Heartworm Symptoms in Dogs: Care and Next Steps

Heartworm disease is one of those conditions I wish every dog parent never had to think about. But as a veterinary assistant here in Texas, I can tell you it is very real, very preventable, and much safer with fewer complications when it is caught early. Heartworms are transmitted by mosquitoes, and adult worms primarily live in the blood vessels of the lungs (the pulmonary arteries). In heavier infections, they can extend into the right side of the heart, where they can cause lasting damage.

If you are noticing changes in your dog’s energy, breathing, or stamina, this overview will help you recognize possible symptoms, understand what they may mean, and know what to do next.

What heartworm does

Heartworm disease is caused by Dirofilaria immitis, a parasite spread through mosquito bites. Over months, immature worms mature into adults and begin to affect the heart, lungs, and blood vessels. The biggest problem is not just the worms themselves, but the inflammation and strain they cause as they grow and disrupt blood flow.

Even when dogs seem fine, heartworm can be quietly progressing. That is why consistent prevention and annual testing matter so much.

Quick note: This article is focused on dogs. Cats can get heartworm too, but it looks different and needs a different approach.

Common symptoms

Symptoms vary depending on how many worms are present, how long your dog has been infected, and your dog’s activity level. Some dogs show subtle signs at first, especially if they are calm couch companions. More athletic dogs may show changes sooner.

Early or mild signs

• Decreased stamina on walks or during play
• Reluctance to exercise or seeming tired sooner than usual
• Soft, occasional cough, especially after activity
• Mild weight loss or reduced appetite

Moderate to advanced signs

• Persistent cough or coughing fits
• Labored breathing or rapid breathing at rest
• Exercise intolerance that is clearly worse than before
• Fainting or collapsing, often with excitement or exertion
• Swollen belly from fluid buildup (a sign of heart strain)
• Weakness, lethargy, and noticeable weight loss

Important note: these symptoms can also occur with other conditions, including kennel cough, pneumonia, asthma-like airway disease, heart disease, and even certain cancers. Symptoms alone cannot confirm heartworm. Confirmation requires veterinary testing, usually an antigen test with or without additional blood tests.

Emergency signs

Some heartworm-related complications can become life-threatening quickly. If you notice any of the signs below, contact an emergency vet right away.

• Collapse, fainting, or severe weakness
• Sudden trouble breathing or blue-tinged gums
• Coughing up blood (or pink, foamy fluid)
• Very swollen abdomen with lethargy
• Dark urine, pale gums, sudden collapse (possible caval syndrome, a critical complication)

If you are unsure, it is always safer to call. Tell the clinic what you are seeing and ask whether you should come in immediately.

How it is diagnosed

Most veterinarians start with a simple in-clinic blood test that detects proteins from adult female heartworms. Depending on the result and your dog’s history, your vet may recommend additional tests to confirm infection and understand severity so treatment is as safe as possible.

Tests your vet may recommend

• Heartworm antigen test (common screening test)
• Microfilariae test to check for circulating baby heartworms
• Chest X-rays to evaluate heart and lung changes
• Bloodwork to assess organ function and overall health
• Echocardiogram (heart ultrasound) in more complex cases

Testing has limits: in the early months after infection (often around the first 6 to 7 months), tests can be negative because the worms are not mature yet. Antigen tests can also miss some low-burden infections. If your vet still suspects heartworm, they may recommend additional testing or retesting later.

Even dogs on prevention can test positive if doses were missed, given late, spit out, or if a product was not a good fit for that dog. No judgment, just a reason to test regularly.

While you wait

If you suspect heartworm, the safest immediate step is to reduce your dog’s activity until your vet can evaluate them. Increased exertion can worsen inflammation and strain on the heart and lungs.

• Keep exercise gentle and brief: leash potty breaks only, no running or rough play.
• Monitor breathing at rest: if your dog is breathing fast, struggling, or seems distressed, seek urgent care.
• Avoid over-the-counter cough meds unless your veterinarian specifically advises them. Many human products are unsafe for dogs or can mask symptoms your vet needs to evaluate.
• Do not start, stop, or switch preventives without veterinary guidance if your dog’s heartworm status is unknown or you suspect infection. Many heartworm-positive dogs stay on prevention, but your vet should guide what to give and when.

Treatment overview

Heartworm treatment is very doable for many dogs, but it must be handled carefully. When adult worms die, they can cause inflammation and clots in the lung vessels. That is why strict activity restriction is not optional, it is protective. This is especially important after adulticide injections.

Most treatment plans take place over several months, with rechecks along the way.

Common parts of a treatment plan

• Stabilization first if the dog is symptomatic (breathing issues, fluid buildup, severe weakness)
• Doxycycline is often prescribed to reduce the Wolbachia bacteria that lives with heartworms, which may lessen complications
• Anti-inflammatory medication when appropriate
• Adulticide injections (melarsomine) to kill adult worms, typically given in a staged protocol
• Ongoing heartworm prevention to prevent new infections and help clear microfilariae

Your vet will choose a protocol based on your dog’s stage of disease, overall health, and risk factors. Please ask about timeline, pain control, and what to watch for at home.

Home care

In my experience, the hardest part for many families is not the medications. It is keeping an active dog quiet for weeks. But this step can truly prevent emergencies.

Ways to enforce rest

• Crate or small-room rest with calm routines
• Food puzzles that do not increase physical activity (ask your vet for safe options)
• Short, slow leash walks only, just for bathroom breaks
• Training brain games like touch, settle, and nose work using tiny treats
• Ask your vet about mild calming support if your dog struggles to stay quiet

Call your vet if you notice

• Worsening cough
• Heavy breathing, panting at rest, or breathing effort
• Lethargy that is more than expected
• Poor appetite lasting more than a day
• Fainting, weakness, or any collapse

Prevention

Heartworm prevention is far safer, easier, and less expensive than treatment. The American Heartworm Society recommends year-round prevention and testing every year, even for dogs who take preventives consistently.

What prevention usually includes

• Monthly chew, monthly topical, or an injectable option given by your vet
• Mosquito control habits (avoid peak mosquito times when possible, remove standing water)
• Annual heartworm test at your veterinary visit

If you are unsure what your dog is taking or whether it covers heartworm, bring the package or a photo to your vet appointment. We are happy to help you make it simple and consistent.

Quick FAQ

Can a dog have heartworms and look normal?

Yes. Many dogs show no obvious signs early on. That is exactly why annual testing is important.

Is heartworm contagious from dog to dog?

No, not directly. It requires a mosquito to spread the parasite.

What if my dog missed a dose of prevention?

Call your vet for guidance. In general, do not panic, but do not guess either. Timing matters, and your vet may recommend testing or a specific schedule.

Bottom line

If you suspect heartworm, take a breath and take action. Restrict activity, schedule a veterinary visit, and get the right test. With early detection and a careful plan, many dogs recover well and go on to live happy, active lives.

If you want, I can help you prepare a short list of questions to bring to your vet appointment based on your dog’s age, weight, lifestyle, and current prevention routine.