Heartworm prevention: where the risk is and what to give
Updated July 29, 2026·4 minper CAPCAnswer — in the first paragraph ↓
DOG · HEARTWORM
Heartworm reaches a dog through a mosquito bite, and the only real defence is regular prevention — one dose a month through mosquito season. Infection rates are highest in Brazil and Argentina, the Caribbean and the southern United States; the risk is growing across Mediterranean Europe and stays sporadic in eastern Europe.
01Where the risk is
Region
How common
Notes
Brazil and Argentina
up to 74% of dogs
the highest rate in the world
Caribbean
around 63%
Puerto Rico, Dominican Republic, Cuba
Mexico, Gulf coast
around 42%
from local surveys
US South and Gulf Coast
above 5%
the zone is spreading west
Canada: Ontario, Quebec
moderate
mosquito season runs June–November
Mediterranean Europe
growing
Italy, Spain, Portugal, Greece
Eastern Europe, southern Russia
sporadic
new cases appearing in the south
Larvae enter the dog’s bloodstream with a mosquito bite, and the adult worms live in the heart and the pulmonary vessels. The disease builds slowly and gives no obvious signs for a long time. Treatment takes months, costs a lot and carries its own risk: dying worms can block a vessel. Prevention is both cheaper and safer than treating the infection.
02What protects a dog
Prevention rests on the macrocyclic lactones: ivermectin, milbemycin, moxidectin and selamectin. They are given monthly as a chew or a spot-on. Often they come as combination products that also cover fleas, ticks and intestinal worms. In the United States and eastern Canada the Companion Animal Parasite Council (CAPC) advises keeping prevention going all year; around the Mediterranean the minimum is May to October, while mosquitoes are flying.
03Why it starts with a test
If a dog is older than 6 months and has never been on prevention, a blood test for heartworm antigen comes before the first dose. Starting without it in an endemic area is risky: the drug works on larvae, not on adult worms, and in an already infected dog that can trigger a severe reaction. A puppy under 6 months needs no test — prevention starts straight away.
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04Frequently asked
Is heartworm prevention needed in the UK?
Cases in the UK are almost always imported — dogs that lived abroad or travelled. Local transmission is not the pattern there. Prevention becomes a real question before a trip to southern Europe or the tropics, and that is worth planning with your vet a few weeks ahead of the journey.
Does ordinary deworming protect against heartworm?
Not always. The familiar intestinal wormers based on fenbendazole, pyrantel or praziquantel do nothing against heartworm. Milbemycin and moxidectin do cover it, and they come inside combination products. The thing to check is the active ingredient, not the words “worm treatment” on the box.
Do cats need this protection too?
Combination spot-ons exist for cats as well, covering heartworm and intestinal parasites alongside fleas and ticks. In a region where heartworm is common it is worth discussing such a product with your vet, especially for a cat that goes outdoors or lives in the countryside.
When does prevention start and stop?
It follows the mosquito season. Around the Mediterranean that means at least May to October; in Ontario and Quebec mosquitoes are active from June to November. In the southern United States and eastern Canada the advice is not to stop at all, because warm days happen in winter too. In the tropics the season barely ends.
DOG · HEARTWORM
