Heartworm in dogs – signs and what to do

Updated 2026-09-30 · 2 sources

Based on: ESCCAP, ESCCAP Deutschland

Contact a vet today

Contact your vet today if you think your dog has this – it can become serious. Heartworm is a serious parasite infection in dogs, spread by mosquitoes.

What to do now

What you can do now

Note when it started, how often it happens and what else you see.
Take a photo or short video to show your vet.
Keep fresh water available.
Let your dog rest somewhere calm.

Don't

Do not give any human medicine.
Do not wait if it gets worse.

What it is and how it spreads

Heartworm (Dirofilaria immitis) is spread by mosquitoes.^[2] It occurs across southern Europe and in Slovenia, Romania and Bulgaria, while it is not widespread in Germany.^[1]^[2] In Germany, the mosquito season is about April to October.^[2]

Signs

Chronic cough
Difficulty breathing
Weakness
Fainting after exercise
Fluid in the abdomen
Weight loss

How pets pick it up

Mosquitoes

Where in Europe it occurs

Country	Occurrence
Italy	Common (endemic)
Portugal	Common (endemic)
Romania	Common (endemic)
Spain	Common (endemic)
France	Present
Germany	Rare

What the experts say

“Clinical signs of the disease caused by Dirofilaria immitis develop gradually and may begin with a chronic cough which may be followed by moderate to severe dyspnoea, weakness and sometimes syncope after exercise or excitement.”
— ESCCAP [1]

“In der Mückensaison von April bis Oktober werden sie von Stechmücken übertragen. In Deutschland ist der Parasit nicht verbreitet”
— ESCCAP Deutschland [2]

If your pet is struggling to breathe, has collapsed, is having a seizure or can't stand up: go to the nearest vet now.

Not sure how serious it is?

OkPaw asks a few quick questions about your pet and tells you in 30 seconds whether it can wait. Free on Google Play for Android.

OkPaw is on Android now – an iPhone version is on its way.

Common questions

How do dogs get heartworm?

Mosquitoes transmit heartworm.^[2]

Where in Europe does heartworm occur?

It is endemic across southern Europe and in Slovenia, Romania and Bulgaria.^[1]

When is the mosquito season in Germany?

The mosquito season is about April to October.^[2]

Sources

How this page was made: written with the help of AI from the sources below, then checked claim by claim against those sources. Urgency levels follow OkPaw’s fixed safety rules. It is guidance, not a diagnosis. How we write our pages