Heartworms in Dogs: Transmission, Symptoms, and Treatment

What are heartworms in dogs?

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

Adult worms can grow to 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 Context

McCall et al. (2008, Advances in Parasitology, PubMed 18394487) provided a comprehensive description of heartworm disease: D. immitis infections follow a multi-stage life cycle—microfilariae are ingested by the mosquito, develop into infective L3 larvae, and are transmitted to the dog upon biting. It takes 6 months from infection to the development of adult worms (prepatent period)—during this phase, rapid tests are negative. Enzyme-linked immunosorbent assays (ELISA) for D. immitis antigen are the gold standard for diagnosis.

Simón et al. (2012, Clinical Microbiology Reviews, PubMed 22232373) analyzed the global spread of heartworm disease: climate change and increased travel with dogs have shifted the range of the disease northward. Northern Italy, France, and parts of Spain are highly endemic. Zoonotic potential: Humans can become infected with D. immitis (rare, mostly asymptomatic pulmonary granulomas). Prevention with monthly macrolides (ivermectin, milbemycin) is highly effective.

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

Vitomalia-Position

Heartworms aren’t a common occurrence in Germany—but for dogs traveling abroad, protection is essential. A Mediterranean vacation without preventive treatment is a calculated risk, and the consequences (a complex, expensive, and risky course of Adulzid treatment) are out of proportion to the simple preventive measures available.

When do heartworms become a concern in dogs?

• If you are planning a trip to areas where heartworm is endemic (Southern Europe, the Mediterranean, the tropics)
• For dogs from international animal welfare organizations (Spain, Greece, Romania): Testing is recommended prior to entry
• If symptoms occur: coughing, intolerance to exercise, weight loss in dogs that travel
• At least 6 months after returning from an endemic area: Antigen test recommended
• As part of travel precautions against ticks, sand flies, and other parasites

Practical application

Heartworm Prevention and Diagnosis:

Symptoms by stage of infection: - Early stage: no symptoms (worms are still young and few in number) - Moderate stage: coughing when exerting oneself, rapid fatigue - Late stage: heart failure, ascites, syncope, embolisms

Common Mistakes & Myths

• “We don’t need heartworm prevention in Germany.” It’s not endemic in Germany—but if you’re traveling to the Mediterranean, it’s essential starting from your first day there. The risk begins with the first mosquito bite.
• “A single injection after your trip provides protection.” Prophylaxis must be administered before exposure—treatment after infection is complicated and risky. Monthly prophylaxis begins BEFORE your trip.
• "The antigen test comes back positive right away." No—there is a 6-month incubation period. A test taken 2 months after returning may yield a false-negative result. A test taken 6+ months after exposure is reliable.

Current State of Research (2026)

The development of resistance to macrolides (ivermectin) has been reported in the United States but is still rare in Europe. Climate change is shifting the range of the disease further north—with an increasing number of isolated indigenous cases in southern France and northern Italy. New testing methods (combination ELISA for multiple heartworms) simplify diagnosis in dogs returning from travel.

Frequently Asked Questions

Does my dog need heartworm prevention for our Mediterranean vacation?

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

How is heartworm disease treated in dogs?

Protocol: Doxycycline for 30 days (to eliminate Wolbachia) + monthly ivermectin, followed by melarsomine injections (adulticide therapy) every 3–4 weeks. Strict restriction of physical activity during treatment is essential—dead worms can cause embolisms during exertion. Total duration: approximately 3–5 months.

How can I tell if my dog has heartworms?

Early stage: no symptoms — therefore, an antigen test is recommended upon return. Late stage: persistent cough, exercise intolerance, weight loss, syncope, abdominal distension (ascites). Diagnosis: ELISA antigen test (> 6 months after exposure) + microfilarial microscopy + echocardiography at the veterinarian’s office.

Related terms

• Heart Failure in Dogs
• Heartworm in Dogs
• Hookworms in Dogs
• Motion sickness in dogs
• Ticks on Dogs

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/