Heartworms in Dogs: Signs and Behavior Changes

Heartworm disease is one of those diagnoses that can feel scary, but knowledge truly helps. As a veterinary assistant in Frisco, Texas, I have seen how much better things go when families recognize early warning signs, get testing done on time, and stay consistent with prevention. Heartworms are spread by mosquitoes, and once inside your dog, they mature into long worms that live in the blood vessels of the lungs and sometimes in the right side of the heart. That is why the symptoms can look like ordinary “slowing down” at first, then gradually become more serious.

This guide walks you through common signs, behavior changes, what to do next, and how to prevent heartworms with practical, vet-aligned steps you can actually follow. For widely accepted guidelines, see the American Heartworm Society (AHS).

What heartworms do

Heartworms (Dirofilaria immitis) enter a dog through a mosquito bite. Over the next several months, the immature worms travel through the body and eventually mature into adults that live mainly in the pulmonary arteries and sometimes in the heart. The damage comes from:

• Inflammation in blood vessels of the lungs
• Reduced blood flow and increased strain on the heart
• Progressive lung injury, which affects stamina and breathing
• Risk of life-threatening complications during advanced disease or treatment if activity is not restricted

Many dogs do not show obvious signs early on. That is why yearly testing and consistent prevention matter so much, even when your dog seems perfectly healthy.

Early signs

In early heartworm disease, symptoms are often subtle. Many families chalk them up to aging, hot weather, or a dog simply feeling less energetic. Trust your instincts if something feels off.

Common early symptoms

• Occasional mild cough, especially after activity or excitement
• Reduced stamina, like tiring sooner on walks
• Less interest in play or needing longer breaks than usual
• Mild breathing changes, like faster breathing after normal activity
• Small appetite changes

If your dog used to be energetic and now seems to run out of steam sooner than normal, that is worth a conversation with your veterinarian.

Behavior changes

Behavior is often the first thing families notice. Heartworms can make dogs feel unwell long before they look obviously sick. Not every behavior change means heartworms, but it should trigger a health check, especially if prevention has been inconsistent.

Behavior shifts to watch for

• More sleeping and less interest in normal routines
• Hanging back on walks, moving behind you instead of leading
• Less social play with other dogs or family members
• Irritability or seeming uncomfortable when picked up or asked to move
• Quieter mood, less enthusiasm at the door, less tail wagging

These changes are easy to miss because they can be gradual. A helpful tip is to think in terms of trends: “Is my dog doing less over the last few weeks or months?”

Late-stage signs

As the worm burden increases and lung and heart strain worsen, symptoms become more obvious and more dangerous.

Advanced symptoms

• Persistent cough
• Clear exercise intolerance, like stopping early on short walks
• Labored breathing or breathing fast at rest
• Weight loss and reduced appetite
• Weakness or collapsing episodes
• Distended belly from fluid buildup in severe cases

Caval syndrome

A particularly severe form of heartworm disease is called caval syndrome. This can happen when a large number of worms obstruct blood flow through the heart. Signs can include sudden weakness, pale gums, difficulty breathing, collapse, and sometimes dark or reddish-brown urine (from blood pigment) and signs of severe anemia. This is an emergency and needs immediate veterinary care.

What to do next

If your dog has symptoms, or if prevention has been missed, do not wait for a cough to “clear up.” In general, outcomes are better and complications are fewer when heartworm disease is found earlier, but treatment can still be lengthy and requires close veterinary guidance.

Step-by-step

• Call your veterinarian and ask for a heartworm test.
• Limit strenuous activity until you have answers, especially if your dog is coughing or tiring easily.
• Bring your prevention history: product name, last dose date, and any missed months.
• Ask what additional tests are recommended. Depending on results and symptoms, your vet may suggest chest X-rays, bloodwork, or an ultrasound.

Important timing note: standard testing may not catch very early infections. It typically takes about 6 to 7 months after infection for the most common antigen tests to reliably turn positive, so your veterinarian may recommend a repeat test based on exposure risk and prevention gaps.

How testing works

Most clinics use a quick blood test that detects heartworm antigen, which mainly comes from adult female worms. Many veterinarians also run a microfilaria test to check for baby heartworms circulating in the blood.

The antigen test can be negative in a few situations, including:

• Early infection during the prepatent period (often about 6 to 7 months)
• Low female worm burden (few or no adult females)
• All-male infections (rare, but possible)

Even if your dog is on prevention, yearly testing is recommended because:

• No prevention is 100% foolproof if doses are missed, vomited, or given late.
• Product failure is rare but possible, and testing catches problems early.
• Early detection lowers risk and helps your veterinarian plan safer treatment.

Prevention

Prevention is far safer, easier, and usually less expensive than treatment. In Texas, mosquito exposure can be persistent, and groups like the American Heartworm Society generally recommend year-round prevention in endemic areas unless your veterinarian advises otherwise.

What prevention looks like

• Monthly chew (often also covers some intestinal parasites)
• Monthly topical medication
• Long-acting injection administered by your veterinarian

Your best choice depends on your dog’s age, lifestyle, health history, and what you can reliably remember. The “best” preventative is the one you will give on schedule.

Before you start or restart

If your dog is older, newly adopted, or has an unknown prevention history, ask your veterinarian whether testing is needed before starting prevention. This is a normal safety step and helps your clinic choose the right timing and plan.

Tips to avoid missed doses

• Set a recurring calendar reminder on your phone.
• Give prevention on the same day each month, like the 1st or 15th.
• Use auto-ship if you tend to run out.
• Ask your clinic about long-acting options if life is busy.

Treatment

Yes, many dogs can be successfully treated, but treatment is not something to DIY or rush. The standard approach is veterinarian-directed and commonly includes:

• Strict exercise restriction (this is essential for safety)
• Medications that may include an antibiotic such as doxycycline and a heartworm preventative (macrocyclic lactone), based on your veterinarian’s protocol
• Adulticide therapy (often melarsomine injections) when appropriate
• Follow-up testing and rechecks to confirm clearance and monitor recovery

Activity restriction is not just a suggestion. When worms die, they can cause inflammation and clots in the lungs, and overexertion increases the risk of serious complications.

If your dog is diagnosed, your veterinarian will outline a plan based on severity. Ask questions and take notes. You are not expected to memorize everything in a stressful moment.

FAQs

Can my dog get heartworms if they stay indoors?

Yes. Mosquitoes can enter homes and garages. Indoor dogs still need prevention in most regions.

Are there signs right after a mosquito bite?

No. Clinical signs typically appear later, after worms mature and damage builds. That is why prevention is so important.

Is a cough always heartworms?

No. Coughing can come from kennel cough, allergies, collapsing trachea, heart disease, pneumonia, and more. But any persistent cough deserves an exam and a discussion about heartworm testing.

What about cats?

This article is focused on dogs. Heartworm can affect cats too, but the disease can look different in cats. If you have a cat at home, ask your veterinarian what prevention is recommended for your area.

Bottom line

Heartworm disease often starts quietly: a little less stamina, an occasional cough, a dog who takes longer to recover after play. Those small clues matter. If you notice changes, schedule a visit and get your dog tested. Then commit to consistent prevention going forward.

Your dog depends on you to notice what they cannot explain. And the good news is that with modern testing, effective preventatives, and early action, you can protect their heart and lungs for the long run.