Heartworm Infection in Cats 🐱🦟

By Dr Duncan Houston, BVSc

Yes—cats absolutely get heartworm. But unlike in dogs, where adult worms live and reproduce in the heart, feline heartworm is primarily a lung disease. The parasite is the same, but cats respond very differently—and often fatally. In this 2025 guide, I’ll explain the dangers of heartworm infection in cats, how to detect it, and—most importantly—how to prevent it. 💉

🔬 The Life Cycle of Heartworm in Cats

It all starts with a mosquito. The insect bites an infected dog, picks up microfilaria (young heartworms), and a few weeks later, bites a cat.

Unlike dogs, cats are not a natural host—so most larvae are destroyed before reaching adulthood. But even a few worms can cause deadly inflammation, especially when they reach the lungs. Cats usually host 1–3 worms (vs. dozens in dogs), but a single worm can be life-threatening. ❗

Fun (but frightening) facts:

• 🧬 Only ~20% of infected cats develop microfilariae
• 📈 Feline infections often involve only immature worms
• ☠️ Many cats die suddenly without prior symptoms

🫁 Heartworm-Associated Respiratory Disease (HARD)

HARD is the term used for the lung-focused inflammatory reaction caused by larval heartworms in cats. It mimics feline asthma but stems from migrating and dying worms. Signs begin 75–90 days after a mosquito bite.

Common symptoms of feline heartworm disease:

• 😿 Coughing, wheezing, or rapid breathing
• 🤮 Vomiting (even without hairballs)
• 🐾 Lethargy or decreased activity
• 🧠 Neurological signs (seizures, ataxia)
• 💔 Sudden death—10–20% of feline cases

🧪 Testing Is Tricky

Testing for heartworm in cats is more difficult than in dogs. Cats rarely have circulating microfilaria and often don’t have enough adult worms to test positive.

Recommended diagnostics:

• 💉 Antigen test – detects adult female worms, but often misses feline infections
• 🧬 Antibody test – detects past or current infection, but can’t confirm active disease
• 📸 Radiographs or ultrasound – assess lung damage or worm presence
• 🩺 Clinical signs – key to guiding testing and treatment decisions

💊 Treatment in Cats

Unlike in dogs, we cannot safely kill heartworms in cats. Medications used in dogs (like melarsomine) are toxic in cats. The approach is symptom management and supportive care.

Common treatments:

• 💊 Prednisolone to reduce lung inflammation
• 🦠 Doxycycline to target Wolbachia (heartworm symbiotic bacteria)
• 💧 Oxygen or fluid therapy during respiratory distress

If the cat is stable, the goal is to monitor and wait for the worms to die naturally (over 2–3 years). Sudden death remains a risk throughout. 🛑

🛡️ Prevention Is Essential

Indoor cats get heartworm too. In fact, 25–30% of cases are in cats that never go outside. Mosquitoes come indoors easily.

The American Heartworm Society recommends monthly prevention for all cats in endemic areas.

Safe, effective preventives include:

• 💊 Heartgard® (ivermectin) – oral chew
• 💊 Interceptor® (milbemycin) – oral chew, also kills intestinal worms
• 🧴 Revolution® or Revolution Plus® (selamectin) – topical, broad-spectrum
• 🧴 Advantage Multi® or Bravecto Plus® (moxidectin) – topical, plus flea/tick protection

All options are prescription-only. Talk to your vet about the right choice based on lifestyle, exposure risk, and parasite history. 🐾

🏥 When to Suspect Heartworm

Any cat with chronic coughing, asthma-like signs, or unexplained vomiting may warrant heartworm testing—especially if they live in or travel to heartworm zones. 🌍

📱 Ask A Vet App Support

Need help choosing a heartworm preventative or concerned about symptoms? The Ask A Vet app can assist you:

• 🧾 Personalized preventive recommendations
• 📸 Symptom analysis and next-step advice
• 💬 Real-time consultation with licensed vets
• 📅 Monthly reminder setup for prevention dosing

🏁 Final Takeaway

Heartworm infection in cats is deadly, hard to diagnose, and has no cure. Monthly prevention is safe, easy, and lifesaving—even for indoor cats. If your cat coughs, wheezes, or vomits regularly, don’t delay—ask your vet about testing and prevention today. 🐱🦟❤️