Quick answer: Heartworm disease is a serious condition where long, thin worms live in a dog's heart and lungs. It spreads through mosquito bites, not directly between dogs, and indoor dogs are at risk too. Early stages often have no signs, while later signs include coughing, tiring easily, weight loss and trouble breathing. Heartworm is almost completely preventable with a vet prescribed monthly chew, topical or long acting injection. Most experts recommend year round prevention and a yearly blood test. Reduce mosquitoes by emptying standing water, fixing screens and avoiding dawn and dusk walks.

What is heartworm disease?

A serious disease caused by a parasite. Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. Adult heartworms live in the heart, lungs and nearby blood vessels of infected animals. They can grow up to about a foot long, and a dog can have dozens or even hundreds of worms at once. Over time, the worms cause damage to the heart, lungs and arteries, which can lead to heart failure, lung disease and damage to other organs.

It develops slowly. After a dog is bitten by an infected mosquito, it takes about six to seven months for the tiny larvae to grow into adult worms. During that time, a dog may show no signs at all. By the time symptoms appear, the disease is often already advanced, which is why prevention and regular testing are so important.

Dogs are natural hosts. Unlike some parasites, heartworms can live, grow and reproduce inside dogs. That means the worms can survive for five to seven years in a dog, causing more and more damage over time.

It is found in many areas. Heartworm disease has been reported in all 50 U.S. states and in many other countries. It is most common in warm, humid areas, but mosquitoes that spread heartworm can be found almost anywhere.

How do dogs get heartworms?

Through mosquito bites. Heartworms are spread by mosquitoes. When a mosquito bites an infected animal, such as a dog, coyote, fox or wolf, it picks up baby worms called microfilariae from the blood. Inside the mosquito, these baby worms develop into infective larvae over about 10 to 14 days. When the mosquito bites another dog, it deposits the larvae onto the skin, where they enter the body through the bite wound.

Not directly from other dogs. Dogs cannot catch heartworms directly from other dogs. Mosquitoes are needed to spread the disease. That means even a dog who never meets other dogs can get heartworm if they are bitten by an infected mosquito.

Indoor dogs are at risk too. Mosquitoes easily get into homes through open doors, windows and screens. Indoor dogs and small dogs who only go out for quick potty breaks can still be bitten.

Wildlife plays a role. Coyotes, foxes and other wild animals can carry heartworms and act as a source of infection in many areas.

Which dogs are at risk for heartworm?

All dogs. Every dog who is not on a heartworm preventive is at risk, regardless of breed, age, size or lifestyle.

Dogs in warm, humid areas. The risk is highest in regions with long mosquito seasons, such as the southeastern United States, along rivers and coasts and in areas with lots of standing water.

Dogs who travel. Dogs who travel to new areas, move from high risk regions or are adopted from areas where heartworm is common may be at greater risk.

Dogs who miss doses. Missing even one dose of preventive can leave a gap in protection.

What are the signs of heartworm disease?

Early stages. Many dogs show no signs in the early stages. This is why regular testing is so important.

Mild to moderate signs. As the disease progresses, dogs may develop a soft, dry cough, tire easily during exercise, lose interest in play, lose weight or have a decreased appetite.

Severe signs. In advanced disease, dogs may have trouble breathing, a swollen belly from fluid buildup, fainting or heart failure.

Caval syndrome. In very severe cases, a large number of worms can block blood flow in the heart. Signs include sudden labored breathing, pale gums and dark, bloody or coffee colored urine. This is a life threatening emergency that needs immediate surgery.

How is heartworm diagnosed?

A simple blood test. Your vet can test for heartworm with a quick blood test that detects proteins from adult female worms. Some vets also look for baby worms in the blood under a microscope.

Test yearly. The American Heartworm Society recommends testing dogs every year, even if they are on prevention year round. Testing helps catch any infections early, such as after a missed dose.

Test before starting prevention. Dogs older than seven months should be tested before starting prevention, because some preventives can cause reactions in dogs who already have adult worms.

