Heartworm disease is caused by the parasitic worm Dirofilaria immitis, transmitted by mosquitoes. Adult worms live in the heart and pulmonary arteries, growing up to 12 inches long. A single infected dog can harbor 30-250 worms. Without treatment, heartworm disease causes progressive heart failure, lung damage, organ failure, and death. Prevention costs $5-$15/month. Treatment costs $1,000-$3,000+ and carries significant risk.
The Lifecycle
- Infected mosquito bites dog, depositing microscopic larvae (microfilariae) into the skin
- Larvae migrate through tissue for 2-3 months, developing into juvenile worms
- Juvenile worms enter the bloodstream and reach the heart/pulmonary arteries
- Worms mature over 3-4 months (6-7 months total from initial infection to adult worms)
- Adult females produce microfilariae that circulate in blood, available for the next mosquito to pick up
- Adults live 5-7 years, continuously damaging heart and lungs
Symptoms by Stage
| Class | Worm Burden | Symptoms |
|---|---|---|
| Class 1 | Low | No symptoms or mild cough |
| Class 2 | Moderate | Cough, exercise intolerance, mild weight loss |
| Class 3 | High | Severe cough, exercise intolerance, difficulty breathing, right-sided heart enlargement |
| Class 4 (Caval Syndrome) | Very high | Cardiovascular collapse, dark brown urine, labored breathing — often fatal without emergency surgical worm extraction |
Prevention Options
| Product | Type | Frequency | Additional Protection | Cost/Month |
|---|---|---|---|---|
| Heartgard Plus | Chewable | Monthly | Roundworms, hookworms | $6-$12 |
| Interceptor Plus | Chewable | Monthly | Roundworms, hookworms, whipworms, tapeworms | $8-$15 |
| Simparica Trio | Chewable | Monthly | Fleas, ticks, roundworms, hookworms | $15-$25 |
| ProHeart 6 | Injection | Every 6 months | Hookworms | $50-$75/injection |
| ProHeart 12 | Injection | Annual | Hookworms | $75-$150/injection |
Testing
- Annual antigen test: Blood test detecting adult female heartworm proteins. Recommended annually even for dogs on prevention (no prevention is 100% effective, and missed doses happen).
- Testing timeline: Antigen tests cannot detect infection until 6-7 months after initial exposure (time required for worms to mature). A negative test today doesn't guarantee the dog wasn't infected 3 months ago.
Treatment (Melarsomine Protocol)
Treatment is risky, painful, and expensive. This is why prevention is vastly preferable:
- Stabilization: Exercise restriction (strict), doxycycline (30 days to weaken worms), prednisone
- Melarsomine injections: Arsenic-based drug injected deep into lumbar muscles. Standard protocol: one injection, then 30 days rest, then two injections 24 hours apart.
- Strict exercise restriction: 6-8 weeks after final injection. Dead worms break into fragments that can embolize (block) pulmonary arteries. Exercise increases blood flow and embolism risk. This is the most dangerous period.
- Follow-up testing: Antigen test 6 months post-treatment to confirm clearance.
Frequently Asked Questions
My dog missed one month of prevention. Is it infected?
Possibly, if exposed to an infected mosquito during that window. Resume prevention immediately and test 6-7 months later (earliest detection point). A single missed month in mosquito season is concerning; multiple missed months dramatically increases risk.
Can heartworm disease be cured?
Yes — treatment kills adult worms. However, heart and lung damage already caused is often permanent, especially in advanced cases. Early detection and treatment before Class 3 produces the best outcomes. Prevention avoids damage entirely.
