Canine Heartworm Disease
Dirofilaria immitis infection
A potentially fatal parasitic disease transmitted by mosquitoes. Adult worms live in the heart and pulmonary arteries, causing severe damage. Prevention is safe, effective, and vastly cheaper than treatment.
Last updated: 2026-04-30
Severity
severe
When to Act
See Vet Soon
Symptoms & Signs
Persistent dry cough
Worsens with exercise; may be the earliest noticeable sign.
Exercise intolerance
Dog tires quickly, may lag behind on walks or refuse activity they previously enjoyed.
Difficulty breathing
Labored breathing after mild exercise; eventually at rest in advanced stages.
Weight loss
Gradual wasting as the disease progresses.
Swollen abdomen
Fluid accumulation (ascites) from right-sided heart failure in advanced disease.
Collapse
Sudden collapse from caval syndrome — worms blocking blood flow through the heart.
Behavioral Changes to Watch For
Pets can't tell us what's wrong. These behavioral changes are often the first clues that something is wrong.
🐾 Progressive lethargy
A normally active dog becomes increasingly sedentary.
What You May Notice:
Your dog that used to run for miles now stops after a short walk, panting heavily.
Causes & Risk Factors
Causes
- •Transmitted by infected mosquitoes carrying Dirofilaria immitis larvae
- •Larvae (L3) enter through the mosquito bite wound
- •Larvae mature over 6 months, migrating to the heart and pulmonary arteries
- •Adult worms can reach 12 inches in length and live 5-7 years
Risk Factors
- ⚠Living in or traveling to mosquito-endemic areas (southeastern US, Mississippi River Valley, but found in all 50 states)
- ⚠Dogs not on monthly heartworm prevention
- ⚠Outdoor dogs with high mosquito exposure
How It's Diagnosed
- 1Antigen test (SNAP 4Dx or similar) — detects adult female worm protein
- 2Microfilaria test (Knott's test) — detects circulating baby worms
- 3Chest radiographs — enlarged pulmonary arteries, right heart enlargement
- 4Echocardiography — visualizes worms in the heart in heavy infections
- 5Blood work — may show anemia, elevated liver enzymes
Treatment Options
Adulticide Therapy (Melarsomine Injections)
The only FDA-approved treatment to kill adult heartworms.
Steps
- 1.Pre-treatment: Doxycycline 30 days to weaken worms by killing Wolbachia bacteria
- 2.Strict exercise restriction for the entire treatment period (2-3 months minimum)
- 3.First melarsomine injection (deep IM into lumbar muscles)
- 4.Second and third injections 24 hours apart, 30 days after the first
- 5.Continued strict rest during worm die-off phase
- 6.Prednisone as needed for pulmonary inflammation from dying worms
Expected Outcome
98% efficacy in eliminating adult worms with the 3-dose protocol.
Precautions
- !Dying worms can cause life-threatening pulmonary thromboembolism
- !STRICT cage rest is mandatory — exercise can cause fatal pulmonary embolism
- !Treatment itself carries risk of complications
- !Expensive: $1,000-$3,000+ vs ~$100/year for prevention
Slow-Kill Method (Alternative)
Monthly heartworm preventive + doxycycline to gradually reduce worm burden.
Steps
- 1.Monthly ivermectin-based preventive
- 2.Doxycycline cycles to eliminate Wolbachia
- 3.May take 1-3 years to clear infection
- 4.Worms continue causing damage during this time
Expected Outcome
Gradual worm reduction over months to years, but ongoing cardiac damage.
Precautions
- !Not recommended by the American Heartworm Society as first-line
- !Worms continue damaging the heart and lungs during treatment
- !Some worms may develop resistance
Common Medications Used
| Medication | Usage | Important Notes |
|---|---|---|
| Doxycycline | Eliminates Wolbachia endosymbiont bacteria, weakening adult worms | Given for 30 days before adulticide treatment. |
| Ivermectin (Heartgard) | Monthly heartworm prevention | Also part of slow-kill protocols. Safe at preventive doses even in most herding breeds. |
Prevention
- ✓Monthly heartworm preventive year-round (oral, topical, or injectable ProHeart)
- ✓Annual heartworm testing even if on prevention
- ✓Mosquito control: eliminate standing water, use pet-safe repellents
- ✓ProHeart 6 or ProHeart 12: injectable prevention lasting 6 or 12 months
When to See a Veterinarian
- ⚠️Cough and exercise intolerance in a dog not on regular prevention
- ⚠️Sudden collapse or severe breathing difficulty
- ⚠️Annual heartworm testing (routine)
Frequently Asked Questions
Is heartworm prevention really necessary year-round?
Prognosis
Good to excellent with early detection and proper adulticide treatment. Guarded in advanced stages with heart failure. Caval syndrome is fatal without immediate surgical worm removal.
References
- [1] American Heartworm Society — Guidelines
- [2] CAPC — Heartworm
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