Heartworm Diagnosis in Dogs and Cats: A 2025 Guide to Testing and Early Detection 🐶🐱🦟

Hello, I’m Dr Duncan Houston BVSc. Heartworm disease is a dangerous condition that can remain hidden in your pet’s body until it causes irreversible damage. Early, accurate diagnosis is essential. In this 2025 guide, we’ll walk through the various testing methods available, what each test reveals, and how to ensure your pet gets the earliest and most effective care. 🧬🩺

🦟 The Basics of Heartworm Detection

Heartworm disease is caused by worms that live in the heart and pulmonary arteries. Diagnosis is about detecting either the adult worms or their offspring (microfilariae). Different tests detect different life stages—and not every infected pet will have all stages. 🧫

🔬 Diagnostic Methods: What They Find

1. 🩸 Direct Blood Smear

Microscopic exam of a drop of blood. Can only detect infections with large numbers of microfilariae. Not recommended as a primary test.

2. 🧪 Difil Test & Knott’s Test

• More sensitive than blood smear
• Concentrate microfilariae from larger blood volume
• Recommended by the American Heartworm Society as part of routine testing in dogs

3. 🧬 Antigen Testing (Main Test for Dogs)

• Detects proteins from adult female worms
• 🧬 Can identify occult infections (no microfilariae)
• 🚫 Cannot detect male-only infections
• ✅ Most in-clinic “snap” tests are antigen-based and give results in minutes

4. 🧫 Antibody Testing (Important for Cats)

• Detects immune response, not the worm itself
• Useful in cats—often infected with only 1–2 worms
• 🐱 Essential because antigen tests may miss feline infections

📉 Why No Microfilariae Might Still Mean Heartworm

A dog may have adult heartworms but no microfilariae due to:

• 🐛 Single-sex infections (all female or all male worms)
• 🛡️ Immune system clearing larvae
• 💊 Use of preventive meds that kill microfilariae but not adults
• 🧬 In cats, microfilariae are rapidly cleared from the bloodstream

This is why antigen and microfilaria tests are now recommended annually for dogs.

📅 Testing Recommendations in 2025

• 🗓️ Test annually—even if your dog is on prevention
• 🐶 Test dogs with both antigen + microfilaria tests
• 🐱 Test cats with both antigen + antibody tests
• 🧑‍⚕️ Wait 7 months after last exposure (e.g., travel) before testing
• 🧪 Don’t test puppies under 5 months—they’re too young to test positive

🖥️ Imaging: Radiographs and Ultrasound

• 📸 Chest X-rays: Reveal enlarged pulmonary arteries or heart enlargement
• 🔍 Ultrasound: Can show live adult worms inside the heart 🐛
• Used when heartworm is suspected in a sick animal with no prior testing

🤔 What About the Other Microfilariae?

Dipetalonema reconditum is a harmless parasite that also produces microfilariae. These can be confused with heartworm larvae on blood tests. However, modern antigen testing eliminates most confusion. 🧬

📱 Ask A Vet: Diagnostic Help Anytime

Need help interpreting your pet’s heartworm test? Ask A Vet provides:

• 📈 Test result reviews
• 🧪 Guidance on antigen vs. antibody testing
• 🐾 Personalized screening plans
• 🧬 Support if you're unsure about exposure history

Get expert help fast at AskAVet.com or through the app. 🩺📲

✅ Summary: What to Remember in 2025

• 🦟 Heartworm testing must detect adult worms and/or larvae
• 🐶 Dogs: annual antigen + microfilaria testing
• 🐱 Cats: antigen + antibody testing due to low worm counts
• 🧪 Test after missed prevention or travel to endemic areas
• 🧬 Imaging helps when blood tests are inconclusive

With early diagnosis and proper testing, heartworm disease can be managed and treated effectively—long before severe damage sets in. 🐾

— Dr Duncan Houston BVSc

For more heartworm advice or personalized screening tips, visit AskAVet.com or download the Ask A Vet app today. 🐶🐱📲