This critter is transmitted by mosquitoes (proper ones, not like leishmaniasis – see article in The Portugal News 23 May 2020).

If a mosquito bites an infected dog (or, more rarely, cat), the mosquito gets a meal of baby heartworm as well as blood. These babies are only 0.3mm long. They stay inside the mosquito for a few weeks, growing. When they have developed enough, they will be injected, the next time the mosquito gets hungry, into an unsuspecting host (your pet). Over the next 2-3 months, these juvenile worms migrate through the dog, until they reach the pulmonary artery and right side of the heart and achieve maturity. As adults, the females can be up to 30cm long (by comparison, at 16cm, the males seem tiny). After approximately 3-6 months of living here, they mate and release babies into the bloodstream – the cycle begins again.

The worms cause trouble in a number of ways – there can be so many that the arteries get physically blocked, and the worms also interfere with the movement of the heart valves, resulting in heart failure. They damage the lining of the blood vessels, which can cause blood clots.

Some dogs with heartworm will look very ill – they may have a cough, and be thin but with a big fluid-filled tummy. Others will have no symptoms at all, and be diagnosed only because a screening test was done.

Heartworm is usually diagnosed by a blood test that detects a substance given off by the female worm’s reproductive tract (grim!). Sometimes they are also seen during heart ultrasound (echocardiography).

Treatment and cure is possible. The success of the treatment depends on how much damage the worms already caused – some dogs need long-term medication for heart failure. Treatment takes several months, since the worms have to be killed gradually – killing them too fast can cause allergic reactions while they decompose, and also potentially fatal blood clots in the lungs (pulmonary thromboembolism). The adults are killed with an arsenic-based injection, and the babies with one of a number of deworming preparations. To reduce the risks, the dog has to be restricted to bed-rest, and take various other medications.

Heartworm in cats is very hard to treat and rarely has a good outcome. Luckily, they are only 10 percent as frequently infected as dogs.

To stop your poor dog from being infected, there are a few things you can do

• Keep it inside at mosquito-o-clock.
• Use a mosquito-repelling collar or pipette.
• Use a heartworm-preventing dewormer. There are several types, including pills, pipettes, and injections.

These measures may need to be carried out all year round – it depends on the climate where you live – in some areas of the Algarve, mosquitoes don’t clear off in winter.
Talk to your vet about the right regime for your dog.
For further advice or information, please contact 124 Vet by calling 282 338 407, or email

Faye Campey