Parasites

Heartworms in Dogs: Transmission, Symptoms & Treatment

Heartworms (Dirofilaria immitis) are filarial nematodes that live in the pulmonary artery and vessels near the heart of dogs. They are transmitted by mosquitoes of the genera Culex, Aedes, and Anopheles. Heartworms are not endemic in Germany - but heartworm prophylaxis is mandatory for dogs traveling to the Mediterranean, Southern Europe, and tropical regions.

Heartworms in dogs: transmission, symptoms & treatment

What are heartworms in dogs?

Heartworms (Dirofilaria immitis) are thread-like parasites that live in the pulmonary artery and the vessels close to the heart in dogs. They are transmitted by mosquitoes of the genera Culex, Aedes, and Anopheles. In Germany, heartworms are not endemic — but for dogs traveling to the Mediterranean region, Southern Europe, and tropical regions, heartworm prophylaxis is essential.

Adult worms can reach 25–30 cm in length and remain in the pulmonary artery for years before clinical symptoms appear. If left untreated, heartworm disease leads to pulmonary hypertension, heart failure, and death.

Background + scientific classification

McCall et al. (2008, Advances in Parasitology, PubMed 18394487) described heartworm disease comprehensively: D. immitis infections follow a multi-stage developmental cycle — microfilariae are taken up by the mosquito, develop into infective L3 larvae, and are transmitted to the dog during the bite. It takes 6 months from infection to adult worms (prepatent period) — during this phase, rapid tests are negative. Enzyme-linked immunosorbent assays (ELISA) for D. immitis antigen are the diagnostic standard.

Simón et al. (2012, Clinical Microbiology Reviews, PubMed 22232373) analyzed the spread of dirofilariasis worldwide: climate change and increased travel with dogs have shifted its distribution area northward. Northern Italy, France, and parts of Spain are highly endemic. Zoonotic potential: humans can be infected with D. immitis (rare, usually asymptomatic pulmonary granulomas). Prevention with monthly macrolides (ivermectin, milbemycin) is highly effective.

Carretón et al. (2014, Parasites & Vectors, PubMed 25016576) evaluated the treatment protocol with prior doxycycline administration: doxycycline eliminates Wolbachia bacteria (intracellular endosymbionts of D. immitis), which makes adulticide therapy with melarsomine less inflammatory and more effective. Pre-adulticide treatment with doxycycline + ivermectin + prednisone is the current standard.

Vitomalia position

Heartworms are not an everyday concern in Germany — but for dogs that travel, protection is not optional. A holiday in the Mediterranean without prophylaxis is a calculated risk, and the possible consequences (complex, expensive, and high-risk adulticide therapy) are out of proportion to simple prevention.

When do heartworms become relevant in dogs?

  • For planned travel to heartworm-endemic areas (Southern Europe, Mediterranean, tropics)
  • For dogs from international animal welfare programs (Spain, Greece, Romania): testing before entry is recommended
  • If symptoms occur: cough, exercise intolerance, weight loss in dogs that have traveled
  • At least 6 months after returning from an endemic area: antigen test recommended
  • As part of travel prophylaxis alongside ticks, sand flies, and other parasites

Practical use

Heartworm prophylaxis and diagnostics:

Measure Timing Medication
Prophylaxis Monthly from 2–4 weeks before travel Ivermectin, milbemycin, moxidectin
Diagnosis Antigen test (ELISA) Veterinarian — valid from 6 months after exposure
Before prophylaxis Test for existing infection In cases of unknown exposure
Treatment of adult worms Melarsomine injection Only under veterinary supervision
Pretreatment Doxycycline 30 days + ivermectin Before adulticide therapy

Symptoms by infection stage: - Early stage: no symptoms (worms still young, low burden) - Middle stage: exertional cough, rapid fatigue - Late stage: heart failure, ascites, syncope, embolisms

Common mistakes & myths

  • “In Germany, we don’t need heartworm prophylaxis." Heartworm is not endemic in Germany — but for trips to the Mediterranean, prophylaxis is essential from the first day on site. The risk starts with the first mosquito bite.
  • “A single injection after the holiday provides protection." Prophylaxis must be given preventively — treatment after infection is complex and risky. Monthly prophylaxis starts BEFORE travel.
  • “The antigen test is positive immediately." No — there is a 6-month prepatent period. A test 2 months after returning can be false-negative. Testing 6+ months after exposure is reliable.

