Heartworms in Dogs: Facts and Tips

Heartworm disease is one of those canine health threats that can feel scary because it is serious. It is also very preventable. As a veterinary assistant in North Texas, I have seen how quickly heartworms can turn an energetic dog into one that tires easily, coughs, or just does not seem like themselves. The good news is that a few simple steps can dramatically reduce your dog’s risk.

What heartworms are

Heartworms are long, spaghetti-like worms (called Dirofilaria immitis) that live primarily in the blood vessels of the lungs and, in more advanced cases, in the right side of the heart. They cause inflammation and damage to the lungs and cardiovascular system over time.

Heartworm disease is not spread from dog to dog directly. It requires mosquitoes to complete the life cycle.

How dogs get heartworms

Heartworm transmission starts when a mosquito bites an infected animal and picks up microscopic baby worms (microfilariae). Over the next about 10 to 14 days, those larvae develop inside the mosquito into an infective stage. In cooler weather, this can take longer. When that mosquito bites your dog, it can deposit infective larvae into your dog’s skin.

From there, larvae mature and migrate through the body. It typically takes about 6 to 7 months after infection before adult worms are present and can be detected on a standard antigen heartworm test. Timing can vary a bit depending on the test and the individual infection.

Why heartworms are a big deal

Heartworms can grow to about a foot long, and dogs can carry dozens (sometimes more) of adult worms. This can lead to:

• Chronic inflammation in the lungs
• Reduced blood flow and higher pressure in lung vessels
• Heart strain that worsens over time
• Lifelong lung and heart changes, even after treatment in some dogs

In severe cases, dogs can develop a life-threatening condition called caval syndrome, where worms obstruct blood flow to the heart. This is an emergency.

Signs to watch for

Many dogs show no obvious signs early on. That is one reason routine testing is so important. As disease progresses, you might notice:

• Soft, persistent cough
• Getting winded on walks or less interest in play
• Fatigue or weakness
• Decreased appetite or weight loss
• Labored breathing
• Swollen belly (fluid buildup in advanced disease)

If your dog suddenly collapses, has pale gums, or seems acutely weak, seek emergency veterinary care.

Testing: when it matters

Heartworm tests are quick, commonly performed blood tests. Veterinarians typically test:

• Yearly, even for dogs on preventives (to catch missed doses or product failures)
• Before starting prevention if a dog is not already on a consistent program
• After a lapse in prevention, based on your veterinarian’s timing guidance

Because heartworms take months to mature, a dog can be infected but test negative early. This “window” is normal, and it is why your veterinarian may recommend a specific retesting schedule if prevention has been inconsistent.

If you have an adult dog with an unknown prevention history, do not start prescription prevention “just in case” without talking to your veterinarian first. Testing and timing guidance help keep prevention both safe and effective.

Prevention: the key point

Heartworm prevention is far safer, easier, and more affordable than heartworm treatment. Preventives are prescription medications that kill immature larvae before they become adult worms. Most work by targeting larvae acquired in the previous weeks, which is why on-time dosing matters so much.

They come in several forms:

• Monthly chewables
• Monthly topical treatments
• Injectable prevention given by your veterinarian on a set schedule

Which one is best depends on your dog’s lifestyle, age, health history, and what you can reliably stick with. The “best” prevention is the one you will give on time, every time.

One more bonus: many prevention products also help control intestinal parasites, although coverage varies by product. Your veterinarian can help you choose based on what is common in your area and your dog’s needs.

Common myths

Myth: “My dog stays indoors, so we do not need prevention.”

Truth: Mosquitoes get inside homes all the time. One bite is enough to transmit infection.

Myth: “Heartworms only happen in humid states.”

Truth: Heartworm disease has been reported in all 50 states. Anywhere mosquitoes live, risk exists.

Myth: “I only need prevention in the summer.”

Truth: Mosquito seasons are longer than many people realize, and weather patterns are unpredictable. In places like North Texas, warm stretches can bring mosquitoes out earlier and keep them around later. Many veterinarians recommend year-round prevention for consistent protection and easier routines.

If a dog tests positive

Hearing “heartworm positive” is overwhelming, but there are established treatment protocols. Your veterinarian will tailor a plan based on your dog’s test results, symptoms, and overall health. Treatment often includes:

• Confirmatory testing and chest imaging when indicated
• Medications to reduce inflammation and complications
• A series of injections to kill adult worms (often a melarsomine-based protocol)
• Strict exercise restriction for weeks to months to reduce the risk of clots and lung complications

That exercise restriction is not optional. When worms die, they break down and can obstruct blood flow in the lungs. Your veterinarian will tell you exactly what “restricted” means for your dog, but it usually looks like short leash potty breaks only and no running, jumping, or play.

You may also hear about “slow-kill” approaches (using preventives over time instead of adulticide injections). These are generally not the preferred option because they allow ongoing damage and can contribute to resistance concerns. If it comes up, ask your veterinarian what approach fits your dog and why.

What you can do today

If you want to take one simple, protective step for your dog, start here:

• Schedule an annual heartworm test and keep it on your calendar.
• Choose a prevention method you can follow consistently, then set phone reminders.
• Ask about mosquito risk in your neighborhood, especially near standing water.
• Use your veterinarian’s guidance for any missed doses, do not guess.

Heartworm disease is a mosquito-borne problem, but it is also a routine-prevention win. Small habits, done consistently, protect your dog’s heart and lungs for the long run.

Quick FAQ

Can puppies get heartworms?

Yes. Puppies can be infected, and your veterinarian will advise when to start prevention and when testing becomes reliable based on age and timing.

Can people catch heartworms from dogs?

Heartworms are not directly contagious. In rare cases, mosquitoes can transmit larvae to humans, but the parasite typically does not mature the same way. Your dog’s prevention is still important, and it helps reduce the number of infected pets in the community. Wildlife (including coyotes and foxes in many regions) can also serve as reservoirs, which is another reason consistent prevention matters.

Is heartworm prevention safe?

For most dogs, preventives are very safe when prescribed appropriately. Always tell your veterinarian about other medications, your dog’s health history, and any past reactions.

Where can I learn more?

For detailed, veterinarian-reviewed guidance, visit the American Heartworm Society at heartwormsociety.org.