Heartworm

Question 1

What’s the lifecycle of Dirofilaria immitis?

Answer 1

• mature adult heartworm lives in the pulmonary arteries
• mates, release microfilaria in the circulation
• mosquito takes a bloodmeal –> goes in, molts 2 more times with the appropriate ambient temperature & Wolbachia pipientis
• mosquito feeds again, L3 goes back to a host
• molts to L4 in the skin, adipose tissue, muscles within 2 weeks, then to S5 (immature adult) in 2-3m
• immature adult enters to vascular system, goes to the heart and lungs –> final maturation & mating

Question 2

How long can a dog be infective for?

Answer 2

• Adult worms can live for 5-7y
• microfilaria up to 30m
• disease starts as soon as S5 is reached –> once in circulation, can cause inflammation and respiratory signs, etc

Question 3

What’s the pathophysiology of heartworm disease?

Answer 3

• worm-induced inflammation, pulmonary hypertension, vasculitis, thromboembolism
• loss of vascular integrity, fibrosis, physical occlusion
• R sided heart failure
• arrhythmia
• glomerulonephritis from antigen-antibody complex

Question 4

What are the clinical signs of heartworm disease in dogs?

Answer 4

Majority of the patients are asymptomatic
- weight loss, decreased exercise tolerance, cough, dyspnea, lethargy, poor condition
- split 2nd heart sound, right sided heart murmur
- R sided heart failure –> distended jugular, ascites

Question 5

How is heartworm diagnosed in dogs?

Answer 5

• direct microfilaria detection
• immunodiagnostic antigen test
• CXR
• ECG
• Echo

Question 6

How does modified Knott’s test work? What’s the limitation?

Answer 6

• it’s basically blood smear evaluation looking for microfilaria, using fresh blood, formaldehyde and methylene blue
• can ddx between D immitis and A reconditum
• A reconditum is not detected on the heartworm antigen test
• not useful in cats, or dogs with no or low numbers of circulating microfilaria, amicrofilaria

Question 7

What diagnostic tests are available for direct detection of microfilaria?

Answer 7

• modified Knott’s test
• millipore filtration
• wet mount

Question 8

How is amicrofilaria produced?

Answer 8

immune mediated destruction of the microfilaria in the lungs (eosinophilic pneumonitis)

Question 9

How does immunodiagnostic antigen tests work for HWI? What are the limitations?

Answer 9

• detects antigens produced by adult female heart worms
• false negative if happens to be all male worms, low number of female worms
• also has a latent period of about 6m for first infection (S5 needs time to mature into adults)
• some provide quantitative data to reflect tumour burden

Question 10

What’s the overall sensitivity and specificity of ELISA snap tests for HWI?

Answer 10

Sensitivity = 79%
Specificity = 97%

Question 11

How can CXR be useful for HWI?

Answer 11

Not used as a screening tool
- but can assess the severity of infection, and pulmonary changes

CXR signs:
- R sided hypertrophy
- prominent main pulmonary artery segment
- increased size and density of pulmonary arteries
- pulmonary artery tortuosity and pruning

• interstitial pattern with pulmonary infiltration

Question 12

What ECG abnormalities would be noted in HWI?

Answer 12

• patterns associated with R ventricular enlargement

Question 13

How does echo help with HWI diagnosis?

Answer 13

• sensitive in finding R sided heart enlargement (deep S wave, prolonged QRS interval)

Question 14

How common is HWI in cats?

Answer 14

not common
- generally not the preferred target host for the feeding mosquitoes
- they will have to feed on a canid to be an effective vector’
- cats are inherently resistant to HWI
- life cycle takes longer in the cat
- however, can still have pulmonary and pulmonary vascular pathology even if an infection never matures

Question 15

What’s HARD?

Answer 15

heartworm associated respiratory disease
- seen in cats
- disease develops even though the heartworm never develops to mature adult

Question 16

What are the most common clinical signs of cats with HWD?

Answer 16

signs are due to worm death/ embolization or aberrant migration
- cough, dyspnea
- hypersalivation, tachycardia, shock, hemoptysis, v/d, syncope, dementia, ataxia, circling, head tilt, blindness, seizure, death

Question 17

How is HWD diagnosed in cats?

Question 18

How does antigen/ antibody positivity relate to feline HWD?

Answer 18

antigen positive = nearly always have mature HWI
antibody positive = typically do NOT have mature HWI
antibody positive/ antigen negative = 50% develop HARD