Heartworm Disease in Dogs: Testing

What is heartworm disease?

Heartworm disease is caused by a parasite called Dirofilaria immitis, better known as heartworm. Heartworms live in the right side of the heart and the pulmonary artery (the large blood vessel that carries blood from the heart to the lungs).

Dogs become infected when they are bitten by an infected mosquito carrying infective heartworm larvae, and the larvae pass from the mosquito into the dog.

Heartworm disease often causes little or no illness in its early stages, so infected dogs may appear healthy. Annual testing allows infection to be detected before serious heart or lung damage develops.

Why does my dog need heartworm testing?

The American Heartworm Society recommends yearly heartworm testing for all dogs over seven months of age, even if they are receiving heartworm preventive medication. Testing is important because:

• Heartworm disease may not cause obvious signs until it is advanced.
• Missed or delayed doses of heartworm preventive can allow infection to develop.
• No preventive medication is 100% effective.
• Early diagnosis allows treatment before more severe damage occurs.

Do all dogs need to be tested?

No, puppies less than six to seven months of age do not need to be tested for heartworm. Adult worms are not present at this age and both antigen tests and microfilariae tests will be negative.

How is heartworm disease diagnosed in dogs?

Heartworm disease is usually diagnosed with a simple blood test. There are two main tests for detecting heartworm infection: one test detects adult worms and the other detects microfilariae (immature worms). The American Heartworm Society recommends yearly testing for all dogs over seven months of age using both the antigen and microfilaria test.

Testing for Adult Worms

The heartworm antigen test detects the presence of adult female heartworms by testing for antigen — proteins released by adult female heartworms that can be detected in the blood. Antigen is detectable for five to seven months after infection and positive results may be obtained with as few as one to three adult females in the heart. Antigen tests will be falsely negative if:

• The infection has been present for less than five months (it is too soon for adult worms to produce detectable antigen)
• The worms are all male or all immature females
• There are very low numbers of worms
• Technical problems occur during testing

Testing for Microfilariae

Any positive or weakly positive antigen test should be followed by a test for microfilariae. The presence of microfilariae confirms that mature adult worms are present and helps guide treatment.

The best tests for detecting microfilariae are concentration tests. The preferred test is the modified Knott's test, which uses a centrifuge (a machine that spins the sample) to concentrate the microfilariae. Another common test is the filter test, which involves passing the sample through a very fine filter that traps the microfilariae. In both methods, microfilariae are identified under a microscope.

Microfilariae tests may be falsely negative if:

• Worms are not yet mature enough to reproduce
• All the adult worms are the same sex
• Too few microfilariae are circulating in the bloodstream to be detected

What about the DNA-PCR test?

The DNA-PCR test, which detects heartworm DNA, is not routinely used to screen dogs for heartworm infection. However, if microfilariae are found and there is any doubt about their identity, then the DNA-PCR is helpful to confirm that they are Dirofilaria immitis and not another kind of blood parasite.

What other tests may be recommended?

In some infected dogs, blood tests are negative even though there are heartworms in the heart. The following tests may give evidence of heartworm disease:

• Blood tests. Abnormalities in the complete blood count (CBC) and blood tests for kidney and liver function may suggest the presence of heartworm disease. These tests are often performed on dogs diagnosed as heartworm-infected, to assess organ function and health status before treatment.
• Radiographs (X-rays). An X-ray of a dog with heartworms will usually show heart enlargement and swelling of the large artery (pulmonary artery) leading from the heart to the lungs. X-rays may also reveal the overall condition of the heart, lungs, and pulmonary vessels. These findings can indicate an increased risk of treatment-related complications.
• Electrocardiogram (EKG or ECG). An electrocardiogram traces the electric currents generated by the heart. It is most useful to determine the presence of abnormal heart rhythms. This test can also detect enlargements in heart chamber size and help determine if a dog can safely undergo heartworm disease treatment.
• Echocardiography (ultrasound of the heart). Echocardiography shows the heart chambers, allowing the general status of the heart to be evaluated. Heartworms can also be seen within the heart and pulmonary artery.

What happens if my dog tests positive? 

If your dog tests positive for heartworm disease, your veterinarian will recommend additional testing to determine the severity of infection and develop the safest treatment plan. Treatment usually includes two types of medication (one to kill the adult heartworms and one to kill the microfilariae), along with strict exercise restriction during recovery.

Additional medications, such as anti-inflammatory drugs, antihistamines, and antibiotics (e.g., doxycycline) may be used to reduce side effects and improve response to treatment. If serious complications arise, hospitalization for further treatment may be needed.

How can I prevent heartworm disease in my dog?

You can prevent your dog from getting heartworms by:

• Using a heartworm preventive approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In the United States, using a preventive is recommended year-round.
• Reducing mosquito exposure. The American Heartworm Society recommends using an FDA- or EPA-approved insecticide designed for dogs to minimize mosquitos. Limiting outdoor activities during the peak mosquito times of dawn and dusk will also help prevent heartworm infection.
• Preventing or reducing standing water where mosquitos breed.

Dogs that have been successfully treated for heartworms are still vulnerable to reinfection. Annual heartworm testing followed by preventive medication is recommended to keep your dog free of heartworm disease. Many excellent preventive products are available to prevent heartworm disease in dogs. Your veterinarian can advise you on which product is most suitable for your pet, and whether year-round treatment is necessary.