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Preventive Care14 min read

Heartworm in Dogs Malaysia: Prevention, Testing and Signs (Cats Too)

By Trusty Tails Editorial Team · 7 October 2026

White and black terrier lying on grass outdoors

Heartworm in dogs is a real concern in Malaysia: a 2022 study of 399 dog blood samples from veterinary hospitals and animal shelters in Malaysia found Dirofilaria immitis, the canine heartworm, in 6.5% of them. The worm is spread by mosquitoes, many infected dogs show no signs at first, and once adult worms are established, treatment is far harder than prevention. This guide explains how heartworm works, why a test usually comes before a preventive, which signs matter, and why cats are not exempt.

This is general information, not a diagnosis. A registered veterinarian who has examined your pet should choose any test, product or schedule.

Quick answer: heartworm in dogs in Malaysia

Heartworm is a parasitic worm spread only by mosquito bites. Larvae enter through the bite wound and grow into adult worms in the heart, lungs and nearby blood vessels. International guidance from the Companion Animal Parasite Council (CAPC) and the American Heartworm Society (AHS) recommends year-round prevention for dogs and annual testing, and AHS and the US FDA advise a test before starting a preventive in dogs aged 7 months or older. Cats can also be infected, and there is no approved drug to clear adult heartworms from a cat, so prevention matters for them too. Ask your veterinarian which preventive suits your pet, and never give a dog or cat a product that has not been prescribed for that animal.

What is heartworm?

Heartworm disease is caused by Dirofilaria immitis. The American Heartworm Society describes it as a serious and potentially fatal disease caused by foot-long worms that live in the heart, lungs and associated blood vessels of affected pets. According to the Merck Veterinary Manual, the worms are only 2.5–4 cm long when they arrive in the lung arteries, then grow rapidly: adult males reach about 15 cm and females about 25 cm.

The dog is a natural host. Inside a dog, heartworms mature into adults, mate and produce offspring, so one dog can carry many worms. CAPC states that heartworms live approximately 5 to 7 years in the dog; AHS gives up to 2 or 3 years in cats.

The Merck Veterinary Manual explains that the disease mainly affects the pulmonary arteries, the vessels carrying blood from the heart to the lungs, causing inflammation and damage to the vessel walls.

How do dogs and cats catch heartworm?

Heartworm is not caught from another pet. The US Food and Drug Administration (FDA) states plainly that the disease is not contagious: a dog cannot catch heartworm from being near an infected dog. The worm needs a mosquito to complete its life cycle.

  1. A mosquito bites an infected animal. It picks up baby worms (microfilariae) circulating in that animal's blood.
  2. The larvae develop inside the mosquito. AHS says they mature into the "infective stage" over 10 to 14 days. CAPC notes that development slows at cooler temperatures and stops below 57°F (about 14°C).
  3. The mosquito bites your pet. CAPC explains that infective larvae are deposited on the skin during feeding and migrate in through the bite wound.
  4. The worms grow for months. AHS states that it takes approximately 6 months for larvae to become sexually mature adult worms. CAPC says infected dogs typically have microfilariae in their blood 6 to 7 months after infection.

This delay matters: a recent infection cannot yet be detected by testing.

How common is heartworm in Malaysia?

There is no single national figure, but published Malaysian studies show that heartworm is present:

  • 6.5% of 399 dogs. A 2022 study in Acta Parasitologica (Vinnie-Siow and colleagues) tested blood from dogs at veterinary hospitals and animal shelters in Malaysia and identified D. immitis in 6.5% and a second filarial worm, Brugia pahangi, in 1.3%.
  • 13.6% of 103 shelter dogs. A 2021 serological survey in Tropical Biomedicine of two animal shelters in central Peninsular Malaysia found 13.6% (14 dogs) seropositive for D. immitis.

Across the tropics, the Tropical Council for Companion Animal Parasites (TroCCAP) wrote in 2023 that the canine heartworm is widespread, with prevalence above 30% in high-risk areas. The same paper points to climate conditions that favour mosquitoes and larval development, and to low compliance with the recommended year-round use of preventives.

Owner awareness is mixed. A 2023 final-year undergraduate project at Universiti Malaysia Kelantan surveyed 144 dog owners in the Klang Valley: 70.1% were aware of heartworm disease in dogs, and cost (40.9%) was the main barrier respondents gave to prevention.

These figures come from specific groups of dogs, often shelter dogs, so they are not a direct estimate of your own pet's risk.

