THE PET HEALTH LIBRARY By Wendy C. Brooks, DVM, DipABVP Educational Director, VeterinaryPartner.com

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The chief issue in the diagnosis of heartworm disease centers around detecting heartworm infection. The following are a list of diagnostic methods that have been and are used in the detection of heartworms within the dog's body.

Because of the 5-month minimum migration time
of the larval heartworm after infection,
it is useless to test dogs
younger than 5 months of age for heartworm.
These puppies are too young to have adult heartworms
and are too young to test positive under any circumstances.

Direct Blood Smear

heartworm microfilaria under the microscope

While this method is a simple screening test, it is not recommended as a regular test method. In this test, a single drop of blood is examined under the microscope to check for live microfilariae. If a large number of microfilariae are present, chances are at least one will be seen swimming by. Of course, detection of heartworm larvae in this way requires large numbers of larvae (less than 20 to 50 microfilariae per ml of blood will not be detected). More subtle infections will be missed.

Difil Test and Knott’s Test

heartworm microfilaria as seen via a Knott's test

These tests represent concentration methods for the detection of microfilariae so that more subtle infections can be detected. In these tests, a larger amount of blood is either centrifuged or filtered to concentrate any microfilariae. Either of these methods is appropriate for a microfilaria test. Nowadays, microfilaria testing is usually done in conjunction with antigen testing (see below). In areas where heartworm infection is not common, more than one type of test should be performed for the sake of accuracy.

Dipetalonema Reconditum: The other Microfilaria

Heartworm is not the only species of worm that has circulating microfilariae as a first stage larva. There is another parasite called Dipetalonema reconditum, which is transmitted by fleas that can be picked up by the microfilaria tests. Subtle differences in the swimming characteristics, nose structure, and tail structure can be used to distinguish this harmless larva from the more serious heartworm microfilaria. Before immunotechnology, there was an element of diagnostic challenge when microfilaria were detected in the blood (was the nose the shape that would justify a harsh and possibly life-threatening treatment for heartworm infection or was it a harmless creature for which no treatment was needed?) The advent of antigen testing has made this distinction much easier to make.

Antigen Testing

Using genetic engineering, it has become possible to create extremely sensitive tests capable of detecting tiny pieces of adult heartworm skin circulating in the blood. In this way, very small numbers of adult worms can be detected - even single worm infections. This kind of testing has made it possible to detect infections in which no microfilariae are present: the so-called occult infections. Most of these immunological tests are available as test kits that can be performed in the veterinarian's office while you wait.

IDEXX snap test for heartworm, an example of an ELISA test for in house use

There are many important reasons why a dog might be infected with adult heartworms yet no microfilariae can be detected:

• Single sex infections or single worm infections
  Younger female worms tend to be resistant to the drugs used to clear adult worms. This means that after the first treatment with anti-heartworm medication, a group of young female worms will be left. If antigen testing is not performed, you could be misled into thinking that the infection had been completely cleared.
• Immunologic destruction of microfilariae
  In cats, the period in which microfilariae can be detected in the blood stream is extremely short as the immune system rapidly groups and clears these larval worms.
• The dog is taking heartworm prevention medication

Milbemycin, ivermectin, moxidectin, and selamectin are able to kill circulating microfilariae (but not adult heartworms). If for some reason, a dog is not properly tested and has missed a dose of medication, it is possible for an infection to establish yet no microfilariae will be detected. People commonly ask why they must continue annual testing in animals that are on preventive medication; this is one reason.

Since Dipetalonema microfilariae can be mistaken for heartworm larvae and since microfilariae can be transferred to unborn puppies (but adult heartworms cannot), it is felt by many specialists that no microfilaria positive dog should be treated for heartworm without a positive antigen test.

Antibody Testing

Antigen tests have limited usefulness when it comes to infections involving only a few worms. One problem is that the antigens detected by the tests are unique to female worms. For most tests, at least three female worms are needed for the test to show a positive result. Animals infected with only male worms will test antigen negative. To get around these limitations, antibody tests have been developed to detect the host's immune response against the parasite. This kind of breakthrough has been especially important in testing for cats in whom infection with one or two worms is the usual situation. Antibody testing may be able to detect infection sooner than can antigen testing and infections involving only male worms can be readily detected. As a general rule, antigen tests are performed for dogs with or without a microfilaria test and cats get antibody tests sometimes with antigen tests as well.

How often Should a Dog be Tested?

This is a hard question to answer. With differences in weather patterns every year (i.e. early spring one year, late freeze another year, etc.), it becomes difficult to figure out in what month heartworm preventive medication should be initiated. If there is any question about when the last possible transmission date was in the previous season, it is probably a good idea to simply test the dog annually.

If there is any question about the dog spitting out any preventive tablets last season , then the dog should be tested before beginning prevention the next season.

If the dog consistently takes monthly heartworm preventive medication all year round, then testing can probably be performed every 2 to 3 years (though an annual physical exam is needed to legally obtain a prescription for medication).

If a dog lives in an area considered not to have heartworm and travels to an area that does, both microfilaria and antigen testing should performed at least seven months after the last day the dog was in the heartworm endemic area.

For more information on the different medications used in the prevention of heartworm infection, see heartworm prevention.

Radiographs and Ultrasound

In areas where heartworm infection is not common, routine testing is not performed and infection is unlikely to be detected until the dog is sick. In these cases, the infection is picked up during a work-up for heart disease.
Radiographs can be strongly indicative of heartworm infection if they show blunted tortuous pulmonary arteries and enlargement of the right side of the heart (the side doing all the extra work to pump blood past the worms).

Ultrasound not only can measure thickening of the right side of the heart but can show the live worms wiggling.

When imaging tests are consistent with heartworm disease, antigen and/or microfilaria testing would be the obvious next step.

See heartworm treatment.

Date Published: 1/1/2001
Date Reviewed/Revised: 07/07/2008