Scientific status 2026

Resistance development to macrolides (ivermectin) is known from the USA, but is still rare in Europe. Climate change is shifting the distribution area further north — isolated autochthonous cases in southern France and northern Italy are increasing. New test formats (combination ELISA for several heart parasites) simplify diagnostics in dogs that have traveled.

Frequently asked questions

Does my dog need heartworm prophylaxis for a Mediterranean vacation?

Yes — Dirofilaria immitis is prevalent in Spain, southern France, Italy, and Greece. Prophylaxis (monthly macrolide) should begin 2–4 weeks before travel and continue until 1–2 months after returning. Contact your veterinarian.

How is heartworm infection in dogs treated?

Protocol: doxycycline for 30 days (eliminates Wolbachia) + ivermectin monthly, followed by melarsomine injections (adulticide therapy) at intervals of 3–4 weeks. Strict exercise restriction during treatment is essential — dead worms can cause embolisms during exertion. Total duration approx. 3–5 months.

How can I recognize heartworms in my dog?

Early stage: no symptoms — this is why an antigen test after returning is important. Late stage: persistent cough, exercise intolerance, weight loss, syncope, distended abdomen (ascites). Diagnosis: ELISA antigen test (> 6 months after exposure) + microfilariae microscopy + echocardiography at the veterinarian.

Related terms

Sources & further reading

  1. McCall, J. W., Genchi, C., Kramer, L. H., Guerrero, J., & Venco, L. (2008). Heartworm disease in animals and humans. Advances in Parasitology, 66, 193–285. https://pubmed.ncbi.nlm.nih.gov/18394487/

  2. Simón, F., Siles-Lucas, M., Morchón, R., González-Miguel, J., Mellado, I., Carretón, E., & Montoya-Alonso, J. A. (2012). Human and animal dirofilariasis: the emergence of a zoonotic mosaic. Clinical Microbiology Reviews, 25(3), 507–544. https://pubmed.ncbi.nlm.nih.gov/22232373/

  3. Carretón, E., Morchón, R., Simón, F., González-Miguel, J., Juste, M. C., & Montoya-Alonso, J. A. (2014). Cardiopulmonary and inflammatory biomarkers in the assessment of the severity of canine dirofilariosis. Parasites & Vectors, 7, 447. https://pubmed.ncbi.nlm.nih.gov/25016576/

Wissenschaftliche Einordnung

McCall et al. (2008, Advances in Parasitology, PubMed 18394487) described heartworm disease comprehensively: D. immitis infections follow a multi-stage developmental cycle – microfilariae are ingested by mosquitoes, develop into infective L3 larvae, and are transmitted to dogs during a bite. Six months pass from infection to adult worms (prepatent period) – rapid tests are negative during this phase. Enzyme-linked immunosorbent assays (ELISA) for D. immitis antigen are the diagnostic standard.

Simón et al. (2012, Clinical Microbiology Reviews, PubMed 22232373) analyzed the global spread of dirofilariasis: climate change and increasing travel with dogs have shifted the distribution area northward. Northern Italy, France, and parts of Spain are highly endemic. Zoonotic potential: humans can be infected with D. immitis (rare, usually asymptomatic pulmonary granulomas). Prevention with monthly macrofilaricides (ivermectin, milbemycin) is highly effective.

Carretón et al. (2014, Parasites & Vectors, PubMed 25016576) evaluated the treatment protocol with prior doxycycline administration: doxycycline eliminates Wolbachia bacteria (intracellular endosymbionts of D. immitis), making adulticide therapy with melarsomine less inflammatory and more effective. Preadulticide treatment with doxycycline + ivermectin + prednisone is the modern standard.