How Do I Know If My Dog Has Heartworms? Symptoms, Stages & Treatment

How Do I Know If My Dog Has Heartworms? A Complete Identification Guide

Heartworm disease in dogs is a serious, potentially fatal condition caused by the parasitic worm Dirofilaria immitis. Transmitted exclusively through the bite of an infected mosquito, these worms mature inside your dog's heart, lungs, and associated blood vessels, where they can grow up to 12 inches long and live 5–7 years. According to the American Heartworm Society (AHS), heartworm disease has been diagnosed in all 50 U.S. states, and an estimated 1 million dogs are currently infected nationwide.

The answer to "how do I know if my dog has heartworms" isn't always straightforward. Early-stage infections often produce no visible symptoms at all, which is exactly why annual veterinary screening is considered the gold standard of detection. By the time clinical signs appear, the disease has typically progressed to a more advanced — and more dangerous — stage. Understanding what to watch for, how diagnosis works, and what treatment involves will help you protect your dog and act quickly if infection is suspected.

What Is Heartworm Disease in Dogs?

Heartworm disease is a parasitic infection in which adult worms inhabit the pulmonary arteries (the blood vessels connecting the heart to the lungs) and, in heavy infections, the right side of the heart itself. The lifecycle begins when a mosquito bites an infected animal — typically a dog, coyote, or fox — and ingests microscopic baby worms called microfilariae. Over 10–14 days inside the mosquito, these develop into infective larvae. When the mosquito bites your dog, the larvae enter through the bite wound and begin a 6–7 month migration through the body, eventually maturing into adult worms in the pulmonary vasculature.

The damage isn't caused only by the worms themselves. The inflammatory response your dog's immune system mounts against the worms — and the substances the worms release — causes progressive damage to the pulmonary arteries, lung tissue, and eventually the heart. In severe cases, a sudden mass of worms can block blood flow entirely, a condition known as caval syndrome, which is a life-threatening emergency.

Heartworm is not directly contagious between dogs. It requires a mosquito vector. However, any dog exposed to mosquitoes in an area where heartworm is endemic is at risk — including indoor dogs, since mosquitoes can enter homes.

Symptoms of Heartworm Disease: What to Watch For

Heartworm symptoms correlate directly with worm burden (number of adult worms), duration of infection, and your dog's individual immune response. Many dogs with low worm counts show zero symptoms. The checklist below is organized by disease class, following the American Heartworm Society staging system.

Key insight for owners: If you're asking "how do I know if my dog has heartworms" because you've noticed a new cough or exercise reluctance, these symptoms overlap with many other conditions — including kennel cough, heart disease, and tracheal collapse. Only a veterinary antigen test can confirm heartworm infection.

Risk Factors: Which Dogs Are Most Vulnerable?

How Is Heartworm Disease Diagnosed?

If your veterinarian suspects heartworm — or during your dog's routine annual wellness visit — diagnosis involves a structured series of tests. Here's what to expect during the diagnostic process:

1. Heartworm Antigen Test (Primary Screening)
   A blood test that detects proteins (antigens) produced by adult female heartworms. This is the standard screening test used by virtually all veterinary clinics. It requires only a small blood sample and results are typically available in 8–10 minutes (in-clinic snap test) or sent to a reference lab. Limitation: It generally cannot detect infections with fewer than 1–3 adult female worms, and infections less than 5–7 months old will test negative because worms haven't matured yet.
   Estimated cost: $35–$75 (often bundled with annual wellness bloodwork)
2. Microfilaria Test (Knott's Test or Filter Test)
   A secondary blood test that looks for baby heartworms (microfilariae) circulating in the bloodstream. A positive result confirms active reproduction by adult worms. Approximately 20% of infected dogs are "occult" — meaning they have adult worms but no circulating microfilariae — so a negative microfilaria test does not rule out infection.
   Estimated cost: $25–$50
3. Chest X-Rays (Thoracic Radiographs)
   Once heartworm is confirmed, X-rays evaluate the extent of damage to the pulmonary arteries, lungs, and heart size. This is critical for staging the disease and planning treatment safely.
   Estimated cost: $150–$350
4. Complete Blood Count (CBC) and Chemistry Panel
   Evaluates kidney function, liver enzymes, red/white blood cell counts, and platelet levels to assess your dog's ability to tolerate treatment. Heartworm disease and its treatment both place demands on these organ systems.
   Estimated cost: $80–$200
5. Echocardiogram (Ultrasound of the Heart)
   Recommended for dogs with Class III disease or suspected caval syndrome. Allows the veterinarian or veterinary cardiologist to visualize worms within the heart chambers and assess cardiac function.
   Estimated cost: $300–$600

