Two types Heartworm in Dogs – Baby (Microfilaris) and Adult (Occult). Which type of treatment is suitable for pets is decided after test. To know more about treatment visit

Heartworm Testing for Dogs

Why Should We Test For Heartworms?

Testing for heartworms is important because heartworm disease is a serious and fatal disease of dogs and cats. The sooner heartworm infection is detected the higher the chance of successful treatment. Heartworm testing is recommended twice a year by Dr. Garner and testing is required once a year for administration of heartworm prevention.

But once your pet has been tested negative and on prevention, why do you need to continually repeat heartworm tests?

  1. The Heartworm Prevention is licensed by the FDA to be administered by a veterinarian to a heartworm negative dog.  The only way a veterinarian can say the pet is negative for heartworm is to perform the test.

  2. Only 44% of pet owners correctly follow the administration instructions of heartworm preventatives.

  3. A small percentage of pets have a false negative result because the infection is too early to be picked up on the test. Regular testing ensures that these infections are not missed.

  4. Administration of most of the heartworm preventatives to heartworm positive pets can cause a severe fatal allergic reaction.

How can a heartworm preventative cause a severe fatal reaction?

1)       Most pets infected with adult heartworms also have baby heartworms called microfilaria.

2)       In many of these pets there are over 5,000 of these baby worms in a cc of blood.  This is 5,000 per thimbleful of blood!

3)       Heartworms are almost a perfect parasite and the dogs system does not “know” or detect the presence of these worms.  These 5,000 worms are invisible.

4)       When heartworm prevention is given these worms become “visible” to the dogs system throwing them into shock – the shock of now knowing there are millions of worms in their blood.

Heartworm Tests – Microfilaria and “Occult”

Microfilaria Tests – Past Present and Future

The microfilaria heartworm tests as it suggests depends on finding microfilaria in the blood of infected dogs.  Microfilaria are baby (L1) heartworms and their presence indicates that there are adult female heartworms present in the heart.  In the past three blood samples tests were collected at different times of the day to maximize the sample collection with the timing of the microfilaria activity in the blood:  One sample collected in the morning one at noon and one in the evening.  The blood could then be processed in one of three ways to find the microfilaria.

1)       The blood smear test uses one drop of blood on a slide covered with a coverslip.  The slide is examined under a microscope for heartworm microfilaria.

2)       The Knotts Test uses 1cc of blood that is fixed with formalin and centrifuged to concentrate the microfilaria.

3)       The Difil Test is a filter test that filters 1cc of lysed blood through a clear plastic filter.  The filter is placed on a slide, stained and examined for microfilaria.

The Knotts Heartworm Test and the Difil Tests are more likely to find microfilaria due to the larger blood sample that is examined.  The blood smear test is diagnostic when microfilaria is found, but may miss infections with fewer numbers of microfilaria.  Since these tests check only for microfilaria, they are limited in their ability to detect heartworm infection and may miss up to 40% of the cases.  The number of microfilaria in the blood does not correlate with either the number of heartworms present or with the severity of the heartworm disease.

Microfilaria testing also has the possibility of diagnosing heartworm disease when there are no adult heartworms.  Microfilaria can be confused with another parasite that is similar in appearance.  Microfilaria may also be present in a dog that had a blood transfusion from an infected dog.  Puppies that have been born from infected mothers have also been reported to have microfilaria without adult heartworms.

The parasite Acanthocheilonema  (previously Dipetalonema reconditum) is very similar to Dirofilaria immitis (heartworm) in appearance but it is not harmful to the dog.  Below is a table that contrasts these two filarid parasites.

Characteristics of Infection By Dirofilaria immitis and

Acanthocheilonema (Dipetalonema) reconditum


 D. Immitis


Hosts  Dogs, Cats, ferrets, humans, Dog  Marine mammals, etc
Vector     Mosquitoes
Adult sites Heart, pulmonary arteries
Disease Yes
Treatment Yes
Head  Tapered
Tail Shape Straight
Thickness Thicker than RBC Thinner than RBC
Length Longer than 290 microns Shorter than 275 u

Microfilaria tests are less popular today because they are less accurate than antibody tests and may falsely indicate heartworm infection where no adult heartworms are present.

Occult Heartworms

What is meant by Occult heartworms. Quite simply this means that adult heartworms may be present in the heart without microfilaria (heartworm babies) being present in the blood.  This is called an occult heartworm infection.  Occult heartworm disease can be caused by the following:

1)       All the adult heartworms are of one sex.

2)       The adult worms have been made sterile or the microfilaria may have been killed by the heartworm prevention medication that the dog may have been previously given.

3)       An antibody reaction in the lungs has killed all the microfilaria.  The most common reason for occult infections is an antibody mediated reaction in the blood vessels of the lungs that kills the microfilaria just as they are born.

4)       The adult heartworms in heart are too young to have produced microfilaria.

Occult Heartworm Tests

How do we test for occult heartworm infection?

For most pets a simple blood test is all that is required for heartworm testing.  If a pet tests negative for heartworms and there are indications that heartworm disease is present ultrasound and radiographs will be performed.

ELISA Serum Tests

Occult heartworm disease literally means “hidden” heartworm disease.  Heartworm infections in the heart that are not producing microfilaria are considered hidden heartworm infections or occult heartworm infections.  Adult heartworms can be diagnosed by a serum test that detects chemical, fecal, and uterine discharges of adult heartworms.  This test uses a technology called Enzyme Linked Immuno Sero Assay (ELISA.) This test uses a antibody that has been manufactured against the female heartworm secretions.  This antibody is attached to an enzyme.  When the antibody comes in contact with the heartworm secretion it changes shape.  This shape change causes the enzyme to glow blue.  A blue color change is a positive reaction for heartworm secretions.  These secretions come only from mature, adult, female heartworms.  Immature heartworms, male heartworms or young L5 stage heartworms will not be detected.


Radiographs of the chest in animals that have no microfilaria but are suspected to have heartworms have long been the diagnostic test relied upon to determine occult heartworm disease.  Radiographs are helpful but the actual heartworms cannot be seen.  The signs of heartworm infection can be seen but the infection may be advanced prior to these signs developing.


Ultrasound can detect heartworms if the infection is severe but it is not a good test for screening dogs for occult heartworms.

How often should pet be tested for heartworms?

Heartworm testing is recommended every six months in heartworm endemic areas such as the Gulf Coast due to the high incidence of heartworms in these areas. In non-endemic areas heartworm testing should be carried out once a year.