What Is Heartworm?
Heartworm disease is caused by Dirofilaria immitis, a parasitic filarial worm transmitted to dogs through the bites of infected mosquitoes. Unlike most parasites that live in the gut or on the skin, adult heartworms live in the heart, pulmonary arteries, and surrounding blood vessels. They can reach 30 centimetres in length. A single dog can harbour dozens of adult worms, and over time the physical presence of these worms causes severe, progressive damage to the heart and lungs — a condition that can be fatal if untreated.
The parasite's life cycle involves mosquitoes as intermediate hosts. When a mosquito bites an infected dog, it ingests microscopic immature worms called microfilariae, which develop within the mosquito over a period of approximately two weeks. When the infected mosquito then bites another dog, it deposits infective larvae into the new host. These larvae migrate through the body over several months, eventually reaching the heart and pulmonary vessels, where they mature into adult worms capable of living for 5 to 7 years.
Where Is Heartworm Found in Europe?
Heartworm is endemic across much of Southern Europe. Countries with well-established heartworm populations include Italy (where the disease has been studied extensively for decades), Spain, Portugal, France, Greece, and Romania. Significant endemic areas also exist in Bulgaria, Hungary, and parts of the Western Balkans.
The distribution of heartworm is closely linked to the distribution of competent mosquito vector species — primarily Culex and Aedes mosquitoes — and to the temperatures that allow larval development within the mosquito. As European summers lengthen and winters grow milder with climate change, the geographic range of heartworm is expanding northwards. Cases are being reported in areas that were previously considered low risk, and the trend is expected to continue.
In the UK, heartworm is not currently endemic. However, dogs that have travelled to Southern Europe can return home with established infections, and there have been rare reports of autochthonous (locally acquired) cases in Southern England. The risk for UK dogs that do not travel is currently very low, but this picture may change over coming decades.
Signs of Heartworm Disease
One of the most dangerous aspects of heartworm disease is that it is often silent in its early stages. A dog can carry adult worms for months or even years before showing any clinical signs, particularly if the worm burden is light or the dog is not highly active. By the time signs become apparent, significant damage to the heart and lungs has often already occurred.
As the disease progresses, affected dogs typically develop a persistent, dry cough — caused by inflammation and physical blockage of the pulmonary vessels. Exercise intolerance follows, with dogs tiring rapidly on walks they previously managed easily. As the condition advances, respiratory distress at rest, weight loss despite maintained appetite, and a swollen abdomen caused by fluid accumulation may be seen. In severe cases, heart failure develops. Sudden collapse and death can occur in dogs with very heavy worm burdens, particularly if they undergo sudden exertion.
Diagnosis
Diagnosing heartworm involves several tests used in combination. An antigen test detects proteins produced by adult female heartworms and is the most commonly used screening test. A microfilaria test examines a blood sample for the presence of circulating larval worms. These blood tests can be run in-clinic or sent to a laboratory. Chest X-rays help assess the degree of heart and lung involvement and the severity of disease. Echocardiography (cardiac ultrasound) can visualise worms directly in the heart and large vessels in heavy infestations, and provides important information about cardiac function.
Importantly, an antigen test must be performed before starting any preventive medication if the dog may have been exposed to heartworm. Starting prevention in a dog with an existing adult worm infection does not kill the adult worms and can, in some circumstances, cause complications.
Treatment: Why Prevention Is So Critical
The treatment of established heartworm disease is one of the most compelling arguments for rigorous prevention. There is no licensed treatment for heartworm disease available in the UK. In countries where adulticide treatment is available — primarily as melarsomine dihydrochloride, sold as Immiticide in some EU countries — the treatment protocol is lengthy, demanding, and carries significant risks.
Treatment typically involves a series of deep muscle injections administered over several months. During and after treatment, strict exercise restriction is essential — sometimes for months — because dying worms can cause life-threatening clots and blockages if the dog exerts itself and increases blood flow through the damaged vessels. Dogs with severe disease may not be stable enough to undergo treatment at all. The process is expensive, stressful for the animal, and not always successful.
This is why prevention is not merely advisable — it is genuinely critical. A once-monthly preventive medication costing a few euros prevents a disease that may be impossible to treat in a jurisdiction where licensed adulticide products are unavailable, and which requires months of intensive management where they are.
Prevention Products
Heartworm prevention relies on macrocyclic lactone drugs, which kill the larval stages of Dirofilaria immitis before they can develop into adult worms. These medications must be given regularly and consistently — missing doses leaves a window of vulnerability.
Milbemycin oxime (found in Milbemax tablets and certain other products) is licensed in the UK and widely available. It is given monthly and also covers intestinal roundworms, tapeworms, and other parasites. Moxidectin, found in the spot-on product Advocate, is also licensed in the UK and provides monthly heartworm prevention alongside flea and lungworm cover. Ivermectin, found in products such as Heartgard in markets where it is available, is another macrocyclic lactone used for heartworm prevention.
In endemic areas, year-round monthly treatment is the standard of care. For UK dogs travelling to heartworm-endemic regions of Europe, prevention should be started before travel and continued for at least a month after return. An antigen test should be performed on return from endemic areas — ideally at least six months after the trip, as antigen tests cannot detect very early infections.
Annual Testing and Ongoing Monitoring
In endemic areas, annual heartworm testing is recommended even for dogs on preventive medication. No prevention is 100% effective if doses are missed or if the product is vomited up unnoticed. Annual testing catches breakthrough infections early, when the worm burden is lighter and the animal's condition is better — improving both the prospects for treatment and the dog's quality of life during the process.
Reducing Mosquito Exposure
Whilst no mosquito control measure is sufficient to replace medical prevention, reducing mosquito exposure provides an additional layer of protection in high-risk areas. Keep dogs indoors at dawn and dusk when mosquitoes are most active. Use screens on windows and doors. In endemic southern European regions, topical repellents licensed for use on dogs may be worth discussing with your vet. The ESCCAP Europe website provides regularly updated guidance on heartworm risk by country and the most current prevention recommendations for travelling and resident dogs.