Treatment Options for Heartworm Disease in Dogs

Heartworm disease can be treated, but treatment is a multi-month commitment that carries inherent risks. The current standard of care, as recommended by the American Heartworm Society, involves a multi-drug protocol centered on melarsomine dihydrochloride (Immiticide®), an arsenic-based compound administered by deep intramuscular injection.

The Standard Treatment Timeline

Understanding the full treatment arc helps you plan logistically and financially:

• Day 1 (Diagnosis): Bloodwork confirmed, staging begins. Doxycycline prescribed (10 mg/kg twice daily for 28 days) to target Wolbachia, a symbiotic bacterium inside heartworms that drives much of the inflammatory reaction.
• Days 1–30: Doxycycline course. Heartworm preventive started (to kill any larvae acquired since infection). Strict exercise restriction begins — leash walks only for bathroom breaks, no running, jumping, or roughhousing.
• Day 30: First melarsomine injection (2.5 mg/kg deep intramuscular, lumbar muscles). Most dogs are hospitalized for 24-hour observation.
• Day 60: Second and third melarsomine injections, 24 hours apart. This is the highest-risk window — dying worms can cause pulmonary thromboembolism (blood clots in the lungs). Hospitalization for 48–72 hours is standard.
• Days 60–120: Continued strict exercise restriction. Your dog must remain calm and confined. Crate rest is often necessary for active dogs.
• Day 120+ (approximately 6 months post-treatment): Follow-up antigen test to confirm all adult worms have been eliminated. If negative, your dog continues on monthly heartworm preventive for life.

Prognosis: Life Expectancy After Heartworm Diagnosis

Prognosis varies significantly based on the stage at diagnosis, your dog's overall health, and how well the treatment protocol is followed. Here's what the evidence shows:

The Merck Veterinary Manual notes that the most common cause of death during treatment is pulmonary thromboembolism — blood clots formed from dying worm fragments — which is why strict exercise restriction is non-negotiable during the 8–12 week treatment window.

Home Care and Daily Management During Treatment

If your dog is undergoing heartworm treatment, your role at home is critical to their survival. Here's what daily management looks like:

Exercise Restriction — The Most Important Rule

This is the single most impactful thing you can do. When adult worms die, their fragments lodge in the pulmonary arteries. Increased heart rate and blood pressure from exercise can dislodge these fragments, causing fatal blockages. For 8–12 weeks after the final melarsomine injection:

• Leash-walk only for bathroom breaks (5–10 minutes maximum)
• No running, jumping on/off furniture, fetch, or play with other dogs
• Crate confinement when unsupervised — use a crate large enough for your dog to stand and turn around
• Carry small dogs up and down stairs
• For highly active dogs, discuss short-term anti-anxiety medication (e.g., trazodone 2–5 mg/kg every 8–12 hours) with your veterinarian to help them rest

Monitoring at Home

• Watch for: Coughing (new or worsening), labored breathing, lethargy, loss of appetite, vomiting, pale gums, dark urine, or collapse
• Check gum color daily: Should be pink, not pale white or blue-tinged
• Monitor breathing rate at rest: Normal is 15–30 breaths per minute. Consistently above 40 at rest warrants a vet call
• Temperature: Low-grade fever (102–103°F / 38.9–39.4°C) is common in the days after injection, but persistent high fever requires veterinary attention

Nutrition and Medications

• Continue feeding a balanced, AAFCO-approved diet. No dietary changes needed unless your vet recommends cardiac-support nutrition for Class III dogs
• Administer all prescribed medications on schedule — doxycycline, heartworm preventive, any anti-inflammatories or cardiac medications
• Ensure fresh water is always available, especially during doxycycline treatment (to prevent esophageal irritation)

How Much Does Heartworm Treatment Cost?

