Summer evenings in the park or on the terrace are also prime time for mosquitoes. And with them comes one of the most serious parasites for our pets: heartworm disease, known in Spain as dirofilariosis. It is caused by Dirofilaria immitis, a parasite that lives in the pulmonary arteries and heart of dogs and, less often, cats. The good news is that preventing it is easy; the bad news is that treating it once established is long and delicate. Here is what you should know.
How it spreads: no mosquito, no heartworm
Heartworm does not pass directly from one dog to another, or from a dog to a cat or a person. It always needs a mosquito in between:
- A mosquito bites an infected dog and takes in blood containing microfilariae, the parasite's microscopic larvae.
- Inside the mosquito, the larvae develop into their infective stage. This only happens when it stays warm for enough days in a row.
- When that mosquito bites another animal, it leaves the larvae on its skin.
- The larvae migrate through the body for months and end up as adult worms in the pulmonary arteries, where they can live for several years.
Several mosquito species can carry it, including the tiger mosquito, which has spread across much of Spain in recent years.
Why summer matters (in Madrid too)
Warm weather means more mosquitoes and faster development of the larvae inside them. That is why the risk is concentrated between spring and autumn, peaking in the hottest months.
Some parts of Spain have a lot of heartworm, such as the Canary Islands, and there is meaningful risk in humid and irrigated areas or near rivers and wetlands. Prevalence in the Madrid region is lower, but it is not zero: cases do occur, dogs travel with their families on holiday, and mosquitoes are increasingly common in urban settings. If your dog spends the summer on the coast, in the Canaries or near water, the risk goes up.
Signs in dogs: silent for a long time
The treacherous thing about heartworm is that early on there are no symptoms. It takes around six months from the bite for adult worms to develop, and even then many dogs look healthy for quite a while as damage builds up in the lungs and heart. When signs do appear, they usually include:
- A persistent cough, especially after exercise.
- Tiring easily on walks and reluctance to exercise.
- Weight loss and poor appetite.
- Fast or laboured breathing.
- In advanced cases, a swollen belly from fluid (right-sided heart failure) or fainting.
There is also a severe complication called caval syndrome, where a large mass of worms blocks blood flow through the heart. The dog suddenly becomes weak, struggles to breathe and may pass dark urine. This is an emergency: go to a veterinary clinic straight away.
Diagnosis: the antigen test
The key test is a blood antigen test, which detects adult female worms. It takes a few minutes in the clinic or can be sent to a lab, and it is usually paired with a check for microfilariae in the blood. If it is positive, your vet will assess the heart and lungs with X-rays, an echocardiogram and blood tests before planning treatment.
One important point: because the worms take months to mature, the test cannot pick up recent infections. So an adult dog starting prevention is usually tested before starting and again a few months later. We walk through this in a case study on when to perform the heartworm antigen test in dogs. For dogs living in risk areas, a yearly check is common, and it is a good opportunity to test for leishmaniasis as well, another insect-borne disease.
Why treating dogs takes so long
Killing adult worms that live inside blood vessels is not simple. When they die, their fragments are carried into the lungs and can cause thromboembolism, the main complication of treatment. That is why current protocols are staged over several months:
- An initial phase with drugs that weaken the worms and clear the larvae.
- Several spaced-out injections of an adulticide drug, given only by the vet.
- Strict rest throughout and for some weeks afterwards: no running, jumping or rough play, because exercise raises the risk of complications.
- Follow-up tests to confirm the infection has cleared.
For a young, energetic dog, months of near-total rest are tough, and the cost adds up. If you want to see how a vet reasons through the process, have a look at our case on the heartworm treatment protocol.
Prevention is far easier
Compared with all that, prevention is simple, safe and much cheaper. It works by killing any larvae your dog may have picked up before they can grow into adults. The most common options are:
- Monthly tablets or spot-ons, often combined with protection against intestinal worms, fleas or ticks.
- A long-acting injection given by your vet that protects for months.
- Mosquito repellents (collars or spot-ons with a repellent effect) as a useful extra, though they do not replace a preventive.
In low-risk areas, covering the mosquito season is usually enough, starting before mosquitoes appear and finishing once they have gone. In high-risk areas or if you travel, your vet may advise year-round protection. What matters most is not missing doses: one forgotten dose in midsummer leaves a gap. And not every parasite product covers heartworm, so check with your vet that yours does.
Cats are different
Cats are poor hosts for this parasite: they get infected less often and usually carry very few worms, or none reach adulthood. That does not mean they are unaffected. The lungs' reaction to the larvae can cause a respiratory disease that is easily mistaken for feline asthma:
- Coughing and episodes of breathing difficulty.
- Vomiting with no obvious cause.
- In some cases, sudden death with no warning signs.
Diagnosis is trickier in cats: the antigen test is often negative even when the cat is affected, so several tests have to be combined. On top of that, there is no approved treatment to kill adult worms in cats; care focuses on controlling inflammation and symptoms. That makes prevention even more important in cats, and there are monthly spot-ons made specifically for them. Mosquitoes come indoors, so it is worth considering for indoor-only cats too if you live in a risk area.
What about people?
Heartworm is an uncommon zoonosis. Humans are accidental hosts: if an infected mosquito bites us, the parasite almost never develops and usually causes no symptoms at all. Rarely, it can leave a small nodule in the lung that turns up by chance on an X-ray. Your dog cannot infect you directly, and protecting it reduces the number of infected animals that can pass the parasite on to local mosquitoes.
When to see the vet
- If your dog coughs, tires sooner on walks or loses weight for no clear reason.
- If your cat coughs, breathes with its mouth open or vomits often.
- Before travelling with your pet to a risk area, to plan prevention.
- As an emergency if there is collapse, marked breathing difficulty or dark urine.
This article is for information only and does not replace an assessment by your veterinarian, who can advise on the right test and prevention plan for your pet and where you live.
