Heartworm (Dirofilaria immitis) is one of the most insidious threats to animal health: it is spread by mosquito bites and can remain completely symptom-free for a long time while the worms develop in the heart and the blood vessels of the lungs. By the time symptoms appear, the damage is often already advanced. In this article we cover how animals become infected, what to look out for, and how heartworm can be screened for and prevented – for dog and cat owners alike.
What is heartworm and why is it dangerous?
Heartworm is a parasitic worm which, in its adult form, settles in the right side of the heart and the large blood vessels of the lungs. A single animal can host dozens of worms, which obstruct blood flow, damage the vessel walls and the lungs, and in the long term can lead to heart failure. The worms can live in the host for years, so the disease worsens slowly and unnoticed.
How do dogs and cats become infected with heartworm?
Microscopic larvae enter the animal's body through the bite of an infected mosquito, then migrate over several months and develop into adult worms in the blood vessels of the heart and lungs. Because a mosquito is required, infection is most common during the warmer mosquito season and in mosquito-rich areas. Hungary – especially its waterside, lowland, mosquito-rich regions – is an affected area, and infection has been spreading in recent years. The risk differs by country and region, so ask your vet about the situation where you live. Important: the disease is not transmitted directly from animal to animal or to humans – it is always carried by mosquitoes.
Symptoms of heartworm
In dogs, the most common signs appear gradually:
- Persistent, dry cough
- Reduced exercise tolerance, tiring easily, breathlessness on walks
- Loss of appetite, weight loss, lethargy
- In advanced cases, fluid build-up in the abdomen and signs of heart failure
In cats the picture is more insidious and harder to recognise: respiratory signs (asthma-like coughing, wheezing), occasional vomiting, sometimes sudden collapse and, rarely, unexpected death. Many animals remain completely symptom-free for a long time, and the disease is only discovered through screening.
Screening: why early detection matters
Because heartworm can hide for years, regular screening is crucial – especially if the animal does not receive continuous preventive treatment. At home, you can screen from a drop of blood within minutes using the heartworm (HW) rapid test, which detects the antigen of adult female worms. Early detection means treatment can be started in time, which is much safer and cheaper than treating an advanced infection.
Two important timing points: the antigen can usually only be detected about 6–7 months after infection (when the worms have matured), and it is worth testing before starting preventive (deworming) treatment, because if there is an active infection, treatment must be agreed with a vet.
We have summarised when, with which method and at what cost it is worth testing here: Heartworm test: when is it worth doing, how does it work and how much does it cost?
Treatment
Treatment for heartworm disease in dogs exists, but it is lengthy, costly and not without risk: killing the worms requires strict exercise restriction, because dying worms can block the blood vessels of the lungs. In cats there is no approved, safe treatment to kill the worms, so prevention and supportive care are what matter most. That is exactly why, with heartworm, prevention is always cheaper and safer than treatment.
Prevention
- Regular preventive products (spot-on, tablet or injection) as recommended by your vet, during mosquito season or all year round.
- Reducing mosquito bites: mosquito nets, timing evening walks, using repellents.
- Annual screening even alongside prevention, because a missed dose or an incorrectly given product can leave a gap in protection.
Frequently asked questions
Can cats get heartworm too?
Yes. It is rarer in cats, but often more severe and harder to diagnose, and there is no safe treatment to kill the worms, so prevention and screening are important for them too.
When should I do a heartworm test?
Once a year, before starting preventive treatment, and if you notice a persistent cough or reduced exercise tolerance. It is also recommended when adopting a new animal.
How long after infection does the test detect it?
The antigen usually becomes detectable about 6–7 months after infection, once the worms have matured. A recent infection may therefore still give a false negative – regular, repeated screening gives an accurate picture.
Is the home heartworm rapid test reliable?
Yes, it works by antigen-based detection. If the result is positive, veterinary confirmation and a treatment plan are needed.
Important: MancsLab rapid tests are intended for preliminary screening at home and do not replace a veterinary diagnosis.