Does My Dog Have Heartworms?

If you are asking, “Does my dog have heartworms?” you are already doing the right thing: paying attention. Heartworm disease can be quiet for months to years, and how quickly signs appear can depend on things like worm burden and your dog’s activity level.

As a veterinary assistant here in Frisco, Texas, I have seen how stressful this question feels for families. The good news is that heartworm disease is detectable and often treatable when caught in time. Treatment can be complex and does carry risks, and outcomes depend on the stage of disease, but many dogs do very well with a veterinarian-guided plan.

Quick note: This article is for education and support, not a diagnosis. If you are worried, your veterinarian is the best next step.

What heartworms are

Heartworm disease is caused by a parasite called Dirofilaria immitis. It is spread by mosquitoes. When an infected mosquito bites your dog, it can deposit immature worms (larvae) that slowly develop and migrate through the body.

Over several months, these worms can mature and live in the blood vessels of the lungs and sometimes the right side of the heart. Adult heartworms can grow long, live for years, and cause progressive damage to the heart and lungs.

Why prevention matters

Heartworm prevention is far safer, easier, and less expensive than treatment. Treatment often requires strict exercise restriction and multiple veterinary visits, because when adult worms die they can cause inflammation and clots in the lung arteries.

Signs at home

One tricky part is that many dogs look normal early on. Symptoms often appear gradually as the worm burden increases and lung vessels become inflamed.

Early signs

• Soft, persistent cough (especially after activity)
• Decreased stamina, tiring faster on walks
• Reluctance to exercise or play like usual
• Mild weight loss or decreased appetite

Advanced signs

• Labored breathing or rapid breathing at rest
• Noticeable fatigue and weakness
• Swollen belly (fluid buildup can happen in severe cases)
• Fainting or collapse, especially during exertion

Important: These signs can also be caused by other conditions like allergies, bronchitis, pneumonia, airway disease, or heart disease. Symptoms alone cannot confirm heartworms. Testing matters.

When it is an emergency

There is a severe, life-threatening form of heartworm disease called caval syndrome. It can occur when a heavy worm burden disrupts blood flow through the right heart and major veins, and it is often associated with red blood cell damage (hemolysis).

Go to an emergency vet now if you see

• Sudden collapse or extreme weakness
• Pale gums
• Difficulty breathing
• Dark, cola-colored urine
• Distended belly with severe lethargy

This situation needs immediate veterinary care.

How dogs get heartworms

Because heartworms are spread by mosquitoes, a dog does not need to visit a dog park or go hiking to be at risk. Mosquitoes get inside homes, garages, and apartments. In warmer regions and mosquito-heavy areas, risk can be year-round.

Risk factors

• Missed doses of heartworm prevention
• Using prevention inconsistently (seasonal only, or skipping “low mosquito” months)
• Living in areas with long mosquito seasons
• Travel to heartworm-endemic regions
• Being a rescue or unknown prevention history

Can prevention fail?

Most veterinary-prescribed preventives are highly effective when used correctly. But yes, infection can still happen in certain situations:

• Missed or late doses: timing matters. Most monthly preventives generally work by eliminating larvae acquired in the previous weeks.
• Spit-out doses or vomiting after dosing: the full dose may not have been absorbed.
• Incorrect weight range: using a product meant for a smaller dog can underdose.
• Product gaps during moves, travel, or budget stress.
• Rare lack of effectiveness: uncommon, but your veterinarian may consider it depending on location and history.

This is one big reason veterinarians recommend annual heartworm testing even for dogs on prevention.

Heartworm testing

If you suspect heartworms, schedule a veterinary visit. Heartworms are not something you can diagnose at home.

Common tests

• Antigen test: a blood test that looks for proteins from adult female heartworms. This is the most common screening test.
• Microfilaria test: checks for baby heartworms in the blood. Not every infected dog has microfilariae in circulation, so this is often paired with an antigen test.
• Chest X-rays: help evaluate changes in the lungs and heart and support staging severity.
• Echocardiogram (ultrasound): sometimes used for advanced cases or to visualize worms and assess heart function.
• Bloodwork: helps evaluate overall health before treatment and checks organ function.

