Tell If Your Dog Has Heartworms: Symptoms, Diagnosis

Heartworm disease is one of the most serious — and entirely preventable — conditions your dog can face. Caused by the parasitic worm Dirofilaria immitis, this disease is transmitted exclusively through mosquito bites. When an infected mosquito bites your dog, it deposits microscopic larvae (L3 stage) into the skin. Over the next 6 to 7 months, these larvae migrate through tissue, mature, and eventually settle in the heart and pulmonary arteries, where adult worms can grow up to 12 inches long and live 5 to 7 years.

According to the American Heartworm Society (AHS), heartworm disease has been diagnosed in all 50 U.S. states, and infection rates in some regions exceed 20% in unprotected dogs. The disease is progressive: early stages often show zero symptoms, which is exactly why knowing how to tell if your dog has heartworms — and keeping up with annual testing — can literally save your dog's life.

What Are Heartworms in Dogs?

Heartworms are foot-long parasitic nematodes that reside in the heart, lungs, and associated blood vessels of infected animals. Dogs are the natural definitive host, meaning the worms complete their full life cycle inside a dog's body. A single infected dog can harbor anywhere from 1 to over 250 adult worms, though the average infection involves about 15 worms.

The life cycle is worth understanding because it explains why prevention timing matters:

1. Microfilariae (baby worms) circulate in an infected dog's bloodstream
2. A mosquito bites the infected dog and ingests microfilariae
3. Inside the mosquito (10–14 days), microfilariae develop into infective L3 larvae
4. The mosquito bites your dog, depositing L3 larvae into the skin
5. L3 → L4 larvae migrate through tissue over 50–70 days
6. L4 → immature adults enter the bloodstream and travel to the heart (67–85 days post-infection)
7. Adult worms mature, mate, and produce microfilariae approximately 6–7 months after initial infection

This 6-to-7-month prepatent period is critical: your dog can be infected and show no signs for half a year or longer. The Merck Veterinary Manual notes that clinical signs typically don't appear until adult worms begin affecting pulmonary artery function and cardiac output.

Symptoms of Heartworm Disease: What to Watch For

One of the most dangerous aspects of heartworm disease is its silent progression. Many dogs show absolutely no symptoms during early infection. When signs do appear, they're often mistaken for aging, kennel cough, or simply being "out of shape." Here's a comprehensive symptom checklist organized by disease stage:

How Veterinarians Diagnose Heartworm Disease

If you're wondering how to tell if your dog has heartworms, the definitive answer comes from veterinary testing — not symptom observation alone. Here's the diagnostic process you can expect:

Step 1: Antigen Test (Primary Screening)

The most common diagnostic tool is a heartworm antigen test, which detects proteins (antigens) produced by adult female heartworms. This test requires a small blood sample and can be run in-clinic in about 10 minutes using SNAP test kits (such as the IDEXX Heartworm IDEXX or Zoetis VetScan).
Cost: $35–$65 as a standalone test, often bundled with annual wellness bloodwork for $80–$150.

Important limitation: Antigen tests typically don't turn positive until approximately 6–7 months post-infection (when adult females are present). Very early infections, male-only infections, or infections with only 1–2 female worms can produce false negatives.

Step 2: Microfilariae Test (Knott's Test or Filter Test)

A microscopic examination of blood checks for circulating microfilariae (baby worms). About 20–30% of heartworm-positive dogs are "occult" — meaning they have adult worms but no circulating microfilariae. This test complements the antigen test.
Cost: $15–$30 (often included with antigen testing).

Step 3: Confirmatory and Staging Diagnostics

Once heartworm is confirmed, your vet needs to stage the disease and assess organ damage before beginning treatment:

The total cost for a complete heartworm diagnostic workup (antigen test, staging bloodwork, X-rays, and echocardiogram) typically ranges from $400 to $1,000+ depending on your region and whether a veterinary cardiologist performs the echocardiogram.

Heartworm Treatment Options and Success Rates

The good news: heartworm disease can be treated, especially when caught early. However, treatment is a serious, multi-month process — not a quick fix. The current gold-standard protocol, recommended by the American Heartworm Society, involves the following:

The Standard Melarsomine Protocol

Melarsomine dihydrochloride (brand name Immiticide® or Diroban®) is the only FDA-approved adulticide — a drug that kills adult heartworms. It's administered as a deep intramuscular injection in the lumbar muscles.