Signs of heartworm in dogs

Many dogs show nothing at first. The Merck Veterinary Manual notes that most infected dogs are subclinically affected, and when signs do appear the most common one is a cough. Signs listed by AHS, CAPC, the FDA and the Merck Veterinary Manual include:

  • a mild, persistent cough
  • reluctance to exercise, or tiring after moderate activity
  • decreased appetite and weight loss
  • laboured or difficult breathing
  • a swollen belly from fluid build-up, a sign of right-sided heart failure
  • fainting or collapse
  • an irregular heart rhythm, which a veterinarian may detect on examination

These signs are not specific to heartworm, and because many infected dogs show no signs at all, the absence of a cough does not rule it out. A veterinarian's examination and testing are needed to answer the question.

See a veterinarian urgently if your dog collapses or faints, struggles to breathe, or has a suddenly swollen belly. The Merck Veterinary Manual describes a severe form called caval syndrome, which combines heart failure with the destruction of red blood cells. Do not wait to see whether these signs settle on their own.

Heartworm in cats: different, and harder to detect

Cats are not ideal hosts. The FDA explains that heartworms do not thrive as well in a cat's body, and the Merck Veterinary Manual notes that most immature worms in cats do not survive to adulthood. That does not make cats safe.

  • Lung disease from young worms. Even worms that never mature can cause inflammation in the lungs and airways. The Merck Veterinary Manual calls this heartworm-associated respiratory disease, or HARD.
  • Sudden death. The FDA warns that some infected cats die suddenly without ever showing signs of illness, and that the death of even one worm can be fatal for a cat.
  • Non-specific signs. AHS lists coughing, asthma-like attacks, periodic vomiting, lack of appetite and weight loss, all of which have many other possible causes.
  • Indoor cats are not exempt. AHS states that both outdoor and indoor cats are at risk, because mosquitoes get indoors.

Testing cats is harder. According to the Merck Veterinary Manual, antigen tests alone are not sensitive enough as the sole screening test for cats, and a positive antibody test shows that an infection has occurred but does not prove a current one. The FDA notes that only about 20% of infected cats have microfilariae in the bloodstream, compared with 80–90% of dogs.

Treatment is the biggest difference. AHS states that there is no approved drug therapy for heartworm infection in cats, and the drug used to treat dogs is not safe for cats; the Merck Veterinary Manual warns that it risks severe lung inflammation and death in cats. For cats, prevention is the main protection. Ask your veterinarian whether a feline heartworm preventive suits your cat's routine.

Why test before starting prevention?

The FDA explains that giving a heartworm preventive to a dog infected with adult heartworms may be harmful or deadly. If microfilariae are circulating, the preventive may kill them suddenly and trigger a shock-like reaction. For that reason, dogs aged 7 months or older should be tested before starting prevention.

For puppies, AHS says those under 7 months of age can start prevention without a test, because an infection would be too young to detect. They should then be tested 6 months after the first visit, again 6 months later, and yearly after that.

What the tests look for

CAPC recommends testing all dogs every year with both an antigen test and a microfilariae test, including dogs already on prevention. The two tests look for different things:

  • Antigen tests look for adult heartworms. CAPC notes that they usually detect only mature infections, older than about 6 months, with at least one female worm.
  • Microfilariae tests look for baby worms in a blood sample.

Your veterinarian will interpret the results alongside your dog's history.

Why test a dog that is already on prevention?

CAPC recommends annual testing even for dogs on prevention, and states that it ensures infected dogs are identified and treated as soon as possible. CAPC also notes that some heartworm strains can develop into adults even in dogs on routine prevention. The Merck Veterinary Manual describes such resistant heartworms as "seemingly rare and geographically limited". An annual test finds a problem early, whatever the cause.

How heartworm prevention works

The preventives in common use belong to a drug group called macrocyclic lactones. CAPC lists oral ivermectin, oral milbemycin oxime, oral, topical or injectable moxidectin, and topical selamectin. These products work against larvae that a mosquito has already deposited, before they grow into adults. The Merck Veterinary Manual states that year-round use of a macrocyclic lactone preventive is the only way to reliably prevent heartworm infection.

