Does My Dog Need a Heartworm Test?

Yes. Heartworm disease is a serious—and potentially fatal—disease in dogs, and all dogs should be tested annually for heartworm infection even if your dog takes a monthly heartworm preventive all year long. Here’s some additional information you’ll want to know about heartworm testing and your dog.

A vet inspects a happy dog while the owner stands near

How Often Should I Get a Heartworm Test for My Dog?

You should get your dog tested for heartworms every year. It’s the only way to know if your dog has heartworms and can typically be done during your dog’s regular exam with your veterinarian.¹

Even if your dog is on a heartworm preventive, they should still be tested to make sure the preventive program is working. Even though preventives are highly effective, they aren’t perfect—and missing just one dose, or giving a dose late, can leave your dog vulnerable.¹ So, annual testing is key to diagnosing infections early.

Heartworm disease has been diagnosed in all 50 states in the US, so the threat of heartworm disease is very real. That’s why the American Heartworm Society recommends that you:

Get your dog tested every 12 months for heartworm
Give your dog heartworm preventive 12 months a year¹

A woman kisses the top of her dog's head

How Much Does a Heartworm Test Cost?

A heartworm test may range anywhere from $35 to $75. However, heartworm disease is an extremely serious condition depending on how far the disease has progressed.²

Annual heartworm tests and year-round heartworm prevention cost far less than the significant expense of treating a heartworm infection in your dog.²

Are There Different Types of Heartworm Tests?

Antigen Tests

These tests detect a substance produced by female heartworms. This is the fastest and most convenient method of testing. This method can diagnose infections older than 6-7 months.

Antigen tests can cause false negatives for several reasons, including:

The infection is less than 6-7 months old, which means the dog is infected but it’s too soon for antigen to be present
The worms are all male or all immature females
Technical difficulties

Microfilaria Tests

These tests examine blood to identify the existence of microfilariae, which are heartworm offspring. Veterinarians typically use this method to validate a positive antigen test. Detection of microfilariae confirms the existence of adult heartworms if the microfilariae are actually from heartworms. There are other filarial worms that infect dogs, so an accurate diagnosis is key.

Microfilaria tests can cause false negatives if:

The worms are not mature enough to mate and produce microfilariae
All adult worms are the same sex, which means reproduction cannot occur
There are too few microfilariae in the bloodstream to be detected

Illustration of a heart with heartworm disease

What Does Heartworm Positive Mean?

If your veterinarian tells you your dog is heartworm positive, it means they detected adult heartworms in your dog. Your veterinarian should use a combination of the above modes of testing to ensure there isn’t a false positive. If the diagnosis is confirmed, treatment should begin as soon as possible to prevent the disease from progressing. Your vet can walk you through your dog’s treatment options and answer any questions you may have.

Can Heartworms Be Cured in Dogs?

Like most medical problems, preventing heartworm disease is a much better option than treating it, as heartworm disease causes permanent damage to the arteries of the lungs.

If your dog does develop heartworm disease, treatment is available but can be quite expensive, especially if the disease is in an advanced stage. Treatment also comes with several risks, and serious complications can occur. Dogs with mild signs of heartworm disease have a high success rate, while dogs with more severe disease have a greater possibility of complications.

The end goal of heartworm treatment is to kill off the adult worms and the microfilariae as safely as possible. This process causes heartworms to die inside your dog’s body and lungs, so further medication may be needed to manage the body’s inflammatory response. It also means that during treatment, restricting your dog’s exercise and activity is an absolute necessity. In the most extreme cases, heartworms may need to be surgically removed. However, this is a high-risk procedure reserved for, and only possible in, more advanced disease stages.³

Can a Dog Get Heartworms From Another Dog?

Only an infected mosquito transmits heartworm disease to a dog. Heartworms require intermediate hosts (mosquitos) to infect new animals, so if a mosquito bites a heartworm positive dog, that mosquito can carry the larvae with it to infect another dog with just one bite. So, if you have a pet with heartworm disease, it’s possible that a mosquito could also infect another pet with the disease.

Can I Give My Dog Heartworm Preventive Without Testing?

For Dogs Younger Than 7 Months

You can start puppies younger than 7 months on heartworm prevention without a heartworm test. That’s because it takes at least 6-7 months for a dog to test positive after infection. However, the puppy should be tested 6 months after the initial visit, tested again 6 months after that, and then tested annually moving forward to ensure they’re heartworm-free.¹

A small brown puppy scampers through the grass

For Dogs Older Than 7 Months

Dogs that are older than 7 months and not already on a heartworm preventive should be tested prior to starting a heartworm medicine. Your veterinarian may recommend testing these dogs 6 months after the initial test, and then annually moving forward.¹

Dogs That Miss a Dose or Stop Taking Heartworm Preventive

If your dog has a lapse in heartworm prevention, you should consult your veterinarian, immediately restart your dog on the monthly preventive, and have another heartworm test taken based on your veterinarian’s recommendation. This waiting period before retesting is important because heartworms must be approximately 7 months old before an infection can be discovered.¹