The current AHS-recommended protocol includes:

1. Pre-treatment (30–60 days before adulticide):
   • Start a heartworm preventive (e.g., ivermectin-based) to kill microfilariae and prevent new infections
   • Doxycycline antibiotic (10 mg/kg twice daily for 30 days) to eliminate Wolbachia, a bacterium that lives inside heartworms and contributes to inflammation when worms die
   • Strict exercise restriction begins immediately and continues through the entire treatment period
   • Prednisone or other anti-inflammatory medication may be prescribed
2. Adulticide injections (Days 60–90):
   • Day 60: First melarsomine injection (kills approximately 50% of adult worms)
   • Day 90: Second injection
   • Day 91: Third injection (24 hours after the second)
   • Hospitalization for 1–3 days after the injection series for monitoring
3. Post-treatment recovery (8–12 weeks after final injection):
   • Continued strict exercise restriction — this is the hardest part for most owners
   • Monitor for complications as dead worms break apart and pass through pulmonary arteries
   • Follow-up antigen test at 6 months post-treatment to confirm all worms are eliminated

Treatment Comparison

Treatment Complications to Know About

The most dangerous period is 7–14 days after adulticide injections, when dead and dying worms break apart and can cause pulmonary thromboembolism (blood clots in the lungs). This is why strict exercise restriction is non-negotiable — even a short game of fetch can trigger a fatal embolism during this window. Signs of post-treatment complications include:

• Fever above 103°F (39.4°C)
• Sudden worsening of cough
• Difficulty breathing or rapid breathing
• Lethargy or refusal to eat
• Coughing up blood

Prognosis by Disease Stage

Your dog's prognosis depends heavily on how many worms are present, how long the infection has been active, and your dog's overall health. Here's a realistic outlook:

Even after successful treatment, previously infected dogs remain at higher risk for pulmonary hypertension and may require periodic cardiac monitoring for the rest of their lives.

Heartworm Treatment Costs: What to Budget For

Heartworm treatment is expensive, and costs vary significantly based on your dog's size, disease stage, and your geographic location. Here's a realistic breakdown:

Compare this to the cost of year-round prevention: $60–$200 per year for products like Heartgard Plus, Interceptor Plus, Simparica Trio, or Revolution Plus. The financial case for prevention is overwhelming.

Risk Factors: Which Dogs Are Most Vulnerable?

While no specific breed is genetically predisposed to heartworm susceptibility (unlike conditions such as hip dysplasia in German Shepherds), breeds that spend more time outdoors — such as Labrador Retrievers, Beagles, Pointers, and Huskies — are statistically overrepresented in heartworm cases simply due to higher mosquito exposure.

Home Care During and After Heartworm Treatment

If your dog is undergoing heartworm treatment, your role at home is critical to their survival. The medication kills the worms, but your management during recovery determines whether those dead worms pass safely or cause a fatal blockage.

Exercise Restriction — The #1 Rule

For the entire treatment period (typically 8–12 weeks from first injection to clearance), your dog must be kept calm. This means:

• Leash walks only — short, 5–10 minute bathroom breaks on a short leash
• No running, jumping, fetching, or roughhousing
• Crate rest when unsupervised — use an appropriately sized crate where your dog can stand, turn, and lie down but not run
• Carry small dogs up and down stairs
• Consider calming aids: Adaptil pheromone diffusers, Kong toys stuffed with frozen peanut butter, or consult your vet about short-term anti-anxiety medication (trazodone is commonly prescribed at 3–5 mg/kg every 8–12 hours)

Monitoring at Home

Keep a daily log during the 7–14 day post-injection window noting:

• Rectal temperature (normal: 101–102.5°F / 38.3–39.2°C) — report anything above 103°F
• Respiratory rate at rest (normal: 15–30 breaths per minute) — count chest rises while sleeping
• Appetite and water intake
• Cough frequency and severity
• Energy level and gum color (should be pink, not pale or blue)

After Treatment: Long-Term Management

Once your vet confirms the infection is cleared (negative antigen test at 6 months post-treatment):

• Resume year-round heartworm prevention immediately — reinfection is always possible
• Schedule annual heartworm testing even while on prevention (no preventive is 100% effective)
• Dogs treated for Class II–III disease may benefit from annual echocardiograms to monitor for pulmonary hypertension
• Gradually return to normal exercise over 2–4 weeks after veterinary clearance — don't jump straight back into intense activity

Prevention: The Only Strategy That Truly Works

Heartworm prevention is one of veterinary medicine's greatest success stories. Modern preventives are safe, effective, and easy to administer. The American Heartworm Society recommends "Think 12": prevent year-round (12 months) and test every 12 months.

