How do dogs get heartworm?

I work with dogs and owners every week, and heartworm is one of those threats that routinely turns a routine checkup into a serious conversation. Understanding how dogs get heartworm helps you make day-to-day choices that protect your companion, avoid complicated treatments later, and keep boarding or travel plans from turning into an emergency.

Why every dog owner should care about heartworm

Heartworm infection can slowly steal a dog’s energy, cause chronic lung and heart problems, and shorten life expectancy. Even dogs that seem fine at first may be carrying adult worms that are quietly damaging the pulmonary arteries and right side of the heart. Those changes may not be reversible, so preventing infection is usually much kinder to the dog than treating established disease.

Treatment for established heartworm can become expensive and stressful. The protocol often involves multiple veterinary visits, injected medications, antibiotics, strict rest for weeks to months, and sometimes hospitalization. I typically see owners surprised at the cost and time commitment when an otherwise healthy dog tests positive; that’s why prevention is a smarter financial and welfare choice for most households.

Prevention versus cure matters beyond money. Once a dog harbors adult heartworms, exercise restriction is essential to reduce the risk of complications — something that affects daily life, boarding decisions, and suitability for some activities. If you board, travel, or adopt from another region, heartworm status and ongoing prevention should be part of the conversation before you commit.

How heartworm reaches dogs — a concise explanation

The simplest explanation is this: a mosquito carrying infective larvae bites a dog and deposits microscopic larvae that enter through the bite wound. Those larvae mature into adult worms that live in the pulmonary arteries and right heart, and infected dogs can then produce microfilariae (tiny offspring) that another mosquito picks up, continuing the cycle. Monthly preventive medicines are designed to stop the immature stages from establishing, so regular prevention usually blocks the infection before it becomes dangerous.

• Mosquito bite transfers infective L3 larvae from the mosquito to the dog.
• Those larvae mature over several months into adult heartworms in the lungs and heart.
• Infected dogs may carry microfilariae in their blood and act as reservoirs for further spread.
• Veterinarian-prescribed preventives, given correctly, interrupt early stages and usually prevent disease.

From mosquito bite to illness: how heartworm develops inside a dog

The life cycle involves two hosts: a mosquito and a canine. When a mosquito feeds on an infected dog, it may ingest circulating microfilariae. Inside the mosquito, those microfilariae usually develop over about 10–14 days (temperature dependent) into infective L3 larvae. When that mosquito later bites another dog, the L3 larvae enter the skin and migrate under the dog’s tissues.

Over the next several months the larvae undergo further development—passing through immature stages—before reaching the pulmonary arteries and the right side of the heart, where they mature into adult worms. Adult worms can be several inches long. Their presence may cause local inflammation, damage to vessel walls, and obstruction of blood flow; this is likely linked to coughing, exercise intolerance, and eventually strain on the right heart.

The damage is both mechanical and inflammatory. Dead or dying worms can trigger clots and a severe inflammatory reaction, which is why treatment programs include staging and careful activity restriction. Killing adult worms too quickly or allowing heavy exercise during treatment may increase the risk of serious complications that may require emergency care.

When and where your dog is most likely to be exposed

Transmission depends on mosquitoes and temperature. Mosquitoes need warm conditions for the microfilariae they carry to develop to the infective L3 stage; development in the mosquito may slow or stop below roughly 57°F (about 14°C) and accelerates in warmer weather. In climates that stay warm year-round, transmission can happen any month. In cooler regions, there may be a distinct mosquito season when risk climbs.

Heartworm prevalence differs by region. Warmer, wetter parts of the country—coastal areas, wet lowlands, and regions with abundant standing water—tend to show higher rates, but pockets of risk exist almost everywhere because local mosquito habitat and stray or untreated dog populations matter. Urban areas can be risky too; small containers, clogged gutters, and storm drains create mosquito breeding sites even in cities.

Travel, relocation, or adopting a dog from an area where heartworm is common can change your pet’s risk profile overnight. I often emphasize that a dog moved from a low-risk to a high-risk area — or vice versa — needs prompt reassessment of prevention and testing plans.

Early warning signs: what to watch for and when to seek urgent care

Early infection may produce no obvious symptoms, which is why screening is so important. Owners should watch for gradual changes that may suggest developing disease: a persistent cough, less enthusiasm for walks, or a subtle weight loss. Those mild signs often precede more notable problems.

