🧪 Heartworm Diagnosis in Dogs and Cats: A 2025 Vet Guide to Testing and Detection 🐾

By Dr Duncan Houston, BVSc

When it comes to protecting our pets from heartworm disease, early and accurate diagnosis is key. 🫀 In this 2025 guide, I’ll walk you through the essential veterinary tests used to detect heartworm infection in dogs and cats, explain what each result means, and offer advice on how to ensure your furry friend stays safe from this dangerous parasite. 🐶🐱

🔍 What Are We Testing For?

Heartworm disease is caused by *Dirofilaria immitis*, a parasite transmitted by mosquitoes. Once inside a host, the larvae mature into adult worms and live in the heart and pulmonary arteries. 🎯 Detecting heartworms involves identifying either the adult worms or their offspring — *microfilariae* (baby worms). Knowing which test identifies what stage is crucial for effective treatment. 🧬

🧫 Direct Blood Smear

This is one of the simplest ways to look for heartworm microfilariae. A drop of blood is placed under a microscope to observe swimming larvae. 🩸

🔹 Pros: Quick and inexpensive
🔹 Cons: Only detects heavy infections (must have 20–50+ microfilariae/mL of blood)

While useful for very obvious infections, this test can miss more subtle or early-stage cases — making it unreliable as a stand-alone screening tool. 🚫

🧪 Concentration Tests: Knott’s and Difil

These tests are more accurate than a direct smear. They involve filtering or centrifuging a larger blood sample to concentrate any microfilariae. 🧫

✔️ Now recommended by the American Heartworm Society as part of routine annual testing in dogs.

These methods help detect infections that a simple blood smear might miss — ideal when you want to be certain! 🎯

🐛 What About Other Parasites? Dipetalonema Reconditum

Not all microfilariae in a dog’s bloodstream are heartworm larvae. One lookalike is *Dipetalonema reconditum*, a harmless parasite transmitted by fleas. 🦟

Vets can distinguish between the two using subtle differences in movement and structure — especially the tail and head shape. 🧠 Thanks to antigen tests, this is now much easier and faster than in the past. 🎉

💉 Antigen Testing: The Gold Standard for Dogs

Antigen tests detect proteins from the skin of female adult heartworms. They can pick up even small infections — as long as at least one female worm is present. 🧬

✅ Performed in-clinic — fast results while you wait
✅ Can detect occult infections — when no microfilariae are present

However, it won’t detect male-only infections or very immature worms, making it essential to combine with microfilaria testing. 🧩

📉 Why Might a Dog Have No Detectable Microfilariae?

There are several reasons, even if the dog is infected:

• 🔸 Single-sex infections — e.g., only males = no microfilariae + no antigen
• 🔸 Young female worms — not producing detectable proteins yet
• 🔸 Immune response — body clears microfilariae quickly
• 🔸 Preventive medication — monthly treatments kill larvae, not adults

This is why annual testing is crucial, even for pets on prevention. A missed dose can allow infection, and microfilariae may no longer be present. 🕵️‍♂️

📣 Updated 2025 Testing Guidelines for Dogs

The American Heartworm Society now recommends:

• 🩺 Annual antigen testing
• 🧫 Annual microfilariae testing (Knott’s or Difil)

Combining both tests ensures better detection — especially in those tricky occult cases. 🧩

🐱 Diagnosing Heartworm in Cats: A Different Challenge

Feline heartworm disease is often harder to diagnose. Cats usually have fewer worms — sometimes just one or two — and often don’t show circulating microfilariae. 😿

🧪 Antigen + Antibody Testing for Cats

Because antigen tests only detect female worms and may miss early infections or male-only cases, vets also use antibody tests in cats. 🧬

✔️ Antibody test: Detects immune system response to heartworm
✔️ Antigen test: Detects actual worm proteins (but may be negative even in infected cats)

This combination increases the odds of catching an infection early — which is critical in cats, where symptoms can appear suddenly and severely. 🐾

📅 How Often Should You Test?

Annual testing is ideal. Even with consistent prevention, missed or delayed doses can open a window for infection. 🗓️

If your dog travels to heartworm-endemic areas or if there's doubt about last season’s prevention schedule, testing should be done 7 months after potential exposure. 🧭

⛔ Don’t Test Too Early!

It takes at least 5 months after infection for heartworms to mature. Puppies younger than 5 months old are unlikely to test positive, so testing before then won’t provide useful results. 🍼

🖼️ Imaging Tests: X-rays and Ultrasound

Sometimes, heartworm is detected when pets show signs of illness. In these cases, imaging plays a role in diagnosis:

• 📷 Radiographs (X-rays): Can show heart enlargement or twisted pulmonary arteries
• 🔍 Ultrasound: May reveal actual worms wriggling inside the heart

These tests are often used when routine screening hasn’t been performed and the dog is already showing symptoms. 🐾

📱 Stay Ahead with Ask A Vet

Keeping up with heartworm prevention and testing doesn’t have to be overwhelming. You can track dates, set reminders, and get support by downloading the Ask A Vet App today! 🐶📅

Whether you have questions about testing, prevention, or symptoms, the experts at AskAVet.com are here to help 24/7. 🐾❤️

🧠 Final Thoughts from Dr Duncan Houston

Heartworm disease remains a serious risk to dogs and cats across the globe. But with accurate testing, early diagnosis, and consistent prevention, your pet can enjoy a long, healthy life free of these harmful parasites. 🐾🫀

Remember: combine antigen and microfilariae testing annually for dogs, use antibody and antigen tests in cats, and never skip preventive care. Your vigilance is their protection. 💪🐕🐈