Heartworm in dogs: prevention in tropical and subtropical climates

Heartworm disease (Dirofilaria immitis) is one of the most serious parasitic conditions dogs can develop. Adult heartworms live in the heart, pulmonary arteries and surrounding blood vessels, causing progressive damage to the cardiovascular and pulmonary systems. In untreated dogs, the disease is eventually fatal. The good news: it is almost entirely preventable with year-round preventive medication.

The parasite is transmitted exclusively through the bite of an infected mosquito. A mosquito bites an infected dog, takes up microfilariae (the larval stage circulating in the blood), and transmits infective larvae when it next feeds. In tropical and subtropical regions — including Singapore, northern and coastal Australia, and Southeast Asia broadly — mosquitoes are present year-round, meaning the transmission risk never has an off-season.

The American Heartworm Society (AHS) recommends year-round prevention for all dogs in endemic areas, testing annually, and testing any dog before starting prevention for the first time. Dogs that are already infected with adult heartworms can suffer serious, potentially fatal reactions to certain preventives — which is why prior testing matters.

Life cycle and why early prevention matters

After a dog is bitten by an infected mosquito, it takes approximately six to seven months for larvae to mature into adult heartworms. Early-stage larvae are killed by preventive medication; adult worms are not. This is why preventives work retroactively on the previous month's exposures (killing larvae before they mature) but cannot eliminate established adult infections. The longer prevention is delayed after exposure, the narrower the window to kill larvae.

• Microfilariae (stage 1 larvae): circulate in the blood of infected dogs; taken up by mosquitoes
• Infective larvae (stage 3): deposited in a new host's skin by a mosquito bite
• Larval stages 4 and 5: migrate through tissue over several months; killed by preventive medication
• Adult worms: live in the heart and pulmonary arteries for 5–7 years; not killed by standard preventives

Prevention options

Several preventive products are available. They differ in formulation, frequency, and whether they also cover other parasites. Availability and prescription rules vary by market — in Australia many oral heartworm preventives are sold over the counter (OTC) under APVMA classification, while in Singapore parasiticides are generally supplied through licensed veterinary channels. Either way, your vet will recommend the most appropriate option for your dog's size, health status and lifestyle, and prior heartworm testing is required before starting.

• Monthly oral chewables (e.g. milbemycin, ivermectin-based): convenient, often combined with intestinal worm coverage
• Monthly topical spot-ons: useful for dogs that resist oral medication
• Six-monthly or twelve-monthly injectable moxidectin (ProHeart 6 / ProHeart 12): administered by a vet, which removes the compliance risk of monthly dosing. The twelve-monthly formulation is APVMA-registered and long-established in Australia; injectable moxidectin is a veterinary-administered product — confirm current availability with your local vet
• Combination products: many preventives also cover fleas, ticks and intestinal parasites — discuss with your vet to avoid unnecessary duplication

Signs of heartworm disease

Early infection is often silent. As adult worm burden increases and disease progresses, clinical signs emerge. The AHS classifies heartworm disease in four classes of severity, from no symptoms (Class 1) to severe disease with right-sided congestive heart failure (Class 4).

• Mild to moderate: persistent dry cough, reduced exercise tolerance, mild lethargy after exertion
• Moderate: coughing, difficulty breathing, fatigue after minimal activity, reluctance to exercise
• Severe: severe coughing, difficulty breathing, heart failure signs (fluid in abdomen, fainting, marked weight loss)
• Caval syndrome (life-threatening): sudden cardiovascular collapse; worms blocking blood flow through the heart — requires emergency surgical removal

Treatment: effective but serious

Treatment of established adult heartworm disease involves a series of intramuscular injections of melarsomine (an arsenic-based compound), strict exercise restriction for months, and careful management to prevent complications from dying worms blocking blood vessels. It is expensive, takes months, carries genuine health risks, and is far more difficult than prevention. The AHS treatment protocol is the established standard of care.