Canine Heartworm: Early Signs, Treatment, and Prevention

I hold my dog's collar while the evening air cools, and I remember how easy it is to miss what we cannot see. Heartworm disease works that way—quiet at first, then demanding, and always worth catching early. This guide gathers what I've learned and what veterinarians teach every day, so you can act with calm, timely care.

Heartworms are parasitic roundworms transmitted by mosquitoes. In dogs, adults live primarily in the vessels that carry blood to the lungs and, with heavier burdens, in the right side of the heart. Left unaddressed, they inflame and obstruct those vessels, strain the heart, and can lead to coughing, exercise intolerance, fluid buildup, and, in advanced cases, life-threatening complications. Knowledge, testing, and prevention save lives.

What Heartworm Really Is

The organism behind canine heartworm is Dirofilaria immitis. After a mosquito bites an infected animal and later bites your dog, immature larvae enter through the skin. Over months, they migrate and mature into adults that primarily occupy the pulmonary arteries—the vessels leading from the heart to the lungs—where they disrupt blood flow and trigger inflammation. The heart works harder; the lungs bear the load.

Because the disease centers on the lung circulation, many early changes look respiratory: occasional soft cough, a dog that slows sooner on walks, or breathing that seems deeper after mild exertion. Disease severity depends on worm burden, how long infection has been present, and the dog's activity level. Sedentary dogs can look fine until they are not; athletic dogs often show signs earlier.

How Dogs Get Infected

Heartworm spreads only through mosquitoes. The insect picks up microscopic "baby" worms (microfilariae) from an infected host; over about two weeks within the mosquito, those develop into infective larvae. When that mosquito feeds again, it deposits larvae onto the skin; the larvae enter through the bite wound and begin their months-long migration toward the heart and lungs.

There is no safe geography or haircut. Indoor dogs go outside to potty; outdoor dogs nap on porches; mosquitoes follow warm, exhaled air wherever it drifts. That is why veterinarians recommend year-round prevention and regular testing even in regions with cooler seasons. Travel, climate shifts, and local wildlife reservoirs keep risk present.

Who Is at Risk

All dogs are susceptible—puppy or senior, short-haired or long-haired, urban or rural. Risk climbs with mosquito exposure and with missed or inconsistent use of preventives. Dogs that exercise heavily while infected tend to show signs sooner because increased blood flow pushes against already stressed lung vessels.

Some lifestyle patterns quietly raise risk: frequent camping or time near standing water; living where mosquitoes flourish for long stretches; adopting or fostering dogs with unknown preventive histories. None of these are reasons for fear; they are cues to test and prevent with discipline.

Early Signs You Can Notice

Pay attention to changes over weeks, not just single days. A dry, persistent cough that wasn't there before; getting tired earlier on familiar routes; reluctance to run or climb stairs; subtle weight loss despite normal meals—these are common early flags. In advancing disease, you might see labored breathing at rest, a swollen belly from fluid, fainting episodes, or sudden collapse.

Because signs overlap with other conditions, testing is essential. Many infected dogs appear outwardly well until the burden grows; catching infection before symptoms escalate makes treatment safer and outcomes better.

Why Testing Matters

Veterinarians screen with an antigen test (to detect proteins from adult female worms) and a microfilaria test (to check for circulating young). Running both improves accuracy, reduces missed infections, and guides safe treatment. Annual testing is advised even for dogs on prevention; in higher-risk areas or after lapses in dosing, additional testing may be recommended.

Before starting or restarting prevention in a dog with unknown status, testing first protects the patient. Certain preventives can cause complications if microfilariae are present at high levels; knowing the status lets your vet stage disease and choose the safest plan.

I steady the leash while the veterinarian listens in calm light.

The Standard Treatment Today

When a dog tests positive, the current standard of care is a staged protocol built around melarsomine dihydrochloride, the only approved adulticide. Vets typically begin with doxycycline to target Wolbachia bacteria that live within heartworms and with a macrocyclic lactone preventive to stop new larvae. After that pre-treatment phase, the recommended adulticide plan is a three-dose melarsomine series: one injection, then two injections 24 hours apart at least a month later. This regimen clears more adult worms and reduces complications compared with older two-dose approaches.

