CHW Ag Test Kit 2.0
What is heartworm disease? How does a dog get infected?
Canine Heartworm antigen (CHW Ag) 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 ventricle 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 immature and infective heartworm larvae.
The larvae pass from the mosquito to the dog and move through body tissues, eventually entering the bloodstream and migrating to the right ventricle of the heart. Inside 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 absorbed by the mosquito. Inside the mosquito, microfilariae develop to become infective larvae. These infective larvae move to the mosquito’s mouthparts, where they wait until the next time the mosquito bites a dog. The heartworm life cycle takes between 5 and 6 and a half months to complete.
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 through the Mississippi River Valley. In Canada, heartworm infection is more restricted, localizing to southern Ontario, southern Manitoba, and southern Quebec, with scattered cases in other parts of the country.
The risk of infection is higher when mosquitoes are actively feeding. This typically requires temperatures above 50°F (10°C). In areas that experience prolonged deadly frosts, the risk of heartworm infection is greatest in the warmer months (late spring to late fall). However, in much of the United States, heartworm infection is a year-round risk.
Can the infection be transmitted directly from dog to dog, from dog to cat, or from dog to person?
No. Dogs can only get heartworm from an infected mosquito. There is no spread of heartworm infection from dog to dog, dog to cat, or dog to person. However, both cats and dogs can become infected with heartworm if bitten by an infected mosquito.
What are the clinical signs of heartworm disease?
In the early stages of the disease, dogs often show no clinical signs, especially if they only carry a small number of worms. As the disease progresses, clinical signs become more prominent and include reluctance to exercise, rapid fatigue with exercise, coughing, and sometimes collapse. In advanced diseases, dogs develop congestive heart failure. Dogs with congestive heart failure lose weight, have poor body condition, are breathing rapidly or wheezing, and develop a buildup of fluid in the abdomen.
How is heartworm disease in dogs diagnosed?
Heartworm disease is usually diagnosed with a simple blood test. There are two main tests to detect heartworm infection; one test detects adult worms and the other detects microfilariae.
Testing for adult heartworms: The American Heartworm Society recommends using the heartworm antigen test as the primary method of testing for adult heartworm infection. This test is specific for adult female heartworms. The antigen is detectable 6½ to 7 months after infection and positive results can be obtained with as few as 1 to 3 adult females in the heart.
Antigen tests will be falsely negative if:
- the infection has been present for less than 5 months (the dog is infected, but it is too early for adults, so no antigen is present).
- the worms are all male or all immature females (no adult female worms).
- there are very low numbers of worms (antigen level too low to detect).
- there are technical difficulties in doing the test itself (the test must be repeated).
Microfilariae tests: Any antigen test that is positive or “weak” positive should be followed up with a microfilariae test. 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 to detect 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 rapidly 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 microfilariae. In both tests, microfilariae are detected and identified using a microscope.
Microfilaria tests can be falsely negative for a number of reasons, including:
- none of the adult worms is mature enough to mate and produce microfilariae.
- all adult worms are of the same sex, so mating cannot occur.
- 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 heartworm DNA, is not yet sensitive enough to detect heartworm infection in dogs. However, if microfilariae are found and there is any doubt about their identity, then DNA PCR is useful to confirm that it is Dirofilaria immitis and not another type of blood parasite.
What other methods are used to detect heartworm infection?
In some infected dogs, blood tests are negative even though heartworms are present in the heart.
- Other blood tests (CBC, blood chemistry, electrolytes). Complete blood count (CBC) abnormalities and blood tests for kidney and liver function may suggest the presence of heartworm disease. These tests are often performed on dogs diagnosed with heartworm disease, to assess organ function and health status prior to treatment.
- Radiographs (X-rays). An x-ray of a dog with heartworms will usually show an enlarged heart and inflammation of the large artery (pulmonary artery) that leads to the lungs from the heart. These signs are considered presumptive evidence of heartworm disease. X-rays can also reveal the condition of the heart, lungs, and pulmonary vessels. This information allows us to predict a greater chance of treatment-related complications.
- Electrocardiogram (EKG or ECG). An electrocardiogram is a monitoring of the electrical currents generated by the heart. It is most useful in determining the presence of abnormal heart rhythms. This test can also detect enlargements in the size of the heart chamber and help determine if a dog can safely undergo heartworm treatment.
- Echocardiography (ultrasound of the heart). An ultrasonic examination shows the chambers of the heart, allowing the overall condition of the heart to be assessed. 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 and microfilariae tests will be negative.
Is there treatment for heartworm disease?
Treatment usually involves two types of drugs: one to kill adult heartworms and one to kill microfilariae. Treatment is usually effective, although there are cases where small numbers of adult heartworms remain after treatment.
There are common side effects associated with heartworm treatment because dead worms are pumped from 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 treated dogs, even if not showing signs of illness, should remain very calm during the treatment period and for 4 weeks afterwards.
Other drugs such as anti-inflammatory drugs, antihistamines, and antibiotics (eg, doxycycline) may be used to reduce side effects and improve response to treatment. If serious complications arise, hospitalization may be necessary for further treatment. For more information on treatment, see our brochure “Heartworm Disease in Dogs – Treatment”.
How can I prevent heartworm disease in my dog?
Annual heartworm testing followed by preventative medication is recommended to keep your dog free of heartworm disease. There are several excellent preventative products available to prevent heartworm disease in dogs. Your vet can advise you on which product is best suited for your pet and whether year-round treatment is necessary.
Dogs in the subtropical United States are at risk of heartworm disease year-round, while dogs in Canada, the northern United States, and Alaska are most at risk during the warmer months (late spring through late fall). Dogs from lower-risk areas traveling to high-risk areas should take preventative medication during the travel period, regardless of the time of year.