• Early/mild signs: occasional cough, reduced stamina on walks, slight weight loss.
• Progressive signs: a chronic cough, clear exercise intolerance, difficulty breathing after exertion.
• Emergency signs: collapse, severe shortness of breath, sudden weakness, or fainting.
• When to seek immediate care: any sudden worsening of breathing, collapse, or signs of shock; otherwise, schedule a vet visit if you notice a persistent cough or declining activity over days to weeks.

Testing is the only reliable way to know. Antigen tests detect a protein from adult female worms and usually become positive several months after infection; some clinics also use microfilaria tests or ultrasound and X-rays for staging when infection is suspected or confirmed. If you see red-flag signs, seek urgent veterinary attention; if you see milder signs, arrange a check and testing within a few days.

If you suspect heartworm, act now — what to do first

If you suspect exposure or see concerning signs, schedule a veterinary exam promptly and ask for heartworm testing. Do not try “home” remedies or treatments bought online without a veterinary prescription. Some products and doses intended for other species or used without guidance can be dangerous.

A typical diagnostic approach may include an antigen test and, if positive, additional tests to stage the disease—such as microfilaria testing, chest X-rays, or ultrasound—to determine the right treatment plan. Treatment is likely to involve an organized series of medications, sometimes antibiotics, and strict exercise restriction to reduce the chance of complications when adult worms die. Follow your veterinarian’s instructions closely; I’ve seen well-intentioned owners make a dog worse by skipping rest or altering dosages.

Also notify anyone who shares care of the dog — household members, kennels, or rescues — so other dogs can be tested and started on prevention if needed. Because mosquitoes are the vector, protecting all dogs in the home reduces the chance of transmission cycling through the local population.

Practical ways to manage your dog’s environment and reduce mosquito exposure

Reducing mosquito exposure lowers the chance of heartworm transmission. The single most effective household step is eliminating small, stagnant water sources: flowerpot saucers, discarded tires, clogged gutters, pet water bowls left outdoors, and low spots that hold rainwater. Check your yard weekly, especially in warm weather.

Physical barriers make a practical difference. Repair torn screens, use door sweeps, and consider screened runs or covered kennels for dogs that spend time outside. I recommend scheduling outdoor exercise to avoid dawn and dusk when many mosquito species are most active, and swapping evening backyard play for early afternoon sessions when mosquitoes may be less numerous.

Community action helps too. Coordinate with neighbors or your homeowners’ association if shared spaces, drains, or communal landscaping create breeding habitat. Local vector-control agencies can advise on broader measures if mosquito numbers are high in your area.

Vet-recommended preventives, repellents and protective gear

Monthly, veterinarian-prescribed heartworm preventives are the cornerstone of protection. These are usually given as oral tablets or topical formulations and are designed to eliminate the early larval stages before they reach the heart. I recommend keeping dogs on prevention year-round in many areas, and always consulting your veterinarian to match product, dose, and schedule to your dog’s weight and health.

There are veterinarian-approved repellents and treated collars or wearables that may reduce mosquito bites on dogs; discuss these with your vet, particularly if your dog will spend extended time outdoors. Use only products labeled for dogs — many human insect repellents are not safe for pets. Physical aids like mosquito nets for crates or covered outdoor runs can add protection for susceptible dogs or during high-mosquito periods.

Avoid untested or DIY remedies. Some household products or essential oils may irritate skin or be toxic if ingested. Your veterinarian can recommend effective, safe options and explain any species-specific cautions (for example, some insecticides are dangerous to cats and should be used carefully around multi-pet households).

References and trusted resources for further information

• American Heartworm Society: 2020 AHS Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs — https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines
• Centers for Disease Control and Prevention: Dirofilaria immitis (Heartworm) — https://www.cdc.gov/parasites/dirofilariasis/
• American Veterinary Medical Association: Heartworm Disease in Animals — https://www.avma.org/resources-tools/pet-owners/petcare/heartworm-disease
• Merck Veterinary Manual: Heartworm Disease (Dirofilariasis) in Dogs — https://www.merckvetmanual.com/circulatory-system/heartworm-disease/overview-of-heartworm-disease
• Companion Animal Parasite Council (CAPC): Heartworm Maps and Regional Risk Information — https://capcvet.org/maps/#/2019/heartworm