Popular FDA-Approved Heartworm Preventives

Important note for herding breeds: Dogs with the MDR1 gene mutation (common in Collies, Australian Shepherds, Shetland Sheepdogs) may be sensitive to ivermectin at higher doses. However, the low doses used in heartworm preventives are generally safe even for MDR1-positive dogs. Consult your veterinarian if you have concerns — they can recommend a milbemycin-based or moxidectin-based alternative.

Why Year-Round Prevention Matters Even in Cold Climates

A common misconception is that you only need prevention during "mosquito season." Here's why that's dangerous:

• Mosquitoes can survive indoors (garages, basements, greenhouses) through winter
• Climate variability means warm spells in December or March can activate mosquitoes unexpectedly
• The American Veterinary Medical Association (AVMA) and the AHS both recommend 12-month prevention regardless of climate
• Year-round preventives also protect against intestinal parasites that are present year-round
• Missing even one month can leave a gap that allows infection to establish

Frequently Asked Questions About Heartworm Disease

Can I get heartworms from my dog?

No. Heartworms are not directly contagious between animals or from dogs to humans. Transmission requires a mosquito as an intermediate host. While humans can rarely be infected by Dirofilaria immitis through a mosquito bite, the worms cannot complete their life cycle in humans and typically cause only small, benign lung nodules.

My dog is on prevention — do they still need annual testing?

Yes. No preventive is 100% effective. Dogs can spit out chewable tablets, topical applications can be applied incorrectly, and rare resistance has been documented in some heartworm populations (particularly in the Mississippi Delta region). Annual antigen testing ensures any breakthrough infection is caught early when treatment is safest and most effective.

What happens if I miss a dose of heartworm prevention?

If you're less than 2 weeks late, administer the missed dose immediately and continue on your regular schedule. If more than 2 weeks have passed, contact your veterinarian — they may recommend testing before restarting, depending on your area's heartworm prevalence and how many doses were missed. The larvae's development timeline means a single missed month in a low-risk area is unlikely to result in infection, but consecutive missed months significantly increase risk.

Can heartworm disease be cured completely?

Yes, in most cases. The 3-injection melarsomine protocol successfully eliminates adult worms in approximately 95–98% of Class I–II cases. However, "cured" means the worms are eliminated — it does not undo the damage they caused. Dogs treated for advanced heartworm disease may have permanent pulmonary scarring, pulmonary hypertension, or cardiac changes that require lifelong management.

Is heartworm treatment painful for dogs?

The melarsomine injections themselves cause temporary soreness at the injection site (deep in the lumbar muscles). Most dogs are uncomfortable for 1–3 days after each injection. Your vet will prescribe pain management (typically NSAIDs or tramadol). The bigger concern isn't pain from the injection but the risk of complications as worms die — this is why exercise restriction is so critical.

How long can a dog live with untreated heartworms?

This varies enormously. A dog with a low worm burden (1–5 worms) might live 2–5 years with minimal symptoms before the disease progresses. A dog with a heavy burden (50+ worms) may deteriorate rapidly within months. Untreated heartworm disease is always progressive and ultimately fatal — the worms will continue to damage the heart and lungs until cardiac failure or caval syndrome occurs.

Can cats get heartworms too?

Yes, cats can be infected with Dirofilaria immitis, though they are atypical hosts and usually harbor fewer worms (1–3). However, even a single worm can cause severe respiratory disease in cats (known as HARD — Heartworm Associated Respiratory Disease). Feline heartworm prevention is available and recommended in endemic areas.

Key Takeaways: How to Tell If Your Dog Has Heartworms

The honest answer to "how to tell if your dog has heartworms" is that you often can't — not without veterinary testing. Early-stage heartworm disease is frequently invisible to even the most attentive owner. By the time symptoms like chronic coughing, exercise intolerance, and abdominal swelling appear, the disease has already caused significant damage.

Your best defense is a three-part strategy:

1. Annual testing — a simple blood test at your yearly wellness visit
2. Year-round prevention — consistent, never-missed monthly or injectable preventives
3. Vigilance — knowing the symptoms and acting quickly if you notice coughing, fatigue, or breathing changes

If your dog hasn't been tested in the past 12 months, schedule a heartworm antigen test at your next vet visit. It takes 10 minutes, costs less than a tank of gas, and could save your dog's life.