Most dogs with heartworms show no symptoms at all in the early stages of infection. A dog can appear perfectly healthy on the outside while worms are growing and multiplying inside the heart and lungs. The only reliable way to know for sure is a blood test at your vet’s office, but there are warning signs worth watching for, especially if your dog has missed doses of preventive medication or has no prevention history.
Why You Can’t Rely on Symptoms Alone
Heartworm disease can take months to produce any visible signs, and some dogs never show obvious symptoms even with a significant number of worms. Sedentary dogs, in particular, may carry a heavy worm burden without tipping you off. The severity of symptoms depends on how many worms are present, how long the infection has lasted, how active your dog is, and how their immune system responds. A couch-potato dog might harbor dozens of worms with barely a cough, while an active hunting dog with the same infection could show dramatic signs.
Symptoms by Stage
Heartworm disease is classified into stages based on how far it has progressed. In the earliest stage, you might notice nothing more than an occasional cough, or no symptoms at all. This is the stage where most infections are caught by routine testing rather than by observation.
As the disease moves into a moderate stage, dogs develop a cough that shows up more regularly and become noticeably tired after moderate exercise. You might see your dog lagging behind on walks that used to be easy, or taking longer to recover after playing.
In more advanced cases, the signs become harder to miss:
- Persistent cough that doesn’t go away
- Exercise intolerance, where even mild activity causes fatigue
- Labored breathing
- Loss of appetite and weight loss
- A sickly, run-down appearance
- Swollen belly from fluid accumulation in the abdomen
- Bluish or purplish gums and skin, indicating poor circulation
In rare, life-threatening cases, dogs may cough up blood, faint, have nosebleeds, or collapse suddenly. These are signs of severe cardiovascular damage that require emergency veterinary care.
How Vets Test for Heartworms
The standard screening is an antigen test, which detects a protein released by adult female heartworms. It requires just a small blood sample and results are often available within minutes at your vet’s office. This is the most sensitive method for detecting an infection, but it has a critical limitation: it takes six to seven months after a mosquito bite for the worms to mature enough to produce detectable levels of antigen. If your dog was infected recently, the test can come back negative even though worms are developing.
Your vet may also run a second blood test that looks for microfilariae, the microscopic baby worms that circulate in the bloodstream. The American Heartworm Society recommends running both tests together, especially for dogs with an unknown prevention history, like shelter adoptions. Finding microfilariae in the blood confirms the infection and tells the vet that your dog could pass heartworms to other animals through mosquito bites.
When Tests Can Miss an Infection
False negatives happen more often than you might expect. There are several reasons a test might come back clean when worms are actually present:
- The infection is too new. Worms need six to seven months to mature to a detectable stage. Testing earlier than that will miss the infection entirely.
- Only male worms are present. The antigen test detects a protein from adult females, so an all-male infection won’t trigger a positive result.
- Very few female worms. Low antigen levels from a small number of females can fall below the test’s detection threshold.
- Immune complexes. In some infected dogs, the body’s own antibodies bind to the heartworm antigen, essentially hiding it from the test. A study of shelter dogs in the southeastern United States found this occurred at a rate of about 7%.
This is exactly why vets sometimes recommend the microfilariae test alongside the antigen test. A dog can test negative on the antigen screen but positive on the microfilariae test, catching infections that would otherwise slip through.
What Happens If the Test Is Positive
A positive antigen test is typically just the starting point. Your vet will likely want additional information to understand how far the disease has progressed and how much damage has already occurred. This usually involves chest X-rays to look at the heart and lungs, blood work to assess organ function, and sometimes an echocardiogram (an ultrasound of the heart) to visualize the worms directly and evaluate heart function.
These results help determine the best treatment approach. A dog caught at an early stage with minimal lung and heart damage has a very different treatment path than one with advanced disease and organ complications.
How Often to Test
The American Heartworm Society recommends annual testing with both the antigen and microfilariae tests, even for dogs on year-round preventive medication. Prevention is highly effective but not foolproof. A dog might spit out a chewable tablet without you noticing, a topical dose might not absorb fully, or a dose might be given late. Annual testing catches any gaps before the infection has time to cause serious damage.
Puppies can start heartworm prevention as young as eight weeks old. Because it takes six to seven months for an infection to become detectable, puppies starting prevention on schedule are generally tested for the first time around seven months of age, then again six months later, and annually after that.
If your dog has been off prevention for any period, or if you’ve recently adopted a dog with an unknown history, testing is especially important. Keep in mind that a negative result today doesn’t rule out an infection from the past several months, so your vet may recommend retesting six months later to cover the detection window.