Lecture 17- Heartworm infection and disease in dogs: part 2 diagnosis and treatment

What are common clinical signs of heartworm disease in an adult dog (>6 months of age)?

Exercise intolerance, trouble breathing with exertion, lethargy, often an outdoor animal, and no consistent history of heartworm prophylaxis.

Why does a dog with heartworm disease have trouble breathing when extra effort is required of the heart?

The presence of adult heartworms in the pulmonary arteries increases resistance to blood flow, leading to pulmonary hypertension and reduced oxygen exchange.

Why might an infected dog appear lethargic and "lay around all day"?

The reduced cardiac output and pulmonary hypertension caused by heartworm infection decrease the dog's ability to tolerate exercise.

What is a common lifestyle risk factor for heartworm disease in dogs?

Being an outdoor animal increases exposure to mosquito vectors that transmit heartworm larvae.

Why is a history of inconsistent heartworm prophylaxis significant in diagnosing heartworm disease?

Lack of regular prevention allows larvae (microfilariae) to mature into adult heartworms, leading to infection and clinical disease.

What type of blood sample is required for a filter test for heartworm diagnosis?

Fresh blood collected in an EDTA (purple top) tube.

What is the purpose of lysing the blood in a ~2% formalin solution in the filter test?

It helps release and concentrate microfilariae for microscopic examination.

How are microfilariae detected in the filter test?

They are concentrated on a membrane and examined microscopically on a glass slide.

How does the Knott's test differ from the filter test in heartworm diagnosis?

The Knott's test uses centrifugation to concentrate microfilariae instead of filtration.

What type of blood sample is required for Knott's test?

Fresh blood collected in an EDTA (purple top) tube.

What is the role of ~2% formalin in the Knott's test?

It lyses the blood, making microfilariae easier to detect under a microscope.

How is microfilariae visualization enhanced in the Knott's test?

A wet mount of the concentrated sample is examined microscopically.

How much more effective are concentration techniques (Knott's test or filter test) compared to direct microscopic examination of a blood drop?

They are approximately 6 to 8 times more effective at detecting microfilariae

Where are microfilariae of Dirofilaria immitis found in an infected dog?

In the circulating blood.

Why is the size of Dirofilaria immitis microfilariae important in diagnosis?

It helps differentiate it from other filarial worms like Acanthocheilonema reconditum.

What are the characteristic measurements of Dirofilaria immitis microfilariae?

Length: 280‐320μm (typically >300μm), Width: 6.1‐7.2μm.

Describe the head and tail morphology of Dirofilaria immitis microfilariae.

Head is tapered, tail is straight.

Why is Acanthocheilonema reconditum important in veterinary medicine?

It is nonpathogenic but resembles Dirofilaria immitis, leading to potential diagnostic confusion.

Where does Acanthocheilonema reconditum reside in the host?

In subcutaneous tissue.

What is the obligate intermediate host for Acanthocheilonema reconditum?

fleas

How do Acanthocheilonema reconditum microfilariae differ from Dirofilaria immitis?

They are smaller, with a blunt anterior end and a hooked tail.

What are the characteristic measurements of Acanthocheilonema reconditum microfilariae?

Length: 215‐270μm, Width: 4.7‐5.8μm.

How can the tail of Acanthocheilonema reconditum microfilariae be described?

It often has a button-hook appearance.

Why is heartworm testing performed in veterinary practice?

To detect infection in symptomatic animals and for screening in prophylactic programs (initiation or renewal).

How has heartworm diagnostic technology shifted in the past 30 years?

There has been a move away from microfilariae detection toward antigen testing.

Why is there renewed interest in microfilariae detection?

Due to the recognition of drug-resistant isolates, making it an important clinical skill.

Who typically performs microfilariae detection in practice?

Trained veterinary staff at local clinics.

What type of heartworm tests are commercially available for dogs and cats?

ELISA-based antigen tests.

What antigen does the ELISA heartworm test detect?

Female heartworm uterine antigen.

How are ELISA heartworm tests interpreted?

By color change in a microwell or development of a dot/line on a reaction device, which can be read visually or with an ELISA reader.

What defines the accuracy of serologic heartworm tests?

Sensitivity and specificity.

What is sensitivity in diagnostic testing?

The ability of the test to correctly identify infected subjects

What factors affect the sensitivity of heartworm antigen tests?

Number of worms present

Sex ratio of worms (only detects female antigen)

Maturity of worms

What happens when a test has poor sensitivity?

It leads to false negatives

What is specificity in diagnostic testing?

The ability of the test to correctly identify non-infected subjects.

What factors affect the specificity of heartworm antigen tests?

False color development

Poor ability to discriminate color vs. non-color

Poor washing technique

Cross-reaction with other parasites due to shared immunodominant proteins

What happens when a test has poor specificity?

It leads to excessive false positives.

What is predictive value in diagnostic testing?

The probability that the test correctly discriminates between infected and non-infected individuals.

What factors influence the predictive value of heartworm tests?

Sensitivity, specificity, and prevalence of infection in the population.

How does disease prevalence affect positive predictive value (PPV)?

Low prevalence leads to poor PPV (excessive false positives).

High prevalence increases PPV (greater accuracy in positive results).

What is the statewide prevalence of heartworm in pet dogs in Tennessee?

1.75%

What is the heartworm prevalence in pet dogs in Claiborne County, TN?

2.65% among animals seen at local vet practices

What is the estimated heartworm prevalence in Harrogate/Cumberland Gap, TN?

~2.3%, based on a summer pedestrian survey

What percentage of pet owners in the Harrogate/Cumberland Gap survey were non-compliant with heartworm prophylaxis?

~42%

What is the heartworm prevalence in shelter dogs from Bell County, KY, and Claiborne County, TN?

8.2%

What is the estimated heartworm prevalence in pet dogs in Claiborne County for modeling serology tests?

2.5%

What are the sensitivity and specificity values for heartworm serology testing in Claiborne County?

Sensitivity: 95%

Specificity: 98%

What is the Positive Predictive Value (PPV) of heartworm serology testing in Claiborne County?

56%

How is Positive Predictive Value (PPV) calculated?

True Positives ÷ Total Positive Tests

Why is heartworm prevalence lower (~1%) in dogs on monthly prophylaxis?

Low natural prevalence in well-cared-for pet populations.

Possible false negatives during early pre-detectable infection stages.

Monthly prophylaxis reduces microfilaremia.

Minimal exposure due to fewer infected reservoirs nearby.

What is the test sensitivity for detecting heartworm in dogs on monthly prophylaxis?

65%, due to:

- Low worm burdens (1-2 worms per host).

- Increased likelihood of single-sex (male-only) infections, which are undetectable by antigen tests.

What is the Positive Predictive Value (PPV) for heartworm serology in dogs on monthly prophylaxis?

25%

What does a 25% PPV mean for heartworm testing in a low-prevalence population?

If a dog tests positive, there is only a 25% chance that it is a true positive.

Why does test sensitivity (~95%) improve when there are >2 female worms?

Higher worm burdens provide more antigen, increasing detection likelihood.

What is the Positive Predictive Value (PPV) of heartworm testing in symptomatic dogs?

98%, meaning the test strongly confirms clinical suspicion

Why is PPV higher in symptomatic dogs compared to dogs on prophylaxis?

Clinical signs strongly suggest heartworm disease.

Radiographs, history, and physical exam findings reinforce diagnosis.

Higher worm burdens provide more detectable antigen.

What is the first question to ask when interpreting a positive heartworm (CHW) test?

"Which population characteristics best describe this client's pet?"

What are the possible population characteristics to consider when interpreting a positive CHW test?

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 unknown

d) Pet dog, clinical signs, past vet care irrelevant

How do you interpret a positive heartworm test after determining the population characteristics?

Use the appropriate Positive Predictive Value (PPV) and confirm with radiographs and other diagnostic tools. Treatment should be based on these findings.

What affects the predictive value in interpreting a positive CHW test?

The disease prevalence in the population. Higher disease prevalence leads to a higher PPV.

Why do some symptomatic animals test negative for heartworm?

These animals may mount a significant antibody response that binds the heartworm antigen, making it unavailable for detection.

How can the presence of microfilaremia affect heartworm diagnosis in symptomatic animals?

Some animals may be microfilaremic, and a Knott's test or DiFil test could be needed to detect microfilariae.

What recent study finding explains why some antigen-negative samples from HW-positive dogs might still show infection?

7.1% of antigen-negative samples were revealed as positive after heat treatment to break antigen-antibody complexes.

What other diagnostic approaches should be considered when a symptomatic dog tests negative for heartworm?

Radiographs and cross-testing with another heartworm serology kit may be necessary to confirm the diagnosis.

What is the likelihood of a false positive result in healthy dogs on heartworm prophylaxis?

The probability of a false positive is high, but occasionally true infections are detected during annual screening.

Why might a healthy dog on heartworm prophylaxis test positive for heartworm?

The dog might have been infected during the last visit, but the previous test was falsely negative because it was conducted during the pre-patent period (susceptibility gap).

How can a dog with a couple of worms be managed after a positive test on prophylaxis?

If the owner has been compliant with the prophylaxis, the infection may be minimal (only a couple of worms) and can typically be managed without significant issues.

How does the probability of false positives influence the belief in prophylaxis failure or drug resistance?

A high probability of false positives can lead to misconceptions about prophylaxis failure or drug resistance and misinterpretations of owner compliance.