Heartworm & Serology

Question 1

What is the lifecycle of Dirofilaria immitus? (4)

Answer 1

1. An infected dog gets bit by a mosquito, then she becomes infected with the microfilariae.
2. After 10-14 days, the now infected mosquito can infect other dogs with the microfilariae when they are bitten.
3. Over the course of 6-7 months, the infective larvae will mature into heartworms.
4. The adult heartworms will mate and release their offspring into the dog’s blood stream.

Question 2

Name the six common tests used to test for heartworm

Answer 2

1. Direct smear
2. ELISA
3. Modified knotts
4. Filter technique
5. PCR
6. Immunochromatography

Question 3

Name the most accurate testing method for heartworm in dogs? Cats?

Answer 3

1. Antigen testing
2. Antibody testing

Question 4

Prepatent period in dogs with heartworm

Answer 4

6-7months

Question 5

Heartworm clinical signs in a dog (7)

Answer 5

1. Coughing
2. Tired after little exercise
3. Weight loss
4. Respiratory issues, difficulty breathing
5. Heart failure signs
6. Visible changes in the lungs under radiographs
7. Caval syndrome

Question 6

Heartworm clinical signs in a cat (6)

Answer 6

1. Respiratory signs (asthma)
2. Coughing
3. Vomiting
4. Decreased activity
5. Weight loss
6. Asymptomatic sometimes

Question 7

Dirofiliaria immitis
Adult description:
Male length & description
Female length & description

Answer 7

1. Long, slender white worms
2. Males: 5-7” with corkscrew tails
3. Females: 10-12” larger due to uterus

Question 8

Microfilaria
Length
Physical description

Answer 8

1. 307-322um long
2. Tapered anterior. Tail straight or hooked.

Question 9

Which is male? Female?

Answer 9

Question 10

Host of heartworm: (6)
Primary host: (1)
Intermediate host:

Answer 10

1. Dog, cat, ferret, seal, sea lion, human
2. Dog
3. Mosquito

Question 11

Lifecycle

Answer 11

Question 12

Diagnosing heartworm (

Answer 12

1. Clinical signs
2. Rads of heart & lungs
3. Ultrasound of heart
4. Blood tests
   - Direct smear
   - Modified Knotts test
   - Filter technique
   - ELISA

Question 13

Microfilaria tests are now viewed as ___ assays, secondary to the more sensitive heartworm ___ tests.
What can its detection be used for? (3)

Answer 13

1. Test:
   - Supplemental
   - Antigen
2. Use:
   - Validate positive antigen test
   - Determine if the dog is a potential reservoir for infection
   - Identify dogs at risk for severe reaction to treatment with microfilaricides

Question 14

What are some limitations for microfilaria tests?

Answer 14

1. About 20% of heartworm-infected dogs are not microfilaremic, usually bc only 1 sex of worm is present of infection is prepatent.
2. The use of macrolide preventives may result in clearance of microfilaria from the blood of treated dogs after several months of use.

Question 15

How to do a direct smear test (3)

Answer 15

1. Place a drop of blood, preferably from an EDTA tube, in the center of the slide.
2. Drop cover slip over top & examine under microscope on 10x power.
3. Dirofilaria will have a stationary, writhing movement. Dipetalonema will exhibit a more rapid, directional movement & will often shoot right off of the field.

Question 15

Modified Knotts Test (6)

Answer 15

1. Wear gloves!! 15ml centrifuge tube, add ~10ml of 2% formalin to 1ml anticoagulant blood. Invert capped tube several times. Centrifuge for 5min at 1000-1500 rpms.
2. After spinning, there will be a whitish plug at the bottom of the tube. Discard of the supernatant in waste container.
3. Pipette a drop of new methylene blue.
4. Mix stain
5. Add drop of mixture to a microscope slide with cover. Examine under 10x.
6. Unlike with the direct smear, the microfilaria will present stationary. Dirofilaria & Dipetalonema will be distinguished by shape & size.

Question 16

Filter technique (3)

Answer 16

1. Principle is to take 1ml of blood, rupture the RBCs & push thru a filter
2. Filter is then examined under the microscope for microfilaria
3. Dirofilaria demonstrated periodicity
   - Microfilaria numbers will increase if BD is done at night

Question 17

Heartworm tests may give false negatives in animals that are infected with only ___ worms or very few ___ worms.
These tests aren’t always precise with regard to correlation between the amount of heartworm ___ in the blood & number of adult ___ worms.

Answer 17

1. Male
2. Female
3. Antigen
4. Female

Question 18

PCR testing is used in microfilaria-___ dogs with low ___ burdens.
The amplification of DNA from circulating microfilaria in a multiplex PCR assay gave reliable ___ results for samples containing as few as __ microfilaria/mL. It also distinguished D ___ infections vs D ___ infections.

Answer 18

1. Negative
2. Worm
3. Positive
4. Four, 4
5. D immitis
6. D repens