Heartworm disease is caused by Dirofilaria immitis, a parasitic worm transmitted by mosquito bite. Larvae injected into the skin migrate through the body for roughly six to seven months, then mature into worms 20–30cm long that live in the pulmonary arteries and right side of the heart, where they damage blood vessels and lungs. Adults survive five to seven years. In Singapore and Malaysia the risk is continuous and year-round — there is no winter to interrupt the mosquito, so there is no month in which prevention can safely lapse.
Why is heartworm a year-round risk in the tropics?
Heartworm larvae must develop inside a mosquito before they become infectious, and that development is temperature-driven — it stalls when temperatures stay below roughly 14°C. Temperate countries therefore get a genuine transmission off-season, and some owners there dose seasonally.
Nothing about that applies here. With 24-hour temperatures in the high twenties and rainfall every month, mosquito breeding and larval development run continuously, and multiple local mosquito genera can carry heartworm. Combine that with a large regional population of unowned and unprotected dogs acting as reservoirs — the strays that welfare groups like SPCA Selangor work with daily — and year-round prevention stops being cautious and becomes the baseline standard of care. Singapore's Animal & Veterinary Service (AVS) and Malaysia's Department of Veterinary Services are the national animal-health authorities on either side of the Causeway.
What are the symptoms of heartworm in dogs?
The cruel part is how quiet it is. A dog can carry worms for a year with no visible signs, which is exactly why testing rather than symptom-watching is the strategy.
| Stage | Signs |
|---|---|
| Class 1 | None, or an occasional mild cough |
| Class 2 | Persistent cough, tiring on walks that used to be easy |
| Class 3 | Marked exercise intolerance, weight loss, laboured breathing, coughing up blood, weakness |
| Class 4 (caval syndrome) | Sudden collapse, pale gums, dark red-brown urine — surgical emergency |
Any adult dog with a new cough, reduced stamina, or unexplained weight loss should be tested, and in this region that applies regardless of prevention history.
Why must a dog be tested before starting prevention?
Two solid reasons, and vets will not skip this:
- Preventives do not kill adult worms. They kill larvae from the previous month or so. Starting prevention on an already-infected dog masks nothing and treats nothing — the adult worms keep damaging the lungs and heart while the owner reasonably assumes the dog is protected.
- Safety. A dog with large numbers of circulating microfilariae can react badly when those larvae die off rapidly, and some preventive products carry label cautions about use in microfilaria-positive dogs.
The standard test is an antigen test detecting proteins from adult female worms, usually paired with a microfilaria check. Because infection takes about six to seven months to become detectable, a puppy can start prevention before seven months of age without testing, then be tested about six months later. Adult dogs and dogs with any gap in prevention are tested first, then annually.
What does heartworm treatment involve?
Treatment works, but it is a serious, months-long medical undertaking:
- Staging — chest imaging and bloodwork to grade disease severity.
- Doxycycline plus a macrocyclic lactone for several weeks, targeting Wolbachia bacteria inside the worms and reducing complications.
- Melarsomine injections — the only approved adulticide, given deep into the lumbar muscles in a specific series. This is painful and requires close monitoring.
- Strict exercise restriction for the whole period. This is non-negotiable, not a suggestion. Dying worms fragment and lodge in lung vessels; exertion is what turns that into fatal thromboembolism. Months of lead-only toilet breaks and crate rest are typical.
- Retesting roughly six months after completing treatment.
The costs are substantial — as of 2026, full treatment for an established infection commonly quotes anywhere from roughly S$1,500 to S$5,000 or more in Singapore, and roughly RM2,000 to RM8,000 in Malaysia, depending on disease stage, dog size, and complications. The dog is unwell throughout, and outcomes in advanced cases are not guaranteed. Prevention, by contrast, runs on the order of S$15–S$40 (roughly RM15–RM60) a month as of 2026, with annual injections in Singapore commonly in the low hundreds of dollars. Owners are sometimes offered a "slow kill" approach using preventives alone over years; veterinary guidelines do not consider this equivalent, because lung damage continues while worms slowly die.
What are the heartworm prevention options?
| Option | Schedule | Notes |
|---|---|---|
| Oral chewable (ivermectin, milbemycin) | Monthly, same date each month | Often combined with intestinal wormers; check the dog actually swallows it |
| Topical spot-on (moxidectin, selamectin) | Monthly | Keep dry for the stated period; useful for dogs that refuse tablets |
| Injectable moxidectin | Every 6 or 12 months, at the clinic | Removes the risk of forgotten doses; requires a negative test first |
All are prescription products and all are dosed by weight, so re-weigh growing puppies. At preventive doses these drugs are safe in herding breeds, but always dose as labelled and never share products between species.
Mosquito reduction helps but does not replace medication: clear standing water from pot trays, gutters, and drains weekly, use screens and fans, and avoid long dusk and dawn loitering in mosquito-heavy areas. Repellent products marketed for dogs vary in evidence — ask your vet before applying anything, and never use human DEET products on a dog.
Set a monthly phone reminder, keep the annual test in your calendar alongside vaccinations, and heartworm becomes a problem you simply never have. Any clinic in the Singapore Veterinary Association's vet centre directory can run the test and dispense prevention on the spot.