Page 2643 - Cote clinical veterinary advisor dogs and cats 4th

1350 Heartworm Serology Testing

Next Diagnostic Steps to Consider Next Diagnostic Steps to Consider Pearls
if Levels Are High if Levels Are Low • Knowledge of microfilarial morphology is
VetBooks.ir and D. repens. A. reconditum larva’s body is an occult infection. some areas of the United States and world, D.
necessary for interpretation of this test, as in
Consider a heartworm antigen test to rule out
Differentiation of D. immitis from A. reconditum
generally smaller in diameter than a canine
immitis is not the only microfilarial that can
Important Interspecies Differences
erythrocyte (≈8 mcm), while D. immitis is
identified microfilariae include Dirofilaria
approximately the same diameter or slightly Cats usually have occult heartworm infections; be identified in circulation. Other commonly
larger. D. repens is an uncommon infection thus other heartworm testing (e.g., antibody repens and Acanthocheilonema reconditum.
in the United States. Confirmation with a and/or antigen test or advanced imaging) is • The availability of this test has become vari-
heartworm antigen test and/or advanced recommended. able in reference laboratories with the advent
imaging (echocardiogram, thoracic radiograph) of more sensitive and specific point-of-care
is recommended. Consider staging the patient Drug Effects Dirofilaria immitis testing.
for heartworm disease using the American Concurrent heartworm preventative medications
Heartworm Society guidelines (https:// decrease the sensitivity of this test secondary AUTHOR: Erin N. Burton, DVM, MS, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
www.heartwormsociety.org/). to destruction of microfilariae.
Causes of Abnormally Low Levels Specimen Collection and Handling
Occult infections (the presence of adult Approximately 1 to 2 mL of anticoagulated
heartworm without a microfilaremia) result whole blood (EDTA, heparin, citrate) is neces-
from single-sex infections, prepatent infec- sary to perform this test.
tion, destruction of microfilariae by monthly
heartworm preventatives, or host immune Relative Cost: $
destruction of microfilariae.

Heartworm Serology Testing

Definition Causes of Abnormally Low Levels • Improper handling or storage of the sample
Antigen and antibody testing for Dirofilaria • Antigen: false-negatives result from low may result in degradation of the antigen.
immitis worm burden (<3 female worms), male-only
infections, or if only immature stages are Specimen Collection and Handling
Synonym present. Natural antibody-antigen complexes • Antigen: whole blood, serum, or plasma
Occult heartworm testing have been documented to interfere with test. depending on kit; specimens may be refriger-
• Antibody: false-negatives result from insuf- ated for up to five days or frozen for longer
Physiology ficient antibody production or conversion storage.
• Antigen tests detect the adult female nema- time (<60 days postinfection). Each antibody • Antibody: serum, 1 mL. Specimens may be
todes of Dirofilaria immitis. test varies at level of sensitivity for different refrigerated for up to five days or frozen for
• Antibody testing detects the species-specific stages of nematode maturation. longer storage.
antibodies resulting from immature, adult
male, adult female, and single-sex nematode Next Diagnostic Steps to Consider Relative Cost: Antigen or antibody $;
D. immitis infections. if Levels Are Low combined antibody and antigen panel $$
If clinical signs are present, consider other
Reference Interval diagnostics such advanced imaging (tho- Pearls
• Antigen tests are reported as positive or no racic radiographs, echocardiogram), CBC/ • The color intensity of antigen tests does
antigen detected. biochemistry panel with a complete urinalysis, not correlate well with the degree of worm
• Antibody test are reported as positive or no or request heat treatment of serum sample burden.
antibodies detected. before antigen testing. • Positive antibody test conveys only exposure.
Not all animals that have had exposure have
Causes of Abnormally High Levels Important Interspecies Differences clinical disease.
• Antigen: infection with adult nematodes; The sensitivity of the heartworm antigen and • False-negative antigen tests after “slow kill”
false-positive results may occur with marked antibody testing in cats is markedly lower than therapy are possible. If suspected, send
hemolysis dogs, thus negative results should be interpreted samples to laboratory for heat treatment
• Antibody: indicates exposure to larvae or with caution. Using both antibody and antigen before testing.
adult nematodes. Seroconversion in cats can testing in cats may improve the accuracy of • Kits for in-clinic testing may decrease cost
be detected as early as 60 days postinfection. diagnosis in this species. per test if a high number of tests are done.
• Current information and guidelines can be
Next Diagnostic Steps to Consider Drug Effects found online from the American Heartworm
if Levels Are High The antigen test can remain positive for up to Society; https://heartwormsociety.org.
Consider staging the patient for heartworm 6 months post-adulticide treatment.
disease using the American Heartworm Society AUTHOR: Erin N. Burton, DVM, MS, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
guidelines (https://www.heartwormsociety.org/ Lab Artifacts
veterinary-resources/american-heartworm- • Improper storage of the antigen test kits may
society-guidelines). interfere with test results.

www.ExpertConsult.com