Heartworm Testing in Dogs: Veterinary Guide 2025🩺🐾

By Dr. Duncan Houston BVSc

Hello, I’m Dr Duncan Houston BVSc, veterinarian and founder of Ask A Vet. Heartworm tests are a critical component of preventive care in 2025, especially since early infections are often silent. This guide covers the types of tests, when and how to use them, their accuracy and limitations, as well as the next steps after a positive result. You’ll also learn how Ask A Vet supports testing reminders, follow-up, and treatment planning. Let’s protect your pup together! 🐶💙

1. Why Testing Matters 🚨

Heartworm disease is caused by the parasite Dirofilaria immitis, which lives in the lungs and heart, causing serious damage—even death. Testing confirms infection before irreversible harm occurs. Prevention alone isn’t enough, as even missed doses can lead to undetected infection.

2. Types of Heartworm Tests

2.1 Antigen Test

• Detects proteins from mature female worms—typically positive around 5–7 months post-infection.
• Side‑room or point-of-care ELISA and immunochromatographic tests boast ~98–99% sensitivity with >96% specificity.
• However, early infections, low worm burden, or male-only infections may yield false negatives.
• Rare false positives arise from antigen persistence post-treatment or cross-reactivity with other parasites.

2.2 Microfilaria Test

• Identifies heartworm larvae (“microfilariae”) circulating in blood via smear, hematocrit, or Modified Knott's method.
• Complements antigen testing—detects occult infections, immune-complex masked antigens, or male-only disease.
• American Heartworm Society and AVMA recommend yearly combination testing.

3. When and How to Test

• ✅ At ≥7 months of age before starting prevention.
• ✅ Annually, even on preventives, to catch breakthrough infections or lapses.
• 🔁 Immediately if prevention lapses or travel to endemic areas occurs.
• 🧪 Retest ~9 months after treatment to confirm clearance.

4. Accuracy & Limitations

4.1 Antigen Test Limitations

• May miss early or low-level infections—sensitivity can drop to ~60–70% for one to two worms.
• Immune complexes can mask antigen—lab heat or acid pretreatment can uncover hidden positives.
• False positives are uncommon but can occur—follow up with confirmatory tests.

4.2 Microfilaria Test Limitations

• Negative microfilaria does not rule out infection—occult infections occur, especially with some preventatives.
• Requires microscopy—Operator-dependent and less sensitive than Knott’s test.

5. Interpreting Results

For discordant or unexpected results, retest with another sample and consider immune-complex unmasking via lab pretreatment.

6. Next Steps if Positive

• Confirm diagnosis—repeat antigen test with fresh sample, consider heat pretreatment if available.
• Assess disease severity: chest X-rays, echocardiography, CBC/chemistry.
• Plan treatment following American Heartworm Society guidelines—usually melarsomine injections, activity restriction, doxycycline, and macrocyclic lactones.
• Retest ~9 months post-treatment to confirm clearance.

7. Role of Ask A Vet

• Ask A Vet: Telehealth consults for interpreting results and planning treatment/testing schedules.

8. 🗓️ Annual Testing Checklist

• ✅ Antigen test if ≥7 months old or after lapse in prevention.
• ✅ Microfilaria test (Knott’s or smear).
• ✅ Confirm positives with repeat lab tests or imaging.
• ✅ Pre-treat and treat as needed—follow AHS protocols.
• ✅ Retest 9 months post-treatment.
• ✅ Continue year-round prevention and testing reminders 

9. Final Thoughts 📝

In 2025, heartworm testing remains essential—no test is perfect, and only combined antigen + microfilaria testing reveals most infections. By understanding test limitations, and follow-up protocols, and integrating smart support tools like Ask A Vet, we can protect dogs from this preventable yet serious disease. Let’s keep your dog safe, tested, and heartworm-free! 🐾💙