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Heartworm Antigen and Antibody Tests



DEFINITION


Antigen test kits detect adult heartworm (Dirofilaria immitis) antigen. Antibody tests detect circulating antibody to either microfilarial cuticular antigen or to adult heartworm antigen.



SYNONYM


Occult heartworm test



TYPICAL NORMAL RANGE


Results are reported as negative or positive. Some tests are semiquantitative, reported as weakly positive. This is not necessarily indicative of worm burden (see causes of abnormally low levels).



PHYSIOLOGY



• Antigen tests detect an antigen primarily derived from adult female heartworm reproductive tract in the circulating blood of the host.

• Antibody tests detect host species-specific circulating antibodies against either adult heartworms or larvae.


CLINICAL APPLICATIONS



CAUSES OF ABNORMALLY HIGH LEVELS



• Antigen: infection with adult heartworms

• Antibody: indicates exposure to D. immitis, but not all infected animals will develop adult heartworms or heartworm disease.

NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS HIGH: Thoracic radiographs, CBC, serum biochemistry panel, urinalysis, +/–echocardiogram (if significant changes in aforementioned tests)



CAUSES OF ABNORMALLY LOW LEVELS



• Antigen test: falsely low levels may be seen with very low worm burden (one or two female worms), single-sex (male) infection, immature worms (<5 months old).

• Antibody test: false negatives may occur <30 days postinfection or if animal fails to produce detectable antibodies.

NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS LOW: Retest in 1-2 months, assess for clinical signs of heartworm disease.


IMPORTANT INTERSPECIES DIFFERENCES: Antigen tests are less sensitive in the cat (low worm burden, single-sex infections). Antibody tests can be useful in cats with clinical signs, but even these may only become positive 2-3 months postinfection and return to negative within 3 months thereafter. Therefore, a combination of an antigen and antibody test, and possibly echocardiography, may improve overall sensitivity in cats compared to the use of individual antibody tests alone.



SPECIMEN AND PROCESSING CONSIDERATIONS


DRUG EFFECTS ON LEVELS: Antigen test may remain positive up to 16 weeks post adulticide treatment.


LAB ARTIFACTS THAT MAY INTERFERE: Severe hemolysis or lipemia may interfere with test (false positives, some false negatives). Antigen may degrade if sample not properly handled or stored.


SPECIMEN: Serum (red-top tube). Store at 2○C-8○C. Some assays may also allow whole blood or plasma.


RELATIVE COST: $



PEARLS


Echocardiography (cardiac ultrasound) rivals or surpasses the heartworm antibody test for sensitivity of detection of adult heartworms in cats.


AUTHOR: LOIS ROTH-JOHNSON


1ST EDITION AUTHOR: FONZIE QUANCE-FITCH



Heartworm Filter Test



DEFINITION


Concentration method for the detection of circulating micriofilariae of Dirofilaria immitis.



SYNONYMS


Difil test, filaria filter test, Knott’s test



TYPICAL NORMAL RANGE


Reported as negative or positive



PHYSIOLOGY


Circulating microfilariae are the larval (L1) stage of adult heartworms (D. immitis). Must be distinguished from circulating larval form of Acanthocheilonema (formerly Dipetalonema) reconditum.



CLINICAL APPLICATIONS


CAUSES OF ABNORMALLY HIGH LEVELS: Circulating microfilariae of D. immitis or A. reconditum


NEXT STEP TO CONSIDER IF LEVELS HIGH: Rule out A. reconditum larva is larger than a red blood cell; (diameter of D. immitis is smaller than a red blood cell); confirm with heartworm antigen test; thoracic radiographs (+/− CBC, serum biochemistry profile, urinalysis, echocardiography) may be useful to stage the disease.


CAUSES OF ABNORMALLY LOW LEVELS: Occult infections (adult heartworms are present but circulating microfilariae are absent) may be present due to single-sex infection, immature worms, immune-mediated destruction of microfilariae, prophylactic administration without initial testing.


NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS LOW: Heartworm antigen test to rule out occult infection


IMPORTANT INTERSPECIES DIFFERENCES: Cats with heartworm disease are frequently microfilaria negative.



SPECIMEN AND PROCESSING CONSIDERATIONS


DRUG EFFECTS ON LEVELS: Animals receiving monthly heartworm prophylaxis may be microfilaremia negative despite having heartworm disease.


SPECIMEN: Anticoagulated whole blood in EDTA (lavender-top tube), citrate (blue-top tube) or heparin (green-top tube)


RELATIVE COST: $



PEARLS


As a screening test, the heartworm microfilaria test is less sensitive than the heartworm antigen test. It has been replaced almost entirely by antigen testing, except in large colony environments where the lower material cost of microfilaria testing makes it the only feasible choice (e.g., certain shelter contexts).


AUTHOR: LOIS ROTH-JOHNSON


1ST EDITION AUTHOR: FONZIE QUANCE-FITCH



Heinz Bodies



DEFINITION


Single or multiple precipitated denatured hemoglobin particles on the red cell (erythrocyte) membrane, often associated with oxidative damage



SYNONYMS


Erythrocyte refractile body or ER body, HB, Schmauch body



TYPICAL NORMAL RANGE


Low proportion seen in red blood cells (RBCs) of normal cats (see below)



PHYSIOLOGY


Oxidants may cause irreversible denaturation of hemoglobin molecule, causing Heinz body (HB) formation. HBs have affinity for membrane protein band 3, forming a complex with it, resulting in clustering of membrane protein band 3 on both the internal and external RBC membrane. External clustering of protein band 3 creates a recognition site for autoantibodies. HBs also make RBCs rigid and less deformable (via cross-linking of spectrin and hemoglobin). These mechanisms make affected RBCs prone to lysis (intravascular hemolysis) or phagocytosis by macrophages in the spleen (extravascular hemolysis).



CLINICAL APPLICATIONS


CAUSES OF ABNORMALLY HIGH LEVELS: Drugs (see drug effects) and chemicals from food (onions, garlic, chives) or other substances (zinc); deficiency of enzymes that protect against oxidants


NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS HIGH:


• Review patient history (diet, drug or toxin exposure).

• Check CBC, urinalysis, chemistry profile for anemia, hemolysis.

IMPORTANT INTERSPECIES DIFFERENCES: Normal in feline RBCs (5%-96% may have HBs) because of innately unstable hemoglobin structure, predisposition to form methemoglobin (low methemoglobin reductase activity), and inefficient removal by spleen. Normally occurring HBs are not associated with anemia or hemolysis. Increased HB formation may also occur without significant anemia in diabetes mellitus, hyperthyroidism, and lymphoma. In other species, increased numbers of HBs may be seen post splenectomy.

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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on H

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