Understanding Heartworm Tests for Dogs
Heartworm disease is a potentially life-threatening condition in dogs, caused by the parasitic worm Dirofilaria immitis. Since early infection often shows no symptoms, regular testing is crucial to catch the disease before it causes severe damage. Let's dive into how heartworm tests work, what they detect, and why they're so important for your dog's health.
What Exactly Is Heartworm Disease?
Heartworms are transmitted by infected mosquitoes. When a mosquito bites a dog, it deposits larvae into the bloodstream. Over about six months, these larvae mature into adult worms that can grow up to 12 inches long and reside in the heart and lungs. Adult worms produce offspring called microfilariae, which circulate in the blood and can be picked up by other mosquitoes, continuing the cycle.
Dogs are natural hosts for heartworms, meaning the parasite completes its entire lifecycle within them. Even dogs that seem healthy can harbor these worms without showing signs—especially in early stages or when only a few worms are present.
Types of Heartworm Tests
- Antigen Test: This blood test detects proteins released by adult female heartworms. Antigen usually becomes detectable five to seven months after infection. It's highly accurate but can miss infections if only male or immature female worms are present, if there are very few worms, or if immune complexes mask the antigen.
- Microfilariae Test: This test looks for microfilariae (heartworm offspring) circulating in the bloodstream using microscopic examination. Not all infected dogs have detectable microfilariae—so-called 'occult' infections occur when adult worms haven't produced offspring or are all of one sex.
The two tests are often used together for best accuracy. Preferred methods for detecting microfilariae include the modified Knott's test (which uses centrifugation) or a filter test; both require microscopic identification.
When Should Dogs Be Tested?
- Puppies under 7 months: They can start on prevention without testing but should be tested six months later, again six months after that, and then annually.
- Dogs over 7 months not previously on prevention: Should always be tested before starting preventive medication.
- Adult dogs on year-round prevention: Still need annual testing because missed doses or vomiting up medication can allow infection.
- If there's a lapse in prevention: Additional testing is recommended if you miss a dose, switch products, adopt a dog with unknown history, or notice possible symptoms like coughing or fatigue.
The Testing Process
Your veterinarian will draw a small blood sample from your dog. For antigen tests, this sample is analyzed to detect proteins unique to adult female heartworms. Microfilariae tests involve preparing the blood sample (often with centrifugation) and examining it under a microscope to look for worm offspring.
If results are positive—or if your dog has symptoms—your vet may recommend additional diagnostics such as chest X-rays, ultrasound (echocardiography), or an ECG to assess organ damage and plan treatment safely.
The Importance of Annual Testing
Annual heartworm testing is recommended even for dogs on consistent preventive medication. Why? Because prevention isn't foolproof: missed doses, late administration, vomiting up pills, or product failures can leave gaps where infection occurs unnoticed. Early detection allows prompt treatment before major organ damage develops—and confirms your preventive program is effective.
Treatment vs Prevention: Why Testing Matters
- Treating established heartworm infection is complex and expensive—it involves medications to kill adult worms (like melarsomine), strict exercise restriction during recovery, and sometimes hospitalization due to complications from dead worms breaking down inside your dog's body.
- Prevention, on the other hand, involves monthly oral/topical medications or periodic injections that kill immature larvae before they become adults. However, these preventives don't kill adult worms—so never start them in older dogs without first testing for infection!
The American Heartworm Society recommends annual testing for all dogs over seven months old regardless of preventive status—and year-round prevention tailored to your region's risk level.
Summary: Key Points About Heartworm Testing
- A heartworm test checks your dog's blood for signs of infection—either antigens from adult female worms or microfilariae offspring.
- Both tests may be used together for best accuracy; sometimes further diagnostics are needed if results are unclear or symptoms appear.
- Puppies should begin prevention early but still get tested as they age; adults need annual tests even with regular prevention.
- Nobody should start preventives in older dogs without an initial test—adverse reactions can occur if given to an already-infected dog.
Catching heartworm disease early through regular testing saves lives and money—protecting your dog's health now and into the future!
