Testing for Heartworm Disease in Dogs

What is heartworm disease? How does a dog become infected?

Heartworm disease is caused by a parasite called Dirofilaria immitis, better known as heartworm. The parasites are long, hair-like worms that live in the right side of the heart and the pulmonary artery, which is 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.

The larvae pass from the mosquito into the dog and move through the tissues of the body, eventually entering the bloodstream and migrating to the right ventricle of the heart. Within the heart, the worms mature into adults, mate, and produce immature worms called microfilariae, which circulate in the bloodstream. 

When the infected dog is bitten by another mosquito, the microfilariae are taken up by the mosquito. Inside the mosquito, the microfilariae develop to become infective larvae. These infective larvae move into the mouth parts of the mosquito, where they wait until the next time the mosquito bites a dog. It takes about 5 to 6.5 months for the heartworm life cycle to be completed.

Where is heartworm infection most common?

Heartworm disease is widespread in the United States and is particularly common along the southeastern and gulf coasts, and along the Mississippi River valley. In Canada, heartworm infection is more restricted and is localized in southern Ontario, southern Manitoba, and southern Quebec, with scattered occurrences elsewhere in the country (including British Columbia and the Atlantic provinces).

"The risk of infection is greatest when mosquitoes are actively feeding."

The risk of infection is greatest when mosquitoes are actively feeding, which typically requires temperatures over 50°F (10°C). In areas that get prolonged killing frosts, the risk of heartworm infection is highest in the warmer months (late spring to late fall). In much of the United States, however, heartworm infection is a year-round risk.

Can infection be spread directly between dogs, from dog to cat, or from dogs to people?

No, dogs can only get heartworm from an infected mosquito. Infection does not spread from dog to dog, dog to cat, or dog to people. However, both cats and dogs can be infected by heartworm if bitten by an infected mosquito.

What are the clinical signs of heartworm disease?

In the early stages of disease, dogs often have no clinical signs, especially if they are carrying only a few worms. As the disease progresses, clinical signs become more noticeable and include reluctance to exercise, rapid fatigue with exercise, coughing, and sometimes collapse. In advanced disease, dogs develop congestive heart failure. Dogs in congestive heart failure lose weight, have a poor body condition, breathe rapidly or with difficulty, and develop a build-up of fluid in the abdomen.

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. The American Heartworm Society recommends yearly testing for all dogs over 7 months using both the antigen and microfilaria test.

Testing for Adult Worms: The Heartworm Antigen Test is specific for adult female heartworms. Antigen is detectable by 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 5 months (dog is infected, but it is too soon for adults, so there is no antigen present);
• the worms are all male or all immature females (no adult female worms);
• there are very low numbers of worms (level of antigen is too low to detect); or
• there are technical difficulties doing the test (test should be repeated).

Testing for Microfilariae: Any antigen test that is positive or "weak" positive should be followed up with a test for microfilariae. The presence of microfilariae confirms that mature adult worms are present in the heart and indicates the need for specific treatment to kill microfilariae. 

The best tests for detecting microfilariae are called concentration tests. The preferred test is the modified Knott's test, which involves using a centrifuge (a machine that spins the sample very quickly in a small circle) 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 tests, microfilariae are detected and identified using a microscope.

Microfilariae tests may be falsely negative for several reasons, including:

• none of the adult worms are mature enough to mate and produce microfilariae;
• all the adult worms are of one sex, so mating cannot occur; or
• there are too few microfilariae in the bloodstream to be detected (mating is just beginning, or there are too few adults to produce large numbers of microfilariae).

What about the DNA-PCR test?

This test, which detects the DNA of the heartworm, 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 methods are used to detect heartworm infection?

In some infected dogs, blood tests are negative even though there are heartworms in the heart.

Other blood tests (CBC, blood chemistries, electrolytes). 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 prior to 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. These signs are considered presumptive evidence of heartworm disease. X-rays may also reveal the condition of the heart, lungs, and pulmonary vessels. This information allows us to predict an increased possibility of complications related to treatment.

Electrocardiogram (EKG or ECG). An electrocardiogram is a tracing of 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). An ultrasonic examination shows the heart chambers, allowing the general status of the heart to be evaluated. Heartworms can also be visualized within the heart and pulmonary artery.

Do all dogs need to be tested?

No. Puppies less than 6–7 months of age do not need to be tested. Adult worms are not present at this age and both antigen tests and microfilariae tests will be negative.

Is there treatment for heartworm disease?  

Treatment usually involves two types of medication: one to kill the adult heartworms and one to kill the microfilariae. Treatment is usually effective, although there are cases where small numbers of adult heartworms remain after treatment.

There are frequently side effects associated with heartworm treatment due to dead worms being pumped out of the heart into the lungs. Side effects are more common in dogs with large numbers of adult heartworms, but all infected dogs are at risk. To minimize side effects, all dogs undergoing treatment must be kept very quiet during the treatment period and for four weeks afterward, even if they are not showing signs of illness. Other drugs, such as anti-inflammatory medication, 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. For further information on treatment, please see the handout "Heartworm Disease in Dogs: Treatment".

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 FDA or Health Canada. In the United States, using a preventive is recommended year-round. Reducing mosquito exposure is also important in prevention. The American Heartworm Society recommends using an FDA or EPA-approved insecticide designed for use on dogs to minimize mosquitos. Prevention or reduction of standing water and limiting outdoor activities during the peak mosquito times of dawn and dusk will also help prevent heartworm infection.

Dogs who 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. Several 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.