Why testing timing matters. Because worms take about six months to mature, a dog who was recently infected may test negative at first. Your vet may recommend a follow up test.

How do I prevent heartworm in my dog?

Use a vet prescribed preventive. Heartworm preventives are available as monthly chewable tablets, monthly topical liquids and long acting injections that last six or twelve months. All require a prescription from your vet.

Give it on time. Set a reminder on your phone or calendar. Giving doses late can leave gaps in protection.

Ask about combined protection. Many heartworm preventives also protect against intestinal parasites and, in some cases, fleas and ticks. Our guide on flea and tick prevention for dogs explains your options.

Choose what works for your family. Your vet can help you choose the option that fits your dog's health, lifestyle and your routine.

Why should my dog have heartworm prevention all year?

Mosquito seasons are unpredictable. Warm spells in winter, indoor mosquitoes and changing climates mean mosquitoes can be active outside the usual season.

Preventives work backward. Most monthly preventives kill larvae that have entered the body in the previous month. Giving prevention year round makes sure no larvae slip through.

It builds good habits. Giving prevention every month all year makes it easier to remember.

It protects against other parasites. Many preventives also protect against intestinal worms year round.

How is heartworm disease treated?

Treatment is possible but difficult. Heartworm treatment usually involves a series of injections to kill adult worms, along with other medications to reduce inflammation and kill young worms.

Strict rest is essential. During and after treatment, dogs must be kept very quiet for several weeks to months. As worms die, they can block blood vessels in the lungs, and exercise increases this risk.

Treatment can be expensive. Treatment can cost far more than years of prevention. Our guide on pet insurance for dogs explained explains how insurance can help with unexpected costs.

Prevention is easier. Preventing heartworm is much safer, easier and less expensive than treating it.

How do I reduce mosquitoes around my home?

Remove standing water. Mosquitoes breed in standing water. Empty buckets, flowerpot saucers, kiddie pools, birdbaths and clogged gutters weekly.

Fix screens. Repair window and door screens to keep mosquitoes out.

Avoid peak times. Mosquitoes are most active at dawn and dusk. Plan walks for other times when possible.

Use pet safe repellents. Ask your vet about pet safe repellents. Never use human insect repellent on dogs, as products with DEET can be toxic.

Can mosquitoes cause other problems for dogs?

Itchy bites. Mosquito bites can cause itchy bumps, especially on thinly furred areas like ears, noses and bellies.

Allergic reactions. Some dogs have allergic reactions to bites. Call your vet if you see swelling, hives or trouble breathing.

West Nile virus. Dogs can be infected with West Nile virus, but serious illness is rare.

Secondary infections. Scratching bites can cause skin infections.

Do I need to think about heartworm when traveling?

Yes. If you travel with your dog to areas with high heartworm risk, make sure your dog is on prevention before you go.

Plan for camping and lakes. Camping trips and lake vacations often mean more mosquito exposure. Our guide on camping with your dog has more tips.

Pack prevention. Bring your dog's preventive if a dose is due while you are away.

Check with your vet. Ask your vet about risks in the area you are visiting.

What should I know about heartworm in adopted dogs?

Ask about testing. Many rescue dogs come from areas with high heartworm rates. Ask the shelter or rescue if your dog was tested and treated.

Follow up with your vet. Schedule a vet visit soon after adoption to review testing and start prevention. Our guide on bringing home a rescue dog has more tips.

Know the history. If your dog was treated for heartworm, ask for records.

Be patient. Dogs recovering from heartworm treatment need quiet, rest and time.

Can cats and other pets get heartworm?

Yes, cats can get heartworm. Cats are not natural hosts, but they can still be infected. Even a few worms can cause serious problems.

There is no approved treatment for cats. Prevention is the only protection.

Ferrets can get heartworm too. Ask your vet about prevention for all pets.

People are rarely affected. Heartworm very rarely causes problems in humans.

What is the heartworm life cycle, in plain English?

