced by adult female D. immitis worms. Highly sensitive and widely available as an in-clinic rapid test. However, cannot detect infections less than six to seven months old.
- Knott's test / blood smear: Examines blood microscopically for circulating microfilariae (larval worms). Useful for confirming infection and differentiating from other Dirofilaria species.
Before starting any preventive medication, dogs in endemic areas or with recent travel history to endemic areas should be tested. Starting a preventive in a dog with an existing adult worm burden can cause serious adverse reactions.
Treatment: Complex, Expensive, and Risky
If a dog is diagnosed with heartworm, treatment is possible but challenging. The gold standard involves melarsomine (Immiticide or Diroban), a drug that kills adult worms. The treatment protocol typically involves several injections over months, combined with strict exercise restriction to prevent embolism from dying worms entering the bloodstream. Treatment can cost several hundred to several thousand euros and carries real risks, including pulmonary thromboembolism and, in severe cases, death.
This is why the veterinary profession universally agrees: preventing heartworm is vastly preferable to treating it.
Prevention: EU-Licensed Options
Several products licensed in Europe provide effective heartworm prevention when used correctly:
- Advocate (imidacloprid + moxidectin): A monthly spot-on that also covers fleas, roundworms, lungworm, and ear mites. One of the most widely used heartworm preventives in Europe. Available on prescription.
- Milprazon / Milbemax (milbemycin oxime + praziquantel): Monthly tablets that prevent heartworm as well as treating intestinal worms and tapeworms. A convenient combination product.
- Heartgard (ivermectin): Available in some European markets. Monthly chewable tablet providing heartworm prevention.
The macrocyclic lactones in these products (moxidectin, milbemycin, ivermectin) work by killing the L3 and L4 larval stages before they can develop into adult worms. They must be given monthly — and consistently — to maintain protective levels. A lapse in treatment can leave a window during which larvae escape and mature.
Testing Before Travel
If you are travelling with your dog to a country or region with higher heartworm risk — such as taking a UK or German dog on holiday to southern Spain or Italy — consult your vet at least one month before departure. Your vet can test your dog for existing infection and prescribe an appropriate preventive for the duration of the trip and for one to three months afterwards, covering the six-month window before any infection acquired on the trip would be detectable.
Summary: Protecting Your Dog Against Heartworm
- Heartworm is endemic in southern Europe and expanding northward
- Prevention with Advocate, Milprazon, or similar products is safe, affordable, and highly effective
- Always test before starting prevention in dogs from endemic areas or with travel history
- Treatment is possible but complex, expensive, and risky — prevention is always better
- Consult your vet before travelling to high-risk regions of Europe
If you live in or regularly visit an area where heartworm risk exists, this is one parasite where monthly prevention should be non-negotiable. Speak to your vet about the right product for your dog's needs and your region's risk level.
Written by Sarah Bennett, animal health writer at ForPetsHealthcare.