Heartworm treatment is one of the more expensive common veterinary procedures. Costs vary by geographic region, clinic type (general practice vs. specialty hospital), and disease severity. Here's a realistic breakdown based on 2025–2026 U.S. pricing data:

Cost-saving tip: Many clinics offer heartworm treatment payment plans. Organizations like RedRover, The Pet Fund, and CareCredit can help with financing. Pet insurance typically does NOT cover heartworm treatment if the condition existed before policy enrollment (pre-existing condition exclusion), which is why prevention is far more economical — monthly preventives cost $5–$20/month ($60–$240/year).

When to See the Vet: Heartworm Red Flags

If your dog is currently undergoing heartworm treatment, any of the above symptoms during the 8–12 week post-injection window should be treated as an emergency. Pulmonary thromboembolism can develop rapidly and requires immediate veterinary intervention.

Prevention: Keeping Your Dog Heartworm-Free

Prevention is dramatically simpler, safer, and cheaper than treatment. The American Heartworm Society recommends a "Think 12" approach: test every 12 months and give preventive 12 months a year, regardless of your location or the season.

Types of Heartworm Preventives

• Monthly oral chewables: Heartgard Plus (ivermectin/pyrantel), Interceptor Plus (milbemycin/praziquantel), Tri-Heart Plus (ivermectin/pyrantel), Sentinel (milbemycin/lufenuron). Cost: $5–$15/dose depending on dog weight.
• Monthly topicals: Revolution Plus (selamectin/sarolaner), Advantage Multi (imidacloprid/moxidectin). Applied to the skin between the shoulder blades. Cost: $12–$25/dose.
• Injectable (ProHeart 6 or ProHeart 12): Administered by your veterinarian — moxidectin sustained-release microspheres provide 6 or 12 months of protection per injection. Cost: $80–$200 per injection depending on duration and dog weight. Ideal for owners who struggle with monthly compliance.

Critical rule: Before starting any heartworm preventive, your dog MUST have a negative antigen test. Giving preventive to an infected dog can trigger a severe reaction as microfilariae die rapidly. Puppies under 7 months can start preventive without testing, but should be tested at 7–8 months and again 6 months later.

Frequently Asked Questions

Can heartworms be transmitted from dog to dog?

No. Heartworm disease requires a mosquito as an intermediate host. Your dog cannot catch heartworms directly from another infected dog or from contact with infected feces, saliva, or urine. However, an infected dog in your neighborhood means mosquitoes in the area are carrying the parasite, increasing risk for all unprotected dogs nearby.

Can I use over-the-counter heartworm treatment?

There are no FDA-approved over-the-counter heartworm treatments. All effective adulticide treatments require veterinary prescription and administration. Products sold online claiming to "cure" heartworm without veterinary involvement are not proven effective and may be dangerous. Heartworm preventives, however, are widely available — some through your vet, others through licensed online pharmacies (verify they require a prescription, which is a sign of legitimacy).

My dog is on preventive year-round. Do I still need annual testing?

Yes. No preventive is 100% effective. Vomiting after a dose, missed doses, drug resistance (documented in some regions, particularly the Mississippi Delta), and product supply chain issues can all create gaps in protection. Annual testing catches breakthrough infections early, when treatment is safest and most successful.

Are certain breeds more prone to heartworm disease?

No breed has a genetic predisposition to heartworm infection itself — any breed can be infected if bitten by a carrier mosquito. However, sporting and working breeds that spend more time outdoors (Labrador Retrievers, German Shorthaired Pointers, Beagles) are statistically overrepresented in heartworm cases due to higher mosquito exposure. Brachycephalic breeds (French Bulldogs, Pugs, Bulldogs) may face more severe complications due to pre-existing respiratory limitations.

Can cats get heartworms too?

Yes, cats can develop heartworm disease, though the infection manifests differently. Cats typically harbor fewer worms, and even 1–2 worms can cause severe respiratory disease (heartworm-associated respiratory disease, or HARD). There is no approved adulticide treatment for cats — management focuses on supportive care and prevention. Monthly feline heartworm preventives like Revolution Plus or Interceptor are recommended for all cats in endemic areas.

What happens if heartworm disease goes untreated?

Untreated heartworm disease is progressive and ultimately fatal. Adult worms continue to reproduce, worm burden increases, and damage to the pulmonary arteries and heart worsens over months to years. Most untreated dogs develop Class III disease within 1–3 years and eventually die from heart failure, pulmonary thromboembolism, or caval syndrome. The timeline varies — some dogs live several years with low worm burdens, but quality of life deteriorates progressively.