Step one: a mosquito bites an infected animal. It all begins when a mosquito bites a dog, coyote, fox or other animal that already has adult heartworms. Female worms in that animal release millions of microscopic babies, called microfilariae, into the bloodstream. When the mosquito drinks blood, it swallows some of those babies too.

Step two: the babies grow inside the mosquito. Over the next 10 to 14 days, depending on the temperature, the babies develop inside the mosquito into infective larvae. Warm weather speeds this up, which is one reason heartworm risk climbs in summer. Below about 57 degrees Fahrenheit, the larvae generally cannot develop.

Step three: the mosquito bites your dog. When the infected mosquito bites your dog, it leaves larvae on the skin, and they wriggle into the body through the bite wound. Over the next few months, the larvae travel through the tissues, grow and eventually reach the heart and lungs.

Step four: adult worms move in. About six to seven months after the bite, the worms are fully grown and can start making babies of their own. This is why a single missed dose of prevention can matter so much, and why vets test once a year to catch problems early.

How much does heartworm prevention cost compared to treatment?

Prevention is affordable. Depending on your dog's size, the product and where you live, heartworm prevention often costs roughly the same as a fancy coffee or two each month. Many products also protect against other parasites, which adds value. Some vet clinics and manufacturers offer rebates, loyalty programs or discounts when you buy several months at once.

Treatment is costly. Treating heartworm disease often involves blood tests, x rays, multiple injections, other medications and follow up visits. The total can run into the hundreds or even thousands of dollars, depending on the severity of the disease and where you live. Severe cases needing surgery can cost much more.

Hidden costs add up. Treatment also means months of strict rest, lost time, possible complications and stress for both you and your dog. Some dogs are left with lasting heart and lung damage even after successful treatment.

Budget for it. Add heartworm prevention to your yearly pet budget so it never feels like a surprise. Our guide on how much does a dog cost per year can help you plan for all the regular costs of dog ownership.

How do I remember to give heartworm prevention?

Pick a memorable day. Choose an easy to remember date, like the first of the month, payday or your dog's birthday date each month. Tie it to something you already do, like paying a bill or changing your calendar page.

Use reminders. Set a recurring phone alarm or calendar event. Many vet clinics and preventive brands offer free email or text reminders. Some families put a sticker on the calendar or the fridge as a visual cue.

Track doses. Keep a simple log of when each dose was given. If you share dog care with family members, use a shared note or chart so everyone knows if the dose has been given. Our guide on sharing dog care as a family has tips for dividing up pet care tasks.

Consider long acting options. If monthly doses are hard to remember, ask your vet about long acting injectable preventives that last six or twelve months. These can be a great fit for busy households.

How do I give heartworm prevention correctly?

Follow the label and your vet's directions. Give the correct product for your dog's weight range. Weigh your dog regularly, especially puppies who are growing quickly, as an underdose may not protect them fully.

Make sure it goes down. With chewables, watch your dog eat the whole tablet. Some dogs hide pills in their cheek and spit them out later, or another pet may steal them. Giving the chew with a meal can help.

Apply topicals correctly. For topical products, part the fur and apply directly to the skin as directed, usually between the shoulder blades. Avoid bathing or swimming for the time stated on the label.

Report problems. If your dog vomits soon after a dose, seems unwell or has a skin reaction, call your vet. They may suggest a different product or redosing.

How can I make my yard less friendly to mosquitoes?

Hunt down standing water weekly. Mosquitoes can breed in just a bottle cap of water. Walk your yard once a week and empty or flip anything that collects water, including buckets, toys, tarps, old tires, plant saucers, wheelbarrows and kiddie pools. Change water in birdbaths and outdoor dog bowls every day or two.

Keep gutters clean. Clogged gutters hold water and leaves, which mosquitoes love. Clean them regularly, especially after storms.

Trim plants and grass. Mosquitoes rest in tall grass, shrubs and shady, damp areas. Keeping grass short and shrubs trimmed reduces resting places.