Points to discuss with your veterinarian:

  • Year-round, not seasonal. CAPC recommends that all dogs, whatever their lifestyle, receive year-round heartworm prevention. The FDA and AHS also recommend year-round prevention.
  • Choose a format you can stick to. Oral, topical and injectable options exist; ask which your household can give reliably on time.
  • Check what the product covers. Some combination products also target intestinal worms, fleas or ticks, while others do not. Do not assume that a heartworm preventive treats intestinal worms, or the reverse, unless the label says so. Our deworming guide explains intestinal worms separately.
  • Use the right product for the right animal. Do not share one pet's product with another pet or another species. Use only a product prescribed for your pet, and follow the label and your veterinarian's instructions exactly. In the United States, the FDA notes that all heartworm preventives require a veterinarian's prescription.
  • Do not improvise. Do not split, combine or substitute products, and never use a farm-animal or human product because it shares an ingredient name. If a dose is missed or late, call your veterinary clinic for advice instead of doubling up.

What happens if a dog tests positive?

A positive result is serious but treatable in many dogs. Your veterinarian will confirm the diagnosis and assess how far the disease has progressed before recommending a plan. In general terms:

  • Treatment uses a specific adulticide. The Merck Veterinary Manual states that the only approved heartworm adulticide is melarsomine dihydrochloride, given by injection, and that doxycycline is usually started at diagnosis. CAPC recommends treating infected dogs with an adulticide as soon as medically practical.
  • Rest is part of treatment. Dead and dying worms can cause blockages in the blood vessels of the lungs (pulmonary thromboembolism). The Merck Veterinary Manual describes exercise restriction as critical to reducing that risk, and CAPC recommends at least a month of strict exercise restriction after adulticide treatment.
  • "Slow kill" is not the recommended approach. CAPC advises against relying on long-term preventive alone to kill adult worms. TroCCAP notes that melarsomine is unavailable in several tropical countries, which can limit the options. Ask your veterinarian what is available and appropriate for your dog.
  • Follow-up testing confirms success. Your veterinarian will advise when to retest and how to continue prevention afterwards.

AHS describes prevention as by far the best option for dogs, and the FDA puts it simply: the best treatment is prevention.

A simple heartworm checklist for Malaysian owners

Situation What to discuss with your veterinarian
New puppy under 7 months Starting prevention, and when the first heartworm test should be done (AHS suggests 6 months after the initial visit)
Adult dog never tested, or prevention lapsed A test before starting or restarting prevention
Dog already on prevention Annual antigen and microfilariae testing, as CAPC recommends
Indoor or outdoor cat Whether a feline preventive suits your cat; AHS states that indoor cats are at risk too
Rescued or adopted dog with unknown history Testing, plus a combined plan for heartworm, ticks and intestinal worms
Travel to an area where heartworm is common The Merck Veterinary Manual advises an antigen and microfilariae test, then starting prevention before travel if the result is negative

Keep a record of the product, date given and test results, and bring it to each wellness visit.

Frequently Asked Questions

Can my dog catch heartworm from another dog? No. Heartworm is spread only through mosquito bites. The FDA states that a dog cannot catch the disease from being near an infected dog.

Does my indoor cat need heartworm prevention? Ask your veterinarian. The American Heartworm Society states that both indoor and outdoor cats are at risk, because mosquitoes get indoors. There is no approved drug to clear adult heartworms from a cat, so prevention is the main protection.

Why does my dog need a blood test before starting a heartworm preventive? The FDA warns that giving a preventive to a dog that already has adult heartworms may be harmful or even deadly. If microfilariae are present, they can die suddenly and trigger a shock-like reaction. AHS says puppies under 7 months can usually start prevention before testing.

My dog is on prevention. Why test every year? Testing finds an infection early whatever the cause, and CAPC notes that some heartworm strains can develop into adults even in dogs on routine prevention. CAPC recommends annual antigen and microfilariae testing for all dogs.

What are the early signs of heartworm in dogs? Often none. When signs appear, the most common is a cough; others include tiring easily, weight loss and difficulty breathing. See a veterinarian for testing.

Does a dewormer protect against heartworm? Not necessarily. Intestinal worms and heartworm are different parasites, and products target specific organisms. Only rely on a product whose label lists heartworm prevention and which your veterinarian has prescribed for your pet.

What should I do if I missed a dose? Contact your veterinary clinic with the product name and the date of the last dose. Do not double up unless the veterinarian tells you to. They may also advise when your dog should next be tested.

For related reading, see our tick fever in dogs guide, deworming guide and dog vaccination schedule. This guide is educational and does not replace an examination or advice from a registered veterinarian who knows your pet.

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