Exam 2 - Lecture 17

Question 1

Diagnosis of Canine Heartworm Disease

Answer 1

-Clinical Presentation:
-Adult dog, >6 months of age

-Exercise intolerant:
– trouble breathing when
extra effort is required of heart
– Usually lays around all day

-Outdoor animal

-No consistent history of HW prophylaxis

Question 2

Filter Test - Heartworm Diagnosis Blood Examination

Answer 2

-Based on examination of fresh blood collected in EDTA (purple top tube)

-Blood lysed in ~2% Formalin solution

-Microfilariae are concentrated on membrane

-Membrane examined microscopically on glass slide

Question 3

Knott’s Test - Heartworm Diagnosis Blood Examination

Answer 3

-Based on examination of fresh blood collected in EDTA (purple top tube)

-Blood lysed in ~2% Formalin solution

-Microfilariae are concentrated by centrifugation

-Wet mount examined on microscope slide

-Either concentration technique is x6 to x8 better than direct microscopic examination of blood drop

Question 4

Dirofilaria immitis - Heartworm - Size

Answer 4

-Microfilaria in the circulating blood

-Size is important!
-Length 280‐320μm >300μm
-Width 6.1‐7.2μm

-Head is tapered

-Tail is straight

Question 5

Acanthocheilonema reconditum (aka Dipetalonema)

Answer 5

-NOT HEARTWORM!!!

-Veterinary importance for similarity to Canine Heartworm

-Nonpathogenic filarial worm:
– Lives in subcutaneous tissue
– Flea is obligate intermediate host

-Microfilaria circulating in peripheral blood

-Size matters!
– Length 215‐270 um
– Width 4.7‐5.8 um

-Slightly smaller than Dirofilaria immitis

-Head is blunt (like broomstick)

-Tail is often has a hooked appearance

Question 6

Diagnostic Issues

Answer 6

-Testing in Companion Animal Practices to detect infection:
-Symptomatic Animals
-Screening patients in relation to prophylactic programs (Initiation, Renewal)

-Technology Shift over last 30 years away from microfilariae detection and reliance on antigen testing

-Renewed interest in microfilariae detection with recognition of drug resistant isolates important clinical skill carried out locally w/ trained staff

Question 7

Serologic Testing for Heartworm

Answer 7

-Commercially available tests kits validated for use in dogs and cats

-ELISA based format directed at female uterine antigen

-Inexpensive and Easy to use

-Positive test indicated by color change in microwell or development of dot/line on reaction device

-Read visually or by O.D. with ELISA reader

-Inherent test accuracy defined by Sensitivity and Specificity

Question 8

Sensitivity - Serologic Testing for Heartworm

Answer 8

-Sensitivity: the ability of the test to detect subjects with infection

-Affected by:
– amount of antigen available for detection
-# of worms
-sex ratio of worm infection
-maturity of worms

-Tests with poor Sensitivity susceptible
too many false negatives

Question 9

Specificity - Serologic Testing for Heartworm

Answer 9

-Specificity: the ability of the test to detect those without infection

-Affected by:
– false color development
– poor ability to discriminate color/non color
– poor washing technique
– cross reaction with other parasitic species (shared immunodominant proteins)

-Poor Specificity leads to excessive false positives

Question 10

Predictive Value - Serologic Testing for Heartworm

Answer 10

-Predictive Value: the probability that the test correctly discriminates between subjects with and without infection

– Function of:
-Sensitivity, specificity, and prevalence of infection in the population

-Low prevalence correlated with poor
positive predictive value (excessive false positives)

-High prevalence of disease correlated w/ greater positive predictive value

Question 11

Heartworm Prevalence in TN Pet Dogs

Answer 11

-1.75% prevalence in pet dogs State wide

-2.65% Claiborne Co prevalence (animals seen @ local vet practices)
– Pet dogs of existing clients; Pet dogs of “new” clients showing signs of HW disease, etc.

-~2.3% Harrogate/ Cumberland Gap prevalence based on summer “door to door” pedestrian survey (~ 42% of pet owners non‐compliant CHW prophylaxis use)

-Bell Co KY/ Claiborne Co shelter dog prevalence 8.2%

Question 12

Modeling Predictive Value of Heart Worm Serology in Pet Dogs from Claiborne County

Answer 12

-2.5% Prevalence:
– 95% Sensitivity
– 98 % Specificity

-Positive Predictive:
-Value = 56%
– True Positives ÷ Total Positive Tests

Question 13

Modeling Predictive Value of Heart Worm Serology in Pet Dogs on Monthly Prophylaxis

Answer 13

-Lower 1% prevalence justified by:
– Low natural prevalence in well‐cared for pet dog population

-May have tested falsely negative while HW were migrating in pre‐detectable portion of lifecycle
– Low propensity for microfilaremia due to monthly medication
– Minimal reservoir of other infected dogs in close proximity

Question 14

Modeling Predictive Value of Heartworm Serology in Pet Dogs on Monthly Prophylaxis

Answer 14

-CHW Prevalence ~ 1%
-Lower Test Sensitivity (65%) justified by:
– low (1‐2/ host) worm burdens
– increased propensity for single sex (male only) infection

-Positive Predictive Value = 25 %

• “If I get a positive test on a dog from this population the chance it is a “true positive” is only 25%”-

Question 15

Modeling Predictive Value of Heart Worm Serology in Dogs with Clinical Signs

Answer 15

-Dogs w/ clinical signs pathognomonic of CHWD

-Test Sensitivity (~95%) with > 2 female worms

-Positive Predictive Value = 98 %

-CHW test confirming what you already suspect from presentation, radiographs, prophylaxis history, etc.

Question 16

Interpretation of Positive CHW Test

Answer 16

-First ask “which population characteristics best describe this client’s pet?”

a) Pet Dog, good health, some veterinary care
b) Pet Dog, good health, great veterinary care
c) Stray/recently adopted, good health, past vet care “who knows?”
d) Pet Dog, clinical signs, past vet care irrelevant Interpret test with appropriate PPV, confirm with Radiographs, etc. and treat accordingly.

-Remember Predictive Value > w/ Disease Prevalence

Question 17

When Symptomatic Animals Test CHW Negative

Answer 17

-Some clinically ill animals are known to mount a significant antibody response with the effect of binding HW antigen in the blood so it is not available for detection:

– Sometimes these cases may be microfilaremic so the Knott’s test or DiFil test can assist diagnosis
– Recent study indicated that 7.1 % of antigen‐negative samples from HW+ dogs were revealed following heat‐treatment to break antigen‐antibody complexes
– Radiographs and cross‐testing with another HW serology kit is also indicated

Question 18

When Healthy Dogs on CHW Prophylaxis Test Positive

Answer 18

-Although the probability of a False Positive is high, Occasionally True Infections are detected during the annual screening:

– Chances are this animal was probably infected during the last visit, but the test was Falsely Negative because it was conducted during the pre‐patent period (susceptibility gap).

– If owner compliance has been reliable the animal may only
have a couple of worms and can be managed without
problems.

– How does the probability of False Positives influence our beliefs about prophylaxis failure (aka “Drug Resistance”) and owner compliance w/ prophylactic programs?