How to Tell if Your Dog Has Heartworms

Heartworm disease is one of those conditions I wish every dog parent understood, because it is common, serious, and often subtle at first. As a veterinary assistant in Frisco, Texas, I have seen how heartbreaking it can be when a sweet dog finally shows symptoms and we realize the infection has been building for months.

The good news is this: heartworm disease is highly preventable, and when it is caught early, outcomes are much better. Let’s walk through what heartworms are, how dogs get them, what signs to watch for, and what to do next.

What heartworms are

Heartworms are long, spaghetti-like worms called Dirofilaria immitis that live in a dog’s heart, lungs, and nearby blood vessels. They can cause inflammation, damage the lungs, strain the heart, and in severe cases lead to heart failure or sudden collapse.

This disease is found across the United States, including Texas. Warm weather and mosquitoes create the perfect conditions for spread.

How dogs get heartworms

Heartworms are spread only through mosquito bites. Here’s the basic cycle:

• A mosquito bites an infected animal and picks up microscopic baby heartworms (microfilariae).
• Those baby worms develop inside the mosquito.
• When the mosquito bites another dog, it deposits immature larvae into the skin.
• Over the next several months, the larvae migrate and mature into adult heartworms that settle into the heart and pulmonary arteries.

Important: heartworms are not spread directly from dog to dog through casual contact. Mosquitoes are required for transmission.

Why symptoms can be hard to spot

One of the tricky things about heartworms is that many dogs look normal early on. After a dog is infected, it often takes about 5 to 7 months before antigen tests reliably detect infection. That means a dog can be infected even if an early test is negative, depending on timing.

During this early phase, some inflammation may begin before tests turn positive, and the bigger, more clinically significant damage tends to build as worms mature and increase in number.

That is why veterinarians stress routine annual testing and consistent prevention, even for dogs that stay mostly indoors.

Signs your dog might have heartworms

Symptoms depend on worm burden (how many worms), how long the dog has been infected, and how active the dog is. A couch potato may hide symptoms longer than an athletic dog.

Early or mild signs

• Soft, persistent cough
• Getting tired sooner on walks
• Less interest in exercise or play
• Mild weight loss or decreased appetite

Moderate signs

• More frequent coughing, especially after activity
• Noticeable exercise intolerance
• Labored breathing or faster breathing at rest
• Weight loss and reduced muscle condition

Severe signs (urgent)

• Swollen belly (fluid build-up)
• Weakness, fainting, or collapse
• Pale gums
• Difficulty breathing
• Coughing up blood (less common, but serious)

Call an emergency vet immediately if your dog collapses, has severe breathing trouble, or suddenly becomes extremely weak. A rare but deadly complication called caval syndrome can occur when a large number of worms obstruct blood flow through the heart. This requires immediate treatment.

What you can do at home (and what you should not do)

You cannot diagnose heartworms at home, but you can spot patterns that help your veterinarian.

Helpful at-home observations

• Track the cough: When does it happen, how often, and does it worsen after activity?
• Monitor breathing: While your dog is asleep, count breaths for 30 seconds and double it. Many healthy dogs are roughly in the 10 to 30 breaths-per-minute range at rest, but what matters most is what is normal for your dog. If resting breathing becomes consistently faster, more effortful, or different than usual, call your vet.
• Watch stamina: If your dog used to handle a 20-minute walk and now struggles at 10, note it.
• Take videos: A short video of coughing or labored breathing can be extremely helpful in an exam room.

Avoid these common mistakes

• Do not start or restart heartworm prevention without veterinary guidance if your dog is overdue. In rare cases, starting prevention in a dog with circulating microfilariae can cause reactions. Your vet may recommend testing first and will advise the safest next step.
• Do not force exercise if you suspect heartworm disease. Activity increases stress on the heart and lungs.
• Do not rely on symptoms alone. Many infected dogs have few visible signs.

How veterinarians test for heartworms

Most clinics use a quick blood test that detects heartworm proteins (antigens) from adult female worms. Many veterinarians also run a microfilariae test to look for circulating baby worms.

If a dog tests positive, your veterinarian may recommend additional diagnostics to understand disease severity and plan treatment, such as:

• Chest X-rays to evaluate the heart and lungs
• Bloodwork to check organ function and overall health
• Ultrasound in select cases

My dog tested positive: now what?

Take a breath. A positive result is scary, but it also means you finally have a clear direction. Treatment is very doable, and your veterinary team will guide you step-by-step.

Typical treatment overview

Treatment plans vary based on your dog’s health, the clinic’s protocol, and regional guidance. Many veterinarians follow American Heartworm Society recommendations, which often include:

• Exercise restriction: This is not optional. It is one of the most important safety steps.
• Medications before adulticide treatment: Your vet may prescribe antibiotics (often doxycycline) and a preventive to reduce complications.
• Adulticide injections: A medication that kills adult heartworms. A common approach is a 3-dose protocol, with one injection followed by two injections about a month later (often given 24 hours apart), though your vet will choose the best plan for your dog.
• Supportive care: Steroids or other meds may be used depending on inflammation and risk.

Why exercise restriction matters so much

When worms die, they break apart and are cleared by the body. Too much activity can increase the risk of clots and lung complications. In many cases, your dog will need weeks of strict rest, including leash-only potty breaks.

Home care tips that make rest more realistic

• Use food puzzles and snuffle mats for quiet mental enrichment.
• Offer short, calm training sessions like “touch,” “chin rest,” and “settle.”
• Set up a comfortable crate or recovery area away from high-energy triggers like windows.
• Ask your vet about safe calming support if your dog is anxious during rest.

Can heartworms be prevented?

Yes, and prevention is far easier, safer, and more affordable than treatment. Heartworm preventives are prescription medications that kill immature stages before they mature into adults. They come in chewables, topicals, and injections.

Many preventives also help protect against other parasites, like certain intestinal worms. Some products also include flea and tick coverage, but it varies by medication, so ask your veterinarian what your dog is actually covered for.

Prevention best practices

• Give prevention on schedule year-round, even in cooler months. Texas weather can be mosquito-friendly for long stretches.
• Test annually or as recommended by your veterinarian, even if your dog is on prevention.
• Use mosquito reduction at home by removing standing water and keeping screens in good repair.

If you are unsure which product is best for your dog’s age, lifestyle, and health history, your veterinarian can tailor a plan that also considers other parasites common in your area.

Note: This article focuses on dogs. Cats can get heartworms too, but the disease can look very different in cats, so talk with your vet about the right prevention plan for your household.

When to call the vet

Reach out to your veterinarian if:

• Your dog is due for an annual heartworm test
• You missed prevention doses or started late this year
• Your dog has a cough that lasts more than a few days
• You notice new fatigue, weight loss, or breathing changes

Action step: If you are behind on prevention, call your vet before giving the next dose. A quick test and a safe plan can protect your dog and give you peace of mind.

Your dog depends on you for the “invisible” healthcare stuff, and heartworm prevention is one of the kindest, most practical gifts you can give. If you want help building a simple monthly routine, your veterinary team can help you make it easy to stick with.