Timing note: After a mosquito bite, it usually takes about 6 to 7 months before a standard antigen test can reliably detect adult heartworms. If a dog was recently exposed, your veterinarian may recommend retesting later, even if the first test is negative.

About accuracy: No test is perfect. Early infections, a low number of female worms, or other factors can lead to false negatives. This is why your veterinarian may recommend confirmatory testing or repeat testing based on risk and symptoms.

If the test is positive

A positive test is scary to hear, but it is not the end of the story. Your veterinarian will recommend confirmatory testing and then a plan based on your dog’s symptoms, activity level, and overall health.

What to expect

• Staging: assessing how advanced the disease is (often with X-rays and bloodwork).
• Exercise restriction: one of the most important ways to reduce complications.
• Medication plan: many protocols include medications to reduce inflammation and address associated parasites (often including antibiotics like doxycycline and a heartworm preventive), followed by adulticide therapy when appropriate.
• Follow-up testing: to confirm the infection is cleared.

There are multiple accepted approaches depending on the case and your veterinarian’s guidance. Avoid trying to “treat naturally” without veterinary supervision. Heartworm disease affects the heart, lungs, and blood vessels, and complications can be life-threatening.

If the test is negative

A negative test can be very reassuring, but it is not always the final answer.

• If your dog has symptoms but tests negative, your veterinarian may look for other causes of coughing or fatigue and may recommend X-rays or other diagnostics.
• If exposure may have been recent, your veterinarian may recommend a retest in several months because early infections may not show up yet.

What to do today

If heartworms are on your mind, here is a calm, practical next step plan:

• Schedule a heartworm test with your veterinarian, especially if prevention has been missed or you adopted recently.
• Bring your prevention history: product name, dose dates, and any gaps. Even a photo of the box helps.
• Write down symptoms: coughing frequency, exercise intolerance, appetite changes, fainting episodes, and when they started.
• Keep activity moderate until you know what is going on, especially if your dog is coughing or easily tired.
• Do not start or restart prevention on your own if your dog is overdue or their status is unknown. Call your vet today for the safest plan, which usually includes testing first.

Prevention options

Heartworm prevention is typically given monthly (chew or topical) or as a long-lasting injection administered by a veterinarian. The “best” option is the one you will use correctly and consistently.

Questions to ask

• What prevention do you recommend for my dog’s age, weight, and lifestyle?
• Does it also protect against intestinal parasites like roundworms and hookworms?
• If my dog has had gaps, what is the safest way to restart?
• Should we test once a year or more often based on local risk?

In North Texas, mosquitoes can be persistent, so many clinics recommend year-round prevention. Protocols can vary by clinic and veterinarian, so if you are unsure, call and ask. We would rather answer early questions than deal with a late surprise.

Quick FAQs

Can humans catch heartworms from dogs?

Heartworms are not transmitted directly from dog to human. Mosquitoes are the middle step. In rare cases, people can develop lung lesions associated with heartworm exposure, but humans are considered dead-end hosts, and it is not the same disease process as in dogs.

Can cats get heartworms too?

Yes. Cats can get heartworms, and even a small number of worms can cause serious disease. Prevention exists for cats as well, and diagnosis is often more challenging than in dogs. Treatment also differs, so cats should be managed closely by a veterinarian.

My dog is coughing. Is it heartworms?

It could be, but coughing has many causes. The fastest, most responsible answer is: get the test and let your veterinarian listen to the lungs and heart.

A gentle final note

If you are worried you missed prevention, or your dog is coughing, or you adopted a pup with an unknown history, please do not beat yourself up. Heartworm disease is common in many regions, and it is exactly why veterinary teams push prevention and testing.

The most powerful thing you can do is take action today: schedule a test, follow your veterinarian’s plan, and stay consistent moving forward.