Heartworm disease is a serious, potentially fatal condition caused by parasitic worms (Dirofilaria immitis) that live in the heart, lungs, and associated blood vessels of affected pets. The disease is spread through the bite of an infected mosquito.
Regular testing is crucial for early detection and effective prevention.
Here’s a breakdown of heartworm tests in dogs:
Why is Heartworm Testing Important?
Early Detection: Heartworm disease often shows no symptoms in its early stages. By the time symptoms like coughing, lethargy, or difficulty breathing appear, the disease is usually advanced and harder to treat.
Treatment Effectiveness: Early diagnosis allows for more effective and safer treatment options, minimizing damage to your dog’s vital organs.
Preventing Spread: If your dog tests positive for microfilariae (baby heartworms), it’s a “reservoir” for the disease, meaning infected mosquitoes can pick up the microfilariae from your dog and spread them to other dogs. Treatment can eliminate these microfilariae.
Safe Prevention: It’s critical to test dogs before starting heartworm prevention, especially if there’s any chance they could be positive. Giving preventive medication to a heartworm-positive dog can, in some cases, lead to severe and potentially fatal reactions due to the rapid death of microfilariae.
Types of Heartworm Tests
Most heartworm tests involve a simple blood sample and are typically performed in-clinic or sent to a veterinary laboratory.
Antigen Test (Most Common & Primary Test):
What it detects: This test looks for proteins (antigens) produced by adult female heartworms.
How long it takes: Results are often available within 10-20 minutes in the clinic.
Limitations:
It will only be positive once adult female worms are present, which takes at least 6-7 months after a dog is infected by a mosquito. This means a dog could be infected but test negative during this “pre-patent period.”
It may not detect very light worm burdens (e.g., only one or two adult female worms) or infections with only male worms (which cannot reproduce).
Microfilaria Test (Often done in conjunction or as a follow-up):
What it detects: This test looks for the presence of microfilariae (microscopic baby heartworms) circulating in the dog’s bloodstream.
How it’s done: This can be a direct smear, a filtration test (e.g., Knott’s test), or a commercial kit.
Why it’s important: If microfilariae are found, it confirms an active infection with breeding adult worms, and it means the dog can transmit the disease to other animals via mosquitoes.
Limitations:
Not all heartworm-positive dogs have circulating microfilariae. This can happen if the infection consists only of male worms, if the worms are too young to reproduce, if the dog has been on certain heartworm preventives that kill microfilariae, or due to the dog’s immune system clearing them. These dogs are still infected and need treatment.
Microfilariae can sometimes be confused with other non-pathogenic parasites in the blood.
Many veterinarians perform both an antigen test and a microfilaria test for the most comprehensive screening.
The Testing Process
Blood Draw: Your veterinarian will draw a small blood sample from your dog (usually from a leg vein).
Processing: The blood is then processed according to the specific test being used.
Results: For in-clinic tests, you’ll often have results before you leave. For samples sent to a lab, results may take a few days.
When Should Dogs Be Tested?
Annually: The American Heartworm Society recommends annual testing for all dogs, even those on year-round prevention.
Why? Prevention isn’t always 100% effective (e.g., missed doses, dog spits out pill, product failure, vomiting after administration). Annual testing ensures that if prevention fails, the infection is caught early.
Before Starting Prevention: Always test a dog for heartworm before initiating a preventive program, especially if the dog is older than 6-7 months or has a questionable history (e.g., rescue dog, unknown past).
After a Lapse in Prevention: If your dog has missed doses or gone for a period without prevention.
Puppies: Puppies born to heartworm-positive mothers can have circulating microfilariae. However, they won’t test positive on an antigen test until they are at least 6-7 months old. Typically, puppies are started on prevention around 8 weeks old and then tested around 7-9 months of age, and annually thereafter.
What if My Dog Tests Positive?
If your dog tests positive for heartworm disease, your veterinarian will likely recommend:
Confirmatory testing: To ensure accuracy.
Additional diagnostics: Such as chest X-rays and blood work to assess the severity of the disease and overall health of your dog.
Treatment plan: Heartworm treatment is complex, lengthy, expensive, and involves medication (often an arsenic-based drug) to kill the adult worms, along with strict exercise restriction to prevent complications from dead worms.
Prevention is Key!
Heartworm disease is preventable. Year-round heartworm prevention is highly effective and significantly safer and less expensive than treating the disease itself. Discuss the best heartworm prevention option for your dog with your veterinarian.
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