What is Heartworm Disease?

Heartworm disease is a serious and potentially fatal disease in pets. It occurs in the United States and many other countries.

• Caused by foot-long worms called heartworms.
• Heartworms live in the heart, lungs, and blood vessels, causing severe lung disease, heart failure, and organ damage.
• Affects dogs, cats, and ferrets, but can also infect wildlife like wolves, coyotes, and foxes, and rarely humans.

Wild animals living near urban areas are important carriers of heartworm disease.

Heartworm in Dogs

Dogs are a natural host for heartworms:

• Heartworms in dogs mature into adults, mate, and produce offspring.
• Untreated dogs can harbor hundreds of worms.
• Heartworm disease causes lasting damage to the heart, lungs, and arteries.

Key point: Prevention is better than treatment, and early treatment is more effective than delayed treatment.

Heartworm in Cats

Cats are an atypical host:

• Most heartworms in cats do not survive to adulthood.
• Cats usually have 1–3 adult worms, and some have none.
• Even immature worms cause heartworm associated respiratory disease (HARD).

Important: Medications for treating heartworms in dogs cannot be used in cats. Prevention is the only protection.

How is Heartworm Transmitted?

Heartworm requires a mosquito to transmit the disease:

1. Infected animal: Adult female heartworms produce microfilaria (baby worms) in the bloodstream.
2. Mosquito bites: Picks up microfilaria while feeding.
3. Larval development: Microfilaria mature in the mosquito over 10–14 days.
4. New host: Mosquito bites another pet, depositing infective larvae on the skin. Larvae enter through the bite wound.
5. Maturation: It takes 6 months for larvae to mature into adult heartworms.

Lifespan: Adult heartworms live 5–7 years in dogs and 2–3 years in cats.

Signs of Heartworm Disease in Dogs

Early infection may show no symptoms. Symptoms develop as the infection progresses:

• Mild persistent cough
• Reluctance to exercise
• Fatigue after moderate activity
• Decreased appetite
• Weight loss

Advanced disease may include:

• Heart failure
• Swollen belly due to fluid buildup
• Caval syndrome (life-threatening cardiovascular collapse)
  • Labored breathing
  • Pale gums
  • Dark or bloody urine

Emergency: Caval syndrome requires surgical removal of worms.

How Significant is the Risk?

Heartworm can occur anywhere in the U.S.:

• Spread by mosquitoes, wildlife, and movement of pets.
• Diagnosed in all 50 states.
• Risk factors include climate, wildlife carriers, travel, and local mosquito activity.

Recommendation:

• Test your pet every 12 months.
• Administer heartworm prevention 12 months a year (“Think 12”).

Heartworm Testing

Testing detects heartworm proteins in a small blood sample:

• Early detection improves treatment success.
• Tests can be done in-clinic or sent to a diagnostic laboratory.
• If positive, further tests may be required.

Dog Testing Guidelines

• Puppies under 7 months: Start prevention; test 6 months later, then annually.
• Adult dogs over 7 months: Test before starting prevention; repeat 6 and 12 months later.
• Missed doses: Test immediately, then again in 6 months.

Even on prevention, testing is essential because medications are highly effective but not 100% guaranteed.

If a Dog Tests Positive

Treatment focuses on stabilization, worm elimination, and minimizing side effects:

1. Confirm diagnosis: Positive tests require an additional test.
2. Restrict exercise: Prevent further heart and lung damage.
3. Stabilize disease: Therapy may take months if symptoms are severe.
4. Administer treatment: Follow a multi-step protocol based on the dog’s condition.
5. Retest: About 6 months after treatment, confirm all worms are eliminated.

Long-term prevention: Continue year-round heartworm prevention.

Summary

• Heartworm disease affects dogs, cats, ferrets, and wildlife.
• Mosquitoes transmit the disease.
• Dogs may carry hundreds of worms; cats often carry fewer but can still be damaged.
• Testing and prevention are essential for all pets.
• Early detection and treatment save lives.