Protocols that rely only on preventives ("slow kill") are discouraged because they allow adult worms to persist for long periods, prolong lung damage, and may contribute to resistance. Timely adulticide therapy shortens the disease course and gives the lungs and heart a real chance to heal.

Your veterinarian will stage disease with exam, bloodwork, and imaging when indicated. Staging informs whether additional medications—such as corticosteroids to soften inflammatory reactions—are warranted and how closely activity must be restricted.

Caring Through Treatment

Exercise restriction is not a suggestion—it is protective medicine. As adult worms die, fragments can lodge in the lung vessels and cause thromboembolic events. Keeping activity low before, during, and for weeks after melarsomine minimizes dangerous surges in blood flow through compromised arteries. Think leash walks for toileting only and quiet time otherwise; your veterinary team will tell you when and how to reintroduce activity.

Expect scheduled rechecks. Coughing can briefly increase as worms die. Call your veterinarian promptly for distress: rapid or difficult breathing, sudden weakness or collapse, severe lethargy, or refusal to eat. With careful staging, the recommended protocol, and strict rest, most treated dogs do well.

Prevention That Fits Your Life

Prevention stops larvae before they mature. Options include monthly oral or topical macrocyclic lactones and an extended-release injectable moxidectin given by a veterinarian once yearly for eligible dogs. Your vet will match product to your dog's age, weight, health status, and household routine.

Whatever you choose, consistency is everything. Give doses on the same date each month, set reminders, and avoid gaps—especially during heavy mosquito seasons. Missed doses call for a conversation with your clinic about testing and how to restart safely.

Prevention is not just cheaper than treatment; it is kinder to the lungs and heart. Dogs already on prevention should still be tested annually, because no product or human schedule is perfect and early detection keeps options open.

Mosquito Control at Home

Reduce standing water in yards, refresh outdoor bowls and birdbaths often, maintain screens and door seals, and use pet-safe repellents as directed by your veterinarian. These steps don't replace prevention, but they lower exposure and support the rest of your plan.

Travel and weather shifts can raise risk in places that once felt safe. If your routine changes—seasonal trips, new hikes, a move—tell your vet so testing and prevention can adjust with you.

When Surgery Is the Right Call

In a rare, severe emergency called caval syndrome, a large mass of worms obstructs blood flow in the right heart and vena cava, causing collapse and profound anemia. This scenario requires immediate stabilization and surgical extraction of worms, followed later by standard adulticide therapy once the dog is stable.

Most patients will never need surgery, but knowing it exists—and that speed matters—helps you move decisively if your veterinarian raises the concern.

A Calm Closing Thought

Heartworm disease is preventable, detectable, and treatable—especially when you act early. Keep a steady rhythm: prevention on time, testing each year, and low activity if treatment ever becomes necessary. Your attention is powerful medicine. It gives the lungs space to heal and keeps the heart's work light.

If something feels off—more cough, less stamina, a change you cannot place—call your veterinarian. Early questions are never wasted; they are how health returns to ordinary days.

References

American Heartworm Society. Canine Heartworm Guidelines: Prevention, Diagnosis, and Management.

American Heartworm Society. The AHS Protocol vs. "Slow Kill".

Companion Animal Parasite Council (CAPC). Heartworm Guidelines and General Parasite Prevention Recommendations.

Merck Veterinary Manual. Heartworm Disease in Dogs, Cats, and Ferrets.

U.S. FDA/Labeling. ProHeart 12 (moxidectin) extended-release injectable for dogs.

CDC. Dirofilariasis: About and Prevention.

Disclaimer

This article is informational and not a substitute for veterinary advice, diagnosis, or treatment. If your dog has trouble breathing, collapses, or seems acutely unwell, contact your veterinarian or an emergency clinic immediately.