Heartworm Testing & False Negatives

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, 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, signs become more noticeable & 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, & 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. Heartworm testing is recommended for all dogs 6-7 months & older if the pet was adopted or found, 6-7 months after missing one or more doses of preventive, or on an annual basis if the pet is on preventive continuously.  

Testing for Adult Worms: The Heartworm Antigen Test is specific for adult female worms. 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) - this is a prepatent infection;
• 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. 

False negative results are most common when a dog is adopted, especially if the pet hasn't been previously given a preventive on a routine basis.  If your pet was adopted & tests negative, he/she should be started on a preventive, but we also recommend repeating a heartworm test 6 months after the first, or at least at the following annual exam. 

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?  

Yes, though it is a lengthy & expensive process, covered in another handout.  Prevention is definitely preferred!  

How can I prevent heartworm disease in my dog?

You can prevent your dog from getting heartworms by using a heartworm preventive. 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.