Use pet safe options. If you use mosquito treatments in your yard, choose products labeled as safe for pets and follow directions carefully. Keep dogs indoors while treatments dry. Our guide on spring lawn and garden safety for dogs explains how to choose pet safe yard products.

When should puppies start heartworm prevention?

Early, usually by eight weeks. The American Heartworm Society recommends starting puppies on heartworm prevention as early as the product label allows, usually around eight weeks of age. Puppies do not need a heartworm test before starting if they are under seven months old, because any infection would be too young to show up on a test.

Test later. Puppies started on prevention are usually tested around six months after their first dose, and then yearly. This checks that no infection slipped through before prevention began.

Watch their weight. Puppies grow quickly and may move into a new weight range within weeks. Weigh your puppy at each vet visit and switch to the right dose as they grow.

Build the habit early. Making prevention part of your puppy's routine from day one sets you up for a lifetime of protection. Our guide on crate training a new puppy has more tips for setting up healthy puppy routines.

Is mosquito season getting longer?

In many places, yes. Warmer temperatures, milder winters and more rainfall in some regions can mean longer mosquito seasons. In some areas, mosquitoes are now active earlier in spring and later into fall than they used to be. This is another reason many vets recommend year round prevention instead of seasonal doses.

Indoor mosquitoes survive winter. Some mosquitoes survive the cold by sheltering in garages, basements and homes, where they can still bite pets on warm winter days.

Urban heat islands. Cities can stay warmer than surrounding areas, giving mosquitoes more time to breed and spread heartworm.

Follow local advice. Your vet knows the risks in your area. Ask them about local heartworm rates and the best plan for your dog. Heartworm maps from organizations like the American Heartworm Society can also show trends in your region.

What are common heartworm myths?

Myth: indoor dogs do not need prevention. Mosquitoes get indoors easily.

Myth: heartworm only happens in the South. Heartworm has been found in all 50 states.

Myth: prevention is only needed in summer. Year round prevention is recommended.

Myth: dogs can catch heartworm from other dogs. Mosquitoes are needed to spread it.

Signs your dog is ready for mosquito season

The porch patrol. Your dog snaps at every bug that flies by, with a success rate of zero.

The zoomie swatter. Your dog runs in circles trying to escape one tiny mosquito.

The itchy ear flop. Your dog shakes their head every five minutes because of one bite.

The treat reminder. Your dog knows the monthly chewable is coming and sits before you even open the box.

Key facts about heartworm and mosquito season

Heartworm is spread by mosquitoes. Dogs cannot catch it from each other.

It is almost completely preventable. Use a vet prescribed preventive.

Test every year. Annual testing catches infections early.

Year round prevention is recommended. Mosquitoes can be active outside summer.

• One mosquito bite can spread heartworm.
• Indoor dogs need prevention too.
• Year round prevention is recommended.
• Test every year, even on prevention.
• Prevention costs less than treatment.

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How do dogs get heartworm?

Through the bite of an infected mosquito. Dogs cannot catch it directly from other dogs.

What are the signs of heartworm in dogs?

Early stages often have no signs. Later signs include coughing, tiring easily, weight loss and trouble breathing.

Do indoor dogs need heartworm prevention?

Yes. Mosquitoes get indoors easily, so indoor dogs are at risk too.

Does my dog need heartworm prevention in winter?

Most experts recommend year round prevention because mosquito activity is unpredictable.

How often should my dog be tested for heartworm?

Every year, even if your dog is on prevention.

Is heartworm treatment safe?

Treatment is possible but involves injections and strict rest. Prevention is safer and cheaper.

Can I use human bug spray on my dog?

No. Products with DEET can be toxic. Ask your vet about pet safe repellents.

What if I miss a dose of heartworm prevention?

Give it as soon as you remember and call your vet. They may recommend a test later.

Can cats get heartworm?

Yes. There is no approved treatment for cats, so prevention is important.

How long does it take for heartworms to grow?

About six to seven months after a mosquito bite.