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The important points, first
- Heartworm is spread by mosquitoes. Prevention should start in spring, before their season, and continue until late autumn.
- In southern Europe ESCCAP recommends protection from May to the end of November, and all year where heartworm is very common.
- At the start of each prevention year the dog should be checked with a blood test.
- Signs develop slowly, starting with a chronic cough, then shortness of breath and weakness after exertion.
- If you travel with your dog to a risk area, ask your veterinarian how to protect it before you leave.
What heartworm is
According to the ESCCAP guidelines, Dirofilaria immitis is the most dangerous filarial worm of dogs and cats. Adult females release microfilariae into the blood, which a mosquito takes up when it bites an infected animal; after developing in the mosquito, the larvae pass to another dog with a new bite. Adult worms live mainly in the pulmonary arteries. That is why, despite its name, heartworm disease is first of all a lung disease, with the right heart involved only in later stages.
Another species, Dirofilaria repens, lives under the skin and is the main cause of human filarial infection in Europe. Cats can also get D. immitis, but far less often than dogs. Where the disease is common, prevalence in cats is generally about a tenth of that in dogs.1
Heartworm in Italy
ESCCAP considers D. immitis endemic across southern Europe. In Italy heartworm was long considered typical of the north and the Po Valley. A study of more than 10,000 heartworm tests performed between 2009 and 2019 by two reference laboratories showed that there are now endemic foci in the centre, the south and the islands too.
Positive results were found in 2.83% of samples from the north, 7.75% from the centre and 4.97% from the south and islands. Across Italy the share of positive tests rose from 0.77% in 2009 to values of up to 8.47% in 2016–2017. According to the authors, prevention is needed even in areas where few cases have been seen so far.13
Signs and diagnosis
In dogs, signs develop gradually. A chronic cough comes first, then varying shortness of breath, weakness and sometimes fainting after exercise or excitement. In advanced stages, when the right side of the heart starts to fail, fluid in the abdomen, loss of appetite, weight loss and dehydration may appear. Sudden death is rare. When many worms die at once, however, they can cause an embolism with severe breathing difficulty. In small dogs worms can move into the heart and cause an acute form that is usually fatal.
Diagnosis uses blood tests that look for microfilariae or adult worm antigens. Antigens can only be found about 6–8 months after infection, and the test for microfilariae can also be negative in an infected dog. Chest X-rays and heart ultrasound help show how severe the disease is.1
How it is prevented
To prevent heartworm, ESCCAP recommends macrocyclic lactones, oral or spot-on, given monthly throughout the transmission season. They kill larvae that entered in the previous 30 days and stop them becoming adults. Prevention starts in spring, before the mosquito season, and continues until late autumn: in southern Europe from May to the end of November and, where heartworm is very common, all year. In some European countries there is also a long-acting injection for dogs over six months, registered to protect for a year.
At the start of each prevention year the dog should be tested for antigens and microfilariae. During the season, adding a mosquito repellent can be useful. If your dog goes from a heartworm-free area to a risk area, ESCCAP advises giving the first dose within 30 days of arrival and the last within a month of returning. Product, schedule and tests are for your veterinarian to decide.12
Sources and checks
Numbers in the text lead to the exact source. Commercial blogs and social posts are not used as evidence.
- 1Veterinary guideline
ESCCAP Guideline 05: Control of Vector-Borne Diseases in Dogs and Cats (5th edition, 2024)
European Scientific Counsel Companion Animal Parasites (ESCCAP) · Biology of D. immitis and D. repens, distribution in Europe, signs in dogs and cats, diagnosis, monthly prevention May–November in southern Europe, travel.
Link checked on 16 Sept 2026 - 2Veterinary guideline
ESCCAP Guideline 01: Worm Control in Dogs and Cats (7th edition, 2025)
European Scientific Counsel Companion Animal Parasites (ESCCAP) · In Dirofilaria-endemic areas, monthly or long-acting injectable prevention during the vector season.
Link checked on 16 Sept 2026 - 3Peer-reviewed research article
Leishmania infantum and Dirofilaria immitis infections in Italy, 2009–2019: changing distribution patterns
Parasites & Vectors (2020) · PubMed PMID 32293524 · Over 10,000 heartworm tests (90,633 in total with leishmaniosis): foci in the centre, south and islands too; positive rates by macro-area and increase over time. One author is employed by a pharmaceutical company.
Link checked on 16 Sept 2026
This guide does not recommend products or doses. Heartworm testing and prevention should be decided with your veterinarian based on where your dog lives and its lifestyle.