What Is Heartworm Disease?
Heartworm disease is a serious and potentially fatal condition caused by Dirofilaria immitis, a parasitic roundworm that lives in the heart, pulmonary arteries, and associated blood vessels of infected dogs. Adult worms can reach 30 centimetres in length, and a single dog may harbour dozens of them. Over time, the physical presence of these worms and the body's inflammatory response cause progressive damage to the heart and lungs, ultimately leading to heart failure if left untreated.
Heartworm is transmitted exclusively by mosquitoes — it cannot spread directly from dog to dog. For European dog owners, the key message is that a disease once considered a distant, warm-climate problem is now moving steadily northward, driven by climate change and the movement of infected animals across borders.
The Life Cycle: Mosquito to Dog
Understanding the heartworm life cycle helps explain both the disease's progression and the logic behind preventive treatment. When a mosquito bites an infected dog, it ingests microscopic immature worms called microfilariae that circulate in the dog's bloodstream. These develop into infective larvae inside the mosquito over a period of 10 to 14 days (dependent on warm temperatures). When the mosquito bites another dog, it deposits these larvae into the skin.
The larvae migrate through the dog's body over several months, eventually reaching the heart and pulmonary arteries as young adult worms. The entire journey from infection to sexually mature adults takes approximately six to seven months — this is why monthly preventative medication is so effective: it kills larvae in the early stages, well before they can reach the heart.
Adult worms can live for five to seven years in a dog. Female worms produce microfilariae, which circulate in the bloodstream and continue the cycle when another mosquito feeds.
Where in Europe Is Heartworm Found?
Historically, D. immitis was considered endemic to the Mediterranean basin: southern Italy (particularly the Po Valley), Spain, Portugal, southern France, and the Balkans. However, the disease's range has shifted considerably in recent decades.
EU veterinary surveillance data and peer-reviewed studies have documented heartworm cases progressively further north and at higher altitudes. France now reports cases well beyond its southern regions. Cases linked to locally resident mosquitoes (rather than travel alone) have been confirmed in Germany and Switzerland. The Balkans — including Serbia, Croatia, Romania, and Bulgaria — report high prevalence rates, making them an important epicentre in central-southern Europe.
The expansion is driven primarily by warmer temperatures, which allow mosquito populations to survive at higher latitudes and for longer seasons, and which speed up larval development within the mosquito. The transport of infected dogs from high-prevalence regions — including rescue dogs imported from southern Europe and the Balkans — has also contributed to cases appearing in previously low-risk northern countries.
Recognising the Symptoms
One of the most insidious aspects of heartworm disease is its slow, silent progression. Dogs may carry worms for a year or more before showing any clinical signs. When symptoms do appear, they reflect the cumulative damage to the heart, lungs, and blood vessels.
- Early stage: mild, occasional cough; slight reduction in exercise tolerance; no other obvious signs
- Moderate disease: persistent cough; exercise intolerance; fatigue after mild activity; occasional fainting
- Severe disease: significant breathing difficulties; distended abdomen (from fluid accumulation); severe weight loss; heart failure; caval syndrome (life-threatening blockage of blood flow)
The American Heartworm Society and European veterinary organisations classify heartworm disease into severity classes (Class I to IV), with Class IV — caval syndrome — requiring immediate surgical intervention as it is rapidly fatal. If your dog develops any unexplained cough or exercise intolerance, especially if they have been in a heartworm-endemic region, seek veterinary advice promptly.
Diagnosis: What Your Vet Will Do
Several diagnostic tools are used to confirm heartworm infection and assess its severity:
- Antigen test: a blood test that detects proteins produced by adult female worms; this is the primary screening test and is highly accurate when worm burdens are moderate to high
- Microfilariae test: a blood test to detect circulating microfilariae; used alongside antigen testing as some infections involve worms of only one sex and produce no microfilariae
- Chest X-rays: reveal enlargement of the heart and pulmonary arteries, lung damage, and fluid in the chest
- Echocardiography (cardiac ultrasound): allows direct visualisation of worms in the heart and pulmonary vessels, and assesses cardiac function
- Blood and urine tests: assess organ function and overall health prior to treatment
Because heartworm treatment carries significant risks, thorough pre-treatment assessment is essential. Your vet will use these results to determine the safest protocol for your individual dog.
Treatment: Complex, Costly, and Not Without Risk
Treating established heartworm infection is a serious undertaking that is very different from administering a simple course of antibiotics. The standard adult-worm treatment protocol used in Europe involves the drug melarsomine dihydrochloride (Immiticide), an arsenical compound given by deep injection into the lumbar muscles.
The standard "split-dose" protocol involves:
- A course of doxycycline (antibiotic) for 30 days, to weaken the worms and reduce microfilariae
- A single melarsomine injection, followed by a 30-day period of strict rest
- Two further melarsomine injections given 24 hours apart
- Strict exercise restriction for 6 to 8 weeks after the final injection
The greatest risk during treatment is pulmonary thromboembolism — dead worms breaking off and blocking blood vessels in the lungs, which can be fatal. This is why absolute rest and activity restriction is critical. The cost of the full treatment protocol, including diagnostic workup, injections, and monitoring, can run to several hundred euros. Not all European countries have reliable access to melarsomine, making prevention even more important.
Prevention: Simple, Effective, and Far Preferable
The good news is that heartworm prevention is straightforward, safe, and inexpensive compared to treatment. Monthly preventative medications — available as oral tablets or spot-on solutions — work by killing heartworm larvae before they can develop into adults. Given monthly without interruption, these products are almost 100% effective.
- Monthly oral tablets or chewables containing macrocyclic lactones (milbemycin, moxidectin, ivermectin) are the backbone of prevention
- Some combination products also protect against intestinal worms and other parasites
- Year-round prevention is recommended for dogs in endemic regions; seasonal prevention (covering mosquito season) may be appropriate in lower-risk areas
- Dogs should be tested before starting prevention if they have previously lived in or travelled to endemic areas without cover
Products suitable for heartworm prevention can be found through your vet or through pet retailers such as Zooplus, which stocks a wide range of parasite prevention treatments. Always confirm with your vet which product is appropriate for your dog's size, health status, and risk level.
Travelling Dogs and Rescue Dogs: A Particular Risk
Dogs travelling from non-endemic northern Europe to southern France, Spain, Portugal, Italy, or the Balkans should be placed on heartworm prevention before travel and maintained on it throughout their stay. Similarly, rescue dogs imported into northern Europe from high-prevalence regions must be tested for heartworm infection before arriving — a point that reputable rescue organisations should ensure as standard practice. If you are adopting a rescue dog from southern Europe or the Balkans, ask specifically about heartworm testing and discuss prevention with your vet as soon as your dog arrives home.