Canine Heartworm Diagnostics

What are clinical signs of heartworm disease?

Fatigues easily; intolerant of exercise

Cough

Cyanotic mucous membranes

Heart murmur

Tachycardia

Ascites

Syncope

What is the life threatening emergency caused by heartworms called?

Caval syndrome

What are the clinical signs of Caval syndrome?

Lethargic, weak

Pale mucus membranes

Severe passive congestion of the liver

Coarse systolic murmur of tricupsid regurgitation

Enlarged jugular vein and bounding jugular pulsations

Labored breathing

Sudden onset

What causes Caval syndrome?

Heartworms cause partial obstruction of blood flow through the tricuspid valve and interference with valve closure

What are physical exam findings of heartworm disease in a dog?

Tachycardia

Weak pusles

Cough

Difficulty breathing

How is Dirofilaria immitis diagnosed?

Detecting antigen in the blood

Recovery of microfilaria from the blood

Clinical signs

What antigen is detected in heartworm disease?

Female uterine antigen

What type of heartworms must be present for a positive antigen test?

Female heartworms

According to the American Heartworm Society, there is no justification to test dogs for heartworm before what age?

7 months of age

What is the prepatent period of Dirofilaria immitis?

6-7 months

When do antibodies start to appear after infection?

7-10 days

What does the detection of antibodies mean?

The animal was exposed

Doesn't mean there's an active infection

What does detection of antigens mean?

There is an active infection

What is needed to test for antigens/antibodies?

Test kit

Blood/serum

Conjugate

Substrate

What happens in an antibody test?

Patient serum is added to a substrate

A conjugated anti-antibody is added and binds to the patient antibody

Patient antibody binds to the antigen and prevents the anti-antibody from being washed away

What happens in an antigen test?

Patient serum is added to a substrate

Conjugated antibodies are added and bind to the antigen

The antigen binds to a different antibody in the substrate and prevents the conjugated antibody from being washed away

What type of test is available for Dirofilaria immitis?

Antigen for canine and feline

Antibody for just feline

What can cause a false negative antigen test?

Low worm burden

Immature females

Male only infection

Not allowing test to warm to room temp

Immune complexes binding antigen

Why do immune complexes cause a false negative antigen test?

Immune complexes don't leave space for binding to the conjugated antibody or the substrate antibody, causing the conjugated antibody to be washed away

When is immune complexes binding antigens seen?

Shelter dogs

Dogs put on a "slow-kill" treatment regimen

How can immune complexes binding antigens be undone?

Heating up the patient's serum to destroy the dog antibodies but leave the antigen free

What can cause a false positive heartworm test?

Well type tests due to inadequate washing

Cross reaction with other antigens

Residual circulating antigen following treatment

Unknown antigens or test errors

How long can residual heartworm antigens remain following treatment?

Four months or longer

How long should you wait before retesting for heartworm after treatment?

6-9 months

Can color intensity of a positive antigen test be used to determine the level of worm burden?

No, only tells you that there's antigen present (active infection)

How long post-infection does it take until microfilaria are produced?

6-7 months

What is the approximate amount of heratworm positive dogs that will not have circulating microfilariae?

20%

What are reasons that microfilariae may not be present in a positive heartworm test?

Prepatent infections (immature worms)

Unisex infection (can't produce with only one sex)

Drug-induced reduction

Immune-mediated clearance

What test results are expected when only female worms are present?

Positive antigen test

Negative microilariae test

What test results are expected when only male worms are present?

Negative antigen and microfilariae tests

What drug class can kill most/all circulating microfilariae and decrease production, resulting in a negative microfilariae test?

Macrocyclic lactones

What can happen if a dog is on macrocyclic latone preventives?

Can kill most/all microfilariae and decrease production

What are tests for microfilariae?

Knott's test

Filter techniques

Direct smear

What is the Knott's test?

Where 1 cc of blood is mixed with 9 ccs of formalin and is centrifuged to concentrate the microfilariae in the blood

What is the filter technique (DiFil Test)?

Use of a millipore filter to capture microfilariae as blood is lysed and then forced through a membrane

What is the main in clinic test for microfilariae?

Direct smear

What microfilariae test can be used to quickly confirm a positive antigen test?

Direct smear

What is the downside of using a direct smear to test for microfilariae?

Low sensitivity

What is a direct smear used for in a heartworm dog?

To confirm a positive antigen test by identifying microfilariae

What is the non-pathogenic microfilariae that can be mistaken for Dirofilaria immitis?

Acanthocheilonema reconditium

What can Dirofilaria immitis microfilariae be mistaken for?

Acanthocheilonema reconditum

What is Acanthocheilonema reconditum?

Non-pathogenic subcutaneous filarid that produces microfilariae that can be mistaken for heartworm

Will a heartworm antigen test detect Acanthocheilonema reconditum?

No, it doesn't not cross react

What are some of the major differences between Dirofilaria immitis and Acanthocheilonema reconditum microfilariae?

D. immitis is tapored anteriorly, has no cephalic hook, and is bigger

A. reconditum is blunt anteriorly, has a cephalic hook, and is smaller

What should you do if you find microfilariae in a patient's blood?

Send it to a reference lab and the let the parasitologist differentiate what species it is

What testing can be used to confirm diagnosis and determine the stage of heartworm disease?

Radiographs

Echocardiograms

What is seen in radiographs during a heartworm infection?

Enlargement of the right side of the heart, main pulmonary artery and branches in the lobes of the lung

Blunting and thickening of the pulmonary arteries

What is the use of an echocardiogram in heartworm disease?

Used to evaluated right ventricular enlargement and to look for the presence of heartworms in the main pulmonary artery

When should dogs first be tested for heartworms?

6-7 months of age

What should be done before heartworm preventives are given to a dog?

Antigen and microfilariae tests

When do you retest for heartworms following the first test?

6 months after

Then ANNUALLY

How often should dogs be tested for heartworms?

Annually

When should you retest dogs for heartworms?

During the annual exam

6 months after their first test

If the pet is from a highly endemic area

New pet not on preventive

Non-compliant

After winter "off"

When changing products

When should heat treatment be used during heartworm testing?

When there are clinical signs but negative antigen and microfilariae test

When antigen is negative but microfilariae is positive

What are reasons for a positive antigen test but negative microfilariae test?

Female only infection

Residual antigens from treatment

Cross-reaction

Intermittent drug use

Immune clearance of microfilariae

What can cross react with a heartworm antigen test?

Spriocerca lupi

How can Spirocerca lupi and Dirofilaria immitis be told apart?

Using radiographs

What are reasons a microfilariae test would be negative?

Male only infection

Prepatent period (immature)

Heartworm negative

What are reasons an antigen test would be negative but a microfilariae test positive?

Immune complexes blocking binding (use a heat test)

Acanthocheilonema